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Old engine for Continuous Time Bayesian Networks. Superseded by reCTBN. 🐍 https://github.com/madlabunimib/PyCTBN
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PyCTBN/venv/lib/python3.9/site-packages/scipy/stats/_continuous_distns.py

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Refactor on docs; Add performance comparison
4 years ago
# -*- coding: utf-8 -*-
#
# Author: Travis Oliphant 2002-2011 with contributions from
# SciPy Developers 2004-2011
#
import warnings
from collections.abc import Iterable
import ctypes
import numpy as np
from scipy._lib.doccer import (extend_notes_in_docstring,
replace_notes_in_docstring)
from scipy._lib._ccallback import LowLevelCallable
from scipy import optimize
from scipy import integrate
from scipy import interpolate
import scipy.special as sc
import scipy.special._ufuncs as scu
from scipy._lib._util import _lazyselect, _lazywhere
from . import _stats
from ._rvs_sampling import rvs_ratio_uniforms
from ._tukeylambda_stats import (tukeylambda_variance as _tlvar,
tukeylambda_kurtosis as _tlkurt)
from ._distn_infrastructure import (get_distribution_names, _kurtosis,
_ncx2_cdf, _ncx2_log_pdf, _ncx2_pdf,
rv_continuous, _skew, valarray,
_get_fixed_fit_value, _check_shape)
from ._ksstats import kolmogn, kolmognp, kolmogni
from ._constants import (_XMIN, _EULER, _ZETA3, _XMAX, _LOGXMAX,
_SQRT_2_OVER_PI, _LOG_SQRT_2_OVER_PI)
# In numpy 1.12 and above, np.power refuses to raise integers to negative
# powers, and `np.float_power` is a new replacement.
try:
float_power = np.float_power
except AttributeError:
float_power = np.power
def _remove_optimizer_parameters(kwds):
"""
Remove the optimizer-related keyword arguments 'loc', 'scale' and
'optimizer' from `kwds`. Then check that `kwds` is empty, and
raise `TypeError("Unknown arguments: %s." % kwds)` if it is not.
This function is used in the fit method of distributions that override
the default method and do not use the default optimization code.
`kwds` is modified in-place.
"""
kwds.pop('loc', None)
kwds.pop('scale', None)
kwds.pop('optimizer', None)
if kwds:
raise TypeError("Unknown arguments: %s." % kwds)
## Kolmogorov-Smirnov one-sided and two-sided test statistics
class ksone_gen(rv_continuous):
r"""Kolmogorov-Smirnov one-sided test statistic distribution.
This is the distribution of the one-sided Kolmogorov-Smirnov (KS)
statistics :math:`D_n^+` and :math:`D_n^-`
for a finite sample size ``n`` (the shape parameter).
%(before_notes)s
Notes
-----
:math:`D_n^+` and :math:`D_n^-` are given by
.. math::
D_n^+ &= \text{sup}_x (F_n(x) - F(x)),\\
D_n^- &= \text{sup}_x (F(x) - F_n(x)),\\
where :math:`F` is a continuous CDF and :math:`F_n` is an empirical CDF.
`ksone` describes the distribution under the null hypothesis of the KS test
that the empirical CDF corresponds to :math:`n` i.i.d. random variates
with CDF :math:`F`.
%(after_notes)s
See Also
--------
kstwobign, kstwo, kstest
References
----------
.. [1] Birnbaum, Z. W. and Tingey, F.H. "One-sided confidence contours
for probability distribution functions", The Annals of Mathematical
Statistics, 22(4), pp 592-596 (1951).
%(example)s
"""
def _pdf(self, x, n):
return -scu._smirnovp(n, x)
def _cdf(self, x, n):
return scu._smirnovc(n, x)
def _sf(self, x, n):
return sc.smirnov(n, x)
def _ppf(self, q, n):
return scu._smirnovci(n, q)
def _isf(self, q, n):
return sc.smirnovi(n, q)
ksone = ksone_gen(a=0.0, b=1.0, name='ksone')
class kstwo_gen(rv_continuous):
r"""Kolmogorov-Smirnov two-sided test statistic distribution.
This is the distribution of the two-sided Kolmogorov-Smirnov (KS)
statistic :math:`D_n` for a finite sample size ``n``
(the shape parameter).
%(before_notes)s
Notes
-----
:math:`D_n` is given by
.. math::
D_n &= \text{sup}_x |F_n(x) - F(x)|
where :math:`F` is a (continuous) CDF and :math:`F_n` is an empirical CDF.
`kstwo` describes the distribution under the null hypothesis of the KS test
that the empirical CDF corresponds to :math:`n` i.i.d. random variates
with CDF :math:`F`.
%(after_notes)s
See Also
--------
kstwobign, ksone, kstest
References
----------
.. [1] Simard, R., L'Ecuyer, P. "Computing the Two-Sided
Kolmogorov-Smirnov Distribution", Journal of Statistical Software,
Vol 39, 11, 1-18 (2011).
%(example)s
"""
def _get_support(self, n):
return (0.5/(n if not isinstance(n, Iterable) else np.asanyarray(n)),
1.0)
def _pdf(self, x, n):
return kolmognp(n, x)
def _cdf(self, x, n):
return kolmogn(n, x)
def _sf(self, x, n):
return kolmogn(n, x, cdf=False)
def _ppf(self, q, n):
return kolmogni(n, q, cdf=True)
def _isf(self, q, n):
return kolmogni(n, q, cdf=False)
# Use the pdf, (not the ppf) to compute moments
kstwo = kstwo_gen(momtype=0, a=0.0, b=1.0, name='kstwo')
class kstwobign_gen(rv_continuous):
r"""Limiting distribution of scaled Kolmogorov-Smirnov two-sided test statistic.
This is the asymptotic distribution of the two-sided Kolmogorov-Smirnov
statistic :math:`\sqrt{n} D_n` that measures the maximum absolute
distance of the theoretical (continuous) CDF from the empirical CDF.
(see `kstest`).
%(before_notes)s
Notes
-----
:math:`\sqrt{n} D_n` is given by
.. math::
D_n = \text{sup}_x |F_n(x) - F(x)|
where :math:`F` is a continuous CDF and :math:`F_n` is an empirical CDF.
`kstwobign` describes the asymptotic distribution (i.e. the limit of
:math:`\sqrt{n} D_n`) under the null hypothesis of the KS test that the
empirical CDF corresponds to i.i.d. random variates with CDF :math:`F`.
%(after_notes)s
See Also
--------
ksone, kstwo, kstest
References
----------
.. [1] Feller, W. "On the Kolmogorov-Smirnov Limit Theorems for Empirical
Distributions", Ann. Math. Statist. Vol 19, 177-189 (1948).
%(example)s
"""
def _pdf(self, x):
return -scu._kolmogp(x)
def _cdf(self, x):
return scu._kolmogc(x)
def _sf(self, x):
return sc.kolmogorov(x)
def _ppf(self, q):
return scu._kolmogci(q)
def _isf(self, q):
return sc.kolmogi(q)
kstwobign = kstwobign_gen(a=0.0, name='kstwobign')
## Normal distribution
# loc = mu, scale = std
# Keep these implementations out of the class definition so they can be reused
# by other distributions.
_norm_pdf_C = np.sqrt(2*np.pi)
_norm_pdf_logC = np.log(_norm_pdf_C)
def _norm_pdf(x):
return np.exp(-x**2/2.0) / _norm_pdf_C
def _norm_logpdf(x):
return -x**2 / 2.0 - _norm_pdf_logC
def _norm_cdf(x):
return sc.ndtr(x)
def _norm_logcdf(x):
return sc.log_ndtr(x)
def _norm_ppf(q):
return sc.ndtri(q)
def _norm_sf(x):
return _norm_cdf(-x)
def _norm_logsf(x):
return _norm_logcdf(-x)
def _norm_isf(q):
return -_norm_ppf(q)
class norm_gen(rv_continuous):
r"""A normal continuous random variable.
The location (``loc``) keyword specifies the mean.
The scale (``scale``) keyword specifies the standard deviation.
%(before_notes)s
Notes
-----
The probability density function for `norm` is:
.. math::
f(x) = \frac{\exp(-x^2/2)}{\sqrt{2\pi}}
for a real number :math:`x`.
%(after_notes)s
%(example)s
"""
def _rvs(self, size=None, random_state=None):
return random_state.standard_normal(size)
def _pdf(self, x):
# norm.pdf(x) = exp(-x**2/2)/sqrt(2*pi)
return _norm_pdf(x)
def _logpdf(self, x):
return _norm_logpdf(x)
def _cdf(self, x):
return _norm_cdf(x)
def _logcdf(self, x):
return _norm_logcdf(x)
def _sf(self, x):
return _norm_sf(x)
def _logsf(self, x):
return _norm_logsf(x)
def _ppf(self, q):
return _norm_ppf(q)
def _isf(self, q):
return _norm_isf(q)
def _stats(self):
return 0.0, 1.0, 0.0, 0.0
def _entropy(self):
return 0.5*(np.log(2*np.pi)+1)
@replace_notes_in_docstring(rv_continuous, notes="""\
This function uses explicit formulas for the maximum likelihood
estimation of the normal distribution parameters, so the
`optimizer` argument is ignored.\n\n""")
def fit(self, data, **kwds):
floc = kwds.pop('floc', None)
fscale = kwds.pop('fscale', None)
_remove_optimizer_parameters(kwds)
if floc is not None and fscale is not None:
# This check is for consistency with `rv_continuous.fit`.
# Without this check, this function would just return the
# parameters that were given.
raise ValueError("All parameters fixed. There is nothing to "
"optimize.")
data = np.asarray(data)
if not np.isfinite(data).all():
raise RuntimeError("The data contains non-finite values.")
if floc is None:
loc = data.mean()
else:
loc = floc
if fscale is None:
scale = np.sqrt(((data - loc)**2).mean())
else:
scale = fscale
return loc, scale
def _munp(self, n):
"""
@returns Moments of standard normal distribution for integer n >= 0
See eq. 16 of https://arxiv.org/abs/1209.4340v2
"""
if n % 2 == 0:
return sc.factorial2(n - 1)
else:
return 0.
norm = norm_gen(name='norm')
class alpha_gen(rv_continuous):
r"""An alpha continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `alpha` ([1]_, [2]_) is:
.. math::
f(x, a) = \frac{1}{x^2 \Phi(a) \sqrt{2\pi}} *
\exp(-\frac{1}{2} (a-1/x)^2)
where :math:`\Phi` is the normal CDF, :math:`x > 0`, and :math:`a > 0`.
`alpha` takes ``a`` as a shape parameter.
%(after_notes)s
References
----------
.. [1] Johnson, Kotz, and Balakrishnan, "Continuous Univariate
Distributions, Volume 1", Second Edition, John Wiley and Sons,
p. 173 (1994).
.. [2] Anthony A. Salvia, "Reliability applications of the Alpha
Distribution", IEEE Transactions on Reliability, Vol. R-34,
No. 3, pp. 251-252 (1985).
%(example)s
"""
_support_mask = rv_continuous._open_support_mask
def _pdf(self, x, a):
# alpha.pdf(x, a) = 1/(x**2*Phi(a)*sqrt(2*pi)) * exp(-1/2 * (a-1/x)**2)
return 1.0/(x**2)/_norm_cdf(a)*_norm_pdf(a-1.0/x)
def _logpdf(self, x, a):
return -2*np.log(x) + _norm_logpdf(a-1.0/x) - np.log(_norm_cdf(a))
def _cdf(self, x, a):
return _norm_cdf(a-1.0/x) / _norm_cdf(a)
def _ppf(self, q, a):
return 1.0/np.asarray(a-sc.ndtri(q*_norm_cdf(a)))
def _stats(self, a):
return [np.inf]*2 + [np.nan]*2
alpha = alpha_gen(a=0.0, name='alpha')
class anglit_gen(rv_continuous):
r"""An anglit continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `anglit` is:
.. math::
f(x) = \sin(2x + \pi/2) = \cos(2x)
for :math:`-\pi/4 \le x \le \pi/4`.
%(after_notes)s
%(example)s
"""
def _pdf(self, x):
# anglit.pdf(x) = sin(2*x + \pi/2) = cos(2*x)
return np.cos(2*x)
def _cdf(self, x):
return np.sin(x+np.pi/4)**2.0
def _ppf(self, q):
return np.arcsin(np.sqrt(q))-np.pi/4
def _stats(self):
return 0.0, np.pi*np.pi/16-0.5, 0.0, -2*(np.pi**4 - 96)/(np.pi*np.pi-8)**2
def _entropy(self):
return 1-np.log(2)
anglit = anglit_gen(a=-np.pi/4, b=np.pi/4, name='anglit')
class arcsine_gen(rv_continuous):
r"""An arcsine continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `arcsine` is:
.. math::
f(x) = \frac{1}{\pi \sqrt{x (1-x)}}
for :math:`0 < x < 1`.
%(after_notes)s
%(example)s
"""
def _pdf(self, x):
# arcsine.pdf(x) = 1/(pi*sqrt(x*(1-x)))
return 1.0/np.pi/np.sqrt(x*(1-x))
def _cdf(self, x):
return 2.0/np.pi*np.arcsin(np.sqrt(x))
def _ppf(self, q):
return np.sin(np.pi/2.0*q)**2.0
def _stats(self):
mu = 0.5
mu2 = 1.0/8
g1 = 0
g2 = -3.0/2.0
return mu, mu2, g1, g2
def _entropy(self):
return -0.24156447527049044468
arcsine = arcsine_gen(a=0.0, b=1.0, name='arcsine')
class FitDataError(ValueError):
# This exception is raised by, for example, beta_gen.fit when both floc
# and fscale are fixed and there are values in the data not in the open
# interval (floc, floc+fscale).
def __init__(self, distr, lower, upper):
self.args = (
"Invalid values in `data`. Maximum likelihood "
"estimation with {distr!r} requires that {lower!r} < x "
"< {upper!r} for each x in `data`.".format(
distr=distr, lower=lower, upper=upper),
)
class FitSolverError(RuntimeError):
# This exception is raised by, for example, beta_gen.fit when
# optimize.fsolve returns with ier != 1.
def __init__(self, mesg):
emsg = "Solver for the MLE equations failed to converge: "
emsg += mesg.replace('\n', '')
self.args = (emsg,)
def _beta_mle_a(a, b, n, s1):
# The zeros of this function give the MLE for `a`, with
# `b`, `n` and `s1` given. `s1` is the sum of the logs of
# the data. `n` is the number of data points.
psiab = sc.psi(a + b)
func = s1 - n * (-psiab + sc.psi(a))
return func
def _beta_mle_ab(theta, n, s1, s2):
# Zeros of this function are critical points of
# the maximum likelihood function. Solving this system
# for theta (which contains a and b) gives the MLE for a and b
# given `n`, `s1` and `s2`. `s1` is the sum of the logs of the data,
# and `s2` is the sum of the logs of 1 - data. `n` is the number
# of data points.
a, b = theta
psiab = sc.psi(a + b)
func = [s1 - n * (-psiab + sc.psi(a)),
s2 - n * (-psiab + sc.psi(b))]
return func
class beta_gen(rv_continuous):
r"""A beta continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `beta` is:
.. math::
f(x, a, b) = \frac{\Gamma(a+b) x^{a-1} (1-x)^{b-1}}
{\Gamma(a) \Gamma(b)}
for :math:`0 <= x <= 1`, :math:`a > 0`, :math:`b > 0`, where
:math:`\Gamma` is the gamma function (`scipy.special.gamma`).
`beta` takes :math:`a` and :math:`b` as shape parameters.
%(after_notes)s
%(example)s
"""
def _rvs(self, a, b, size=None, random_state=None):
return random_state.beta(a, b, size)
def _pdf(self, x, a, b):
# gamma(a+b) * x**(a-1) * (1-x)**(b-1)
# beta.pdf(x, a, b) = ------------------------------------
# gamma(a)*gamma(b)
return np.exp(self._logpdf(x, a, b))
def _logpdf(self, x, a, b):
lPx = sc.xlog1py(b - 1.0, -x) + sc.xlogy(a - 1.0, x)
lPx -= sc.betaln(a, b)
return lPx
def _cdf(self, x, a, b):
return sc.btdtr(a, b, x)
def _ppf(self, q, a, b):
return sc.btdtri(a, b, q)
def _stats(self, a, b):
mn = a*1.0 / (a + b)
var = (a*b*1.0)/(a+b+1.0)/(a+b)**2.0
g1 = 2.0*(b-a)*np.sqrt((1.0+a+b)/(a*b)) / (2+a+b)
g2 = 6.0*(a**3 + a**2*(1-2*b) + b**2*(1+b) - 2*a*b*(2+b))
g2 /= a*b*(a+b+2)*(a+b+3)
return mn, var, g1, g2
def _fitstart(self, data):
g1 = _skew(data)
g2 = _kurtosis(data)
def func(x):
a, b = x
sk = 2*(b-a)*np.sqrt(a + b + 1) / (a + b + 2) / np.sqrt(a*b)
ku = a**3 - a**2*(2*b-1) + b**2*(b+1) - 2*a*b*(b+2)
ku /= a*b*(a+b+2)*(a+b+3)
ku *= 6
return [sk-g1, ku-g2]
a, b = optimize.fsolve(func, (1.0, 1.0))
return super(beta_gen, self)._fitstart(data, args=(a, b))
@extend_notes_in_docstring(rv_continuous, notes="""\
In the special case where both `floc` and `fscale` are given, a
`ValueError` is raised if any value `x` in `data` does not satisfy
`floc < x < floc + fscale`.\n\n""")
def fit(self, data, *args, **kwds):
# Override rv_continuous.fit, so we can more efficiently handle the
# case where floc and fscale are given.
floc = kwds.get('floc', None)
fscale = kwds.get('fscale', None)
if floc is None or fscale is None:
# do general fit
return super(beta_gen, self).fit(data, *args, **kwds)
# We already got these from kwds, so just pop them.
kwds.pop('floc', None)
kwds.pop('fscale', None)
f0 = _get_fixed_fit_value(kwds, ['f0', 'fa', 'fix_a'])
f1 = _get_fixed_fit_value(kwds, ['f1', 'fb', 'fix_b'])
_remove_optimizer_parameters(kwds)
if f0 is not None and f1 is not None:
# This check is for consistency with `rv_continuous.fit`.
raise ValueError("All parameters fixed. There is nothing to "
"optimize.")
# Special case: loc and scale are constrained, so we are fitting
# just the shape parameters. This can be done much more efficiently
# than the method used in `rv_continuous.fit`. (See the subsection
# "Two unknown parameters" in the section "Maximum likelihood" of
# the Wikipedia article on the Beta distribution for the formulas.)
if not np.isfinite(data).all():
raise RuntimeError("The data contains non-finite values.")
# Normalize the data to the interval [0, 1].
data = (np.ravel(data) - floc) / fscale
if np.any(data <= 0) or np.any(data >= 1):
raise FitDataError("beta", lower=floc, upper=floc + fscale)
xbar = data.mean()
if f0 is not None or f1 is not None:
# One of the shape parameters is fixed.
if f0 is not None:
# The shape parameter a is fixed, so swap the parameters
# and flip the data. We always solve for `a`. The result
# will be swapped back before returning.
b = f0
data = 1 - data
xbar = 1 - xbar
else:
b = f1
# Initial guess for a. Use the formula for the mean of the beta
# distribution, E[x] = a / (a + b), to generate a reasonable
# starting point based on the mean of the data and the given
# value of b.
a = b * xbar / (1 - xbar)
# Compute the MLE for `a` by solving _beta_mle_a.
theta, info, ier, mesg = optimize.fsolve(
_beta_mle_a, a,
args=(b, len(data), np.log(data).sum()),
full_output=True
)
if ier != 1:
raise FitSolverError(mesg=mesg)
a = theta[0]
if f0 is not None:
# The shape parameter a was fixed, so swap back the
# parameters.
a, b = b, a
else:
# Neither of the shape parameters is fixed.
# s1 and s2 are used in the extra arguments passed to _beta_mle_ab
# by optimize.fsolve.
s1 = np.log(data).sum()
s2 = sc.log1p(-data).sum()
# Use the "method of moments" to estimate the initial
# guess for a and b.
fac = xbar * (1 - xbar) / data.var(ddof=0) - 1
a = xbar * fac
b = (1 - xbar) * fac
# Compute the MLE for a and b by solving _beta_mle_ab.
theta, info, ier, mesg = optimize.fsolve(
_beta_mle_ab, [a, b],
args=(len(data), s1, s2),
full_output=True
)
if ier != 1:
raise FitSolverError(mesg=mesg)
a, b = theta
return a, b, floc, fscale
beta = beta_gen(a=0.0, b=1.0, name='beta')
class betaprime_gen(rv_continuous):
r"""A beta prime continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `betaprime` is:
.. math::
f(x, a, b) = \frac{x^{a-1} (1+x)^{-a-b}}{\beta(a, b)}
for :math:`x >= 0`, :math:`a > 0`, :math:`b > 0`, where
:math:`\beta(a, b)` is the beta function (see `scipy.special.beta`).
`betaprime` takes ``a`` and ``b`` as shape parameters.
%(after_notes)s
%(example)s
"""
_support_mask = rv_continuous._open_support_mask
def _rvs(self, a, b, size=None, random_state=None):
u1 = gamma.rvs(a, size=size, random_state=random_state)
u2 = gamma.rvs(b, size=size, random_state=random_state)
return u1 / u2
def _pdf(self, x, a, b):
# betaprime.pdf(x, a, b) = x**(a-1) * (1+x)**(-a-b) / beta(a, b)
return np.exp(self._logpdf(x, a, b))
def _logpdf(self, x, a, b):
return sc.xlogy(a - 1.0, x) - sc.xlog1py(a + b, x) - sc.betaln(a, b)
def _cdf(self, x, a, b):
return sc.betainc(a, b, x/(1.+x))
def _munp(self, n, a, b):
if n == 1.0:
return np.where(b > 1,
a/(b-1.0),
np.inf)
elif n == 2.0:
return np.where(b > 2,
a*(a+1.0)/((b-2.0)*(b-1.0)),
np.inf)
elif n == 3.0:
return np.where(b > 3,
a*(a+1.0)*(a+2.0)/((b-3.0)*(b-2.0)*(b-1.0)),
np.inf)
elif n == 4.0:
return np.where(b > 4,
(a*(a + 1.0)*(a + 2.0)*(a + 3.0) /
((b - 4.0)*(b - 3.0)*(b - 2.0)*(b - 1.0))),
np.inf)
else:
raise NotImplementedError
betaprime = betaprime_gen(a=0.0, name='betaprime')
class bradford_gen(rv_continuous):
r"""A Bradford continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `bradford` is:
.. math::
f(x, c) = \frac{c}{\log(1+c) (1+cx)}
for :math:`0 <= x <= 1` and :math:`c > 0`.
`bradford` takes ``c`` as a shape parameter for :math:`c`.
%(after_notes)s
%(example)s
"""
def _pdf(self, x, c):
# bradford.pdf(x, c) = c / (k * (1+c*x))
return c / (c*x + 1.0) / sc.log1p(c)
def _cdf(self, x, c):
return sc.log1p(c*x) / sc.log1p(c)
def _ppf(self, q, c):
return sc.expm1(q * sc.log1p(c)) / c
def _stats(self, c, moments='mv'):
k = np.log(1.0+c)
mu = (c-k)/(c*k)
mu2 = ((c+2.0)*k-2.0*c)/(2*c*k*k)
g1 = None
g2 = None
if 's' in moments:
g1 = np.sqrt(2)*(12*c*c-9*c*k*(c+2)+2*k*k*(c*(c+3)+3))
g1 /= np.sqrt(c*(c*(k-2)+2*k))*(3*c*(k-2)+6*k)
if 'k' in moments:
g2 = (c**3*(k-3)*(k*(3*k-16)+24)+12*k*c*c*(k-4)*(k-3) +
6*c*k*k*(3*k-14) + 12*k**3)
g2 /= 3*c*(c*(k-2)+2*k)**2
return mu, mu2, g1, g2
def _entropy(self, c):
k = np.log(1+c)
return k/2.0 - np.log(c/k)
bradford = bradford_gen(a=0.0, b=1.0, name='bradford')
class burr_gen(rv_continuous):
r"""A Burr (Type III) continuous random variable.
%(before_notes)s
See Also
--------
fisk : a special case of either `burr` or `burr12` with ``d=1``
burr12 : Burr Type XII distribution
mielke : Mielke Beta-Kappa / Dagum distribution
Notes
-----
The probability density function for `burr` is:
.. math::
f(x, c, d) = c d x^{-c - 1} / (1 + x^{-c})^{d + 1}
for :math:`x >= 0` and :math:`c, d > 0`.
`burr` takes :math:`c` and :math:`d` as shape parameters.
This is the PDF corresponding to the third CDF given in Burr's list;
specifically, it is equation (11) in Burr's paper [1]_. The distribution
is also commonly referred to as the Dagum distribution [2]_. If the
parameter :math:`c < 1` then the mean of the distribution does not
exist and if :math:`c < 2` the variance does not exist [2]_.
The PDF is finite at the left endpoint :math:`x = 0` if :math:`c * d >= 1`.
%(after_notes)s
References
----------
.. [1] Burr, I. W. "Cumulative frequency functions", Annals of
Mathematical Statistics, 13(2), pp 215-232 (1942).
.. [2] https://en.wikipedia.org/wiki/Dagum_distribution
.. [3] Kleiber, Christian. "A guide to the Dagum distributions."
Modeling Income Distributions and Lorenz Curves pp 97-117 (2008).
%(example)s
"""
# Do not set _support_mask to rv_continuous._open_support_mask
# Whether the left-hand endpoint is suitable for pdf evaluation is dependent
# on the values of c and d: if c*d >= 1, the pdf is finite, otherwise infinite.
def _pdf(self, x, c, d):
# burr.pdf(x, c, d) = c * d * x**(-c-1) * (1+x**(-c))**(-d-1)
output = _lazywhere(x == 0, [x, c, d],
lambda x_, c_, d_: c_ * d_ * (x_**(c_*d_-1)) / (1 + x_**c_),
f2 = lambda x_, c_, d_: (c_ * d_ * (x_ ** (-c_ - 1.0)) /
((1 + x_ ** (-c_)) ** (d_ + 1.0))))
if output.ndim == 0:
return output[()]
return output
def _logpdf(self, x, c, d):
output = _lazywhere(
x == 0, [x, c, d],
lambda x_, c_, d_: (np.log(c_) + np.log(d_) + sc.xlogy(c_*d_ - 1, x_)
- (d_+1) * sc.log1p(x_**(c_))),
f2 = lambda x_, c_, d_: (np.log(c_) + np.log(d_)
+ sc.xlogy(-c_ - 1, x_)
- sc.xlog1py(d_+1, x_**(-c_))))
if output.ndim == 0:
return output[()]
return output
def _cdf(self, x, c, d):
return (1 + x**(-c))**(-d)
def _logcdf(self, x, c, d):
return sc.log1p(x**(-c)) * (-d)
def _sf(self, x, c, d):
return np.exp(self._logsf(x, c, d))
def _logsf(self, x, c, d):
return np.log1p(- (1 + x**(-c))**(-d))
def _ppf(self, q, c, d):
return (q**(-1.0/d) - 1)**(-1.0/c)
def _stats(self, c, d):
nc = np.arange(1, 5).reshape(4,1) / c
#ek is the kth raw moment, e1 is the mean e2-e1**2 variance etc.
e1, e2, e3, e4 = sc.beta(d + nc, 1. - nc) * d
mu = np.where(c > 1.0, e1, np.nan)
mu2_if_c = e2 - mu**2
mu2 = np.where(c > 2.0, mu2_if_c, np.nan)
g1 = _lazywhere(
c > 3.0,
(c, e1, e2, e3, mu2_if_c),
lambda c, e1, e2, e3, mu2_if_c: (e3 - 3*e2*e1 + 2*e1**3) / np.sqrt((mu2_if_c)**3),
fillvalue=np.nan)
g2 = _lazywhere(
c > 4.0,
(c, e1, e2, e3, e4, mu2_if_c),
lambda c, e1, e2, e3, e4, mu2_if_c: (
((e4 - 4*e3*e1 + 6*e2*e1**2 - 3*e1**4) / mu2_if_c**2) - 3),
fillvalue=np.nan)
return mu, mu2, g1, g2
def _munp(self, n, c, d):
def __munp(n, c, d):
nc = 1. * n / c
return d * sc.beta(1.0 - nc, d + nc)
n, c, d = np.asarray(n), np.asarray(c), np.asarray(d)
return _lazywhere((c > n) & (n == n) & (d == d), (c, d, n),
lambda c, d, n: __munp(n, c, d),
np.nan)
burr = burr_gen(a=0.0, name='burr')
class burr12_gen(rv_continuous):
r"""A Burr (Type XII) continuous random variable.
%(before_notes)s
See Also
--------
fisk : a special case of either `burr` or `burr12` with ``d=1``
burr : Burr Type III distribution
Notes
-----
The probability density function for `burr` is:
.. math::
f(x, c, d) = c d x^{c-1} / (1 + x^c)^{d + 1}
for :math:`x >= 0` and :math:`c, d > 0`.
`burr12` takes ``c`` and ``d`` as shape parameters for :math:`c`
and :math:`d`.
This is the PDF corresponding to the twelfth CDF given in Burr's list;
specifically, it is equation (20) in Burr's paper [1]_.
%(after_notes)s
The Burr type 12 distribution is also sometimes referred to as
the Singh-Maddala distribution from NIST [2]_.
References
----------
.. [1] Burr, I. W. "Cumulative frequency functions", Annals of
Mathematical Statistics, 13(2), pp 215-232 (1942).
.. [2] https://www.itl.nist.gov/div898/software/dataplot/refman2/auxillar/b12pdf.htm
.. [3] "Burr distribution",
https://en.wikipedia.org/wiki/Burr_distribution
%(example)s
"""
def _pdf(self, x, c, d):
# burr12.pdf(x, c, d) = c * d * x**(c-1) * (1+x**(c))**(-d-1)
return np.exp(self._logpdf(x, c, d))
def _logpdf(self, x, c, d):
return np.log(c) + np.log(d) + sc.xlogy(c - 1, x) + sc.xlog1py(-d-1, x**c)
def _cdf(self, x, c, d):
return -sc.expm1(self._logsf(x, c, d))
def _logcdf(self, x, c, d):
return sc.log1p(-(1 + x**c)**(-d))
def _sf(self, x, c, d):
return np.exp(self._logsf(x, c, d))
def _logsf(self, x, c, d):
return sc.xlog1py(-d, x**c)
def _ppf(self, q, c, d):
# The following is an implementation of
# ((1 - q)**(-1.0/d) - 1)**(1.0/c)
# that does a better job handling small values of q.
return sc.expm1(-1/d * sc.log1p(-q))**(1/c)
def _munp(self, n, c, d):
nc = 1. * n / c
return d * sc.beta(1.0 + nc, d - nc)
burr12 = burr12_gen(a=0.0, name='burr12')
class fisk_gen(burr_gen):
r"""A Fisk continuous random variable.
The Fisk distribution is also known as the log-logistic distribution.
%(before_notes)s
Notes
-----
The probability density function for `fisk` is:
.. math::
f(x, c) = c x^{-c-1} (1 + x^{-c})^{-2}
for :math:`x >= 0` and :math:`c > 0`.
`fisk` takes ``c`` as a shape parameter for :math:`c`.
`fisk` is a special case of `burr` or `burr12` with ``d=1``.
%(after_notes)s
See Also
--------
burr
%(example)s
"""
def _pdf(self, x, c):
# fisk.pdf(x, c) = c * x**(-c-1) * (1 + x**(-c))**(-2)
return burr._pdf(x, c, 1.0)
def _cdf(self, x, c):
return burr._cdf(x, c, 1.0)
def _sf(self, x, c):
return burr._sf(x, c, 1.0)
def _logpdf(self, x, c):
# fisk.pdf(x, c) = c * x**(-c-1) * (1 + x**(-c))**(-2)
return burr._logpdf(x, c, 1.0)
def _logcdf(self, x, c):
return burr._logcdf(x, c, 1.0)
def _logsf(self, x, c):
return burr._logsf(x, c, 1.0)
def _ppf(self, x, c):
return burr._ppf(x, c, 1.0)
def _munp(self, n, c):
return burr._munp(n, c, 1.0)
def _stats(self, c):
return burr._stats(c, 1.0)
def _entropy(self, c):
return 2 - np.log(c)
fisk = fisk_gen(a=0.0, name='fisk')
# median = loc
class cauchy_gen(rv_continuous):
r"""A Cauchy continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `cauchy` is
.. math::
f(x) = \frac{1}{\pi (1 + x^2)}
for a real number :math:`x`.
%(after_notes)s
%(example)s
"""
def _pdf(self, x):
# cauchy.pdf(x) = 1 / (pi * (1 + x**2))
return 1.0/np.pi/(1.0+x*x)
def _cdf(self, x):
return 0.5 + 1.0/np.pi*np.arctan(x)
def _ppf(self, q):
return np.tan(np.pi*q-np.pi/2.0)
def _sf(self, x):
return 0.5 - 1.0/np.pi*np.arctan(x)
def _isf(self, q):
return np.tan(np.pi/2.0-np.pi*q)
def _stats(self):
return np.nan, np.nan, np.nan, np.nan
def _entropy(self):
return np.log(4*np.pi)
def _fitstart(self, data, args=None):
# Initialize ML guesses using quartiles instead of moments.
p25, p50, p75 = np.percentile(data, [25, 50, 75])
return p50, (p75 - p25)/2
cauchy = cauchy_gen(name='cauchy')
class chi_gen(rv_continuous):
r"""A chi continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `chi` is:
.. math::
f(x, k) = \frac{1}{2^{k/2-1} \Gamma \left( k/2 \right)}
x^{k-1} \exp \left( -x^2/2 \right)
for :math:`x >= 0` and :math:`k > 0` (degrees of freedom, denoted ``df``
in the implementation). :math:`\Gamma` is the gamma function
(`scipy.special.gamma`).
Special cases of `chi` are:
- ``chi(1, loc, scale)`` is equivalent to `halfnorm`
- ``chi(2, 0, scale)`` is equivalent to `rayleigh`
- ``chi(3, 0, scale)`` is equivalent to `maxwell`
`chi` takes ``df`` as a shape parameter.
%(after_notes)s
%(example)s
"""
def _rvs(self, df, size=None, random_state=None):
return np.sqrt(chi2.rvs(df, size=size, random_state=random_state))
def _pdf(self, x, df):
# x**(df-1) * exp(-x**2/2)
# chi.pdf(x, df) = -------------------------
# 2**(df/2-1) * gamma(df/2)
return np.exp(self._logpdf(x, df))
def _logpdf(self, x, df):
l = np.log(2) - .5*np.log(2)*df - sc.gammaln(.5*df)
return l + sc.xlogy(df - 1., x) - .5*x**2
def _cdf(self, x, df):
return sc.gammainc(.5*df, .5*x**2)
def _ppf(self, q, df):
return np.sqrt(2*sc.gammaincinv(.5*df, q))
def _stats(self, df):
mu = np.sqrt(2)*sc.gamma(df/2.0+0.5)/sc.gamma(df/2.0)
mu2 = df - mu*mu
g1 = (2*mu**3.0 + mu*(1-2*df))/np.asarray(np.power(mu2, 1.5))
g2 = 2*df*(1.0-df)-6*mu**4 + 4*mu**2 * (2*df-1)
g2 /= np.asarray(mu2**2.0)
return mu, mu2, g1, g2
chi = chi_gen(a=0.0, name='chi')
## Chi-squared (gamma-distributed with loc=0 and scale=2 and shape=df/2)
class chi2_gen(rv_continuous):
r"""A chi-squared continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `chi2` is:
.. math::
f(x, k) = \frac{1}{2^{k/2} \Gamma \left( k/2 \right)}
x^{k/2-1} \exp \left( -x/2 \right)
for :math:`x > 0` and :math:`k > 0` (degrees of freedom, denoted ``df``
in the implementation).
`chi2` takes ``df`` as a shape parameter.
%(after_notes)s
%(example)s
"""
def _rvs(self, df, size=None, random_state=None):
return random_state.chisquare(df, size)
def _pdf(self, x, df):
# chi2.pdf(x, df) = 1 / (2*gamma(df/2)) * (x/2)**(df/2-1) * exp(-x/2)
return np.exp(self._logpdf(x, df))
def _logpdf(self, x, df):
return sc.xlogy(df/2.-1, x) - x/2. - sc.gammaln(df/2.) - (np.log(2)*df)/2.
def _cdf(self, x, df):
return sc.chdtr(df, x)
def _sf(self, x, df):
return sc.chdtrc(df, x)
def _isf(self, p, df):
return sc.chdtri(df, p)
def _ppf(self, p, df):
return 2*sc.gammaincinv(df/2, p)
def _stats(self, df):
mu = df
mu2 = 2*df
g1 = 2*np.sqrt(2.0/df)
g2 = 12.0/df
return mu, mu2, g1, g2
chi2 = chi2_gen(a=0.0, name='chi2')
class cosine_gen(rv_continuous):
r"""A cosine continuous random variable.
%(before_notes)s
Notes
-----
The cosine distribution is an approximation to the normal distribution.
The probability density function for `cosine` is:
.. math::
f(x) = \frac{1}{2\pi} (1+\cos(x))
for :math:`-\pi \le x \le \pi`.
%(after_notes)s
%(example)s
"""
def _pdf(self, x):
# cosine.pdf(x) = 1/(2*pi) * (1+cos(x))
return 1.0/2/np.pi*(1+np.cos(x))
def _cdf(self, x):
return 1.0/2/np.pi*(np.pi + x + np.sin(x))
def _stats(self):
return 0.0, np.pi*np.pi/3.0-2.0, 0.0, -6.0*(np.pi**4-90)/(5.0*(np.pi*np.pi-6)**2)
def _entropy(self):
return np.log(4*np.pi)-1.0
cosine = cosine_gen(a=-np.pi, b=np.pi, name='cosine')
class dgamma_gen(rv_continuous):
r"""A double gamma continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `dgamma` is:
.. math::
f(x, a) = \frac{1}{2\Gamma(a)} |x|^{a-1} \exp(-|x|)
for a real number :math:`x` and :math:`a > 0`. :math:`\Gamma` is the
gamma function (`scipy.special.gamma`).
`dgamma` takes ``a`` as a shape parameter for :math:`a`.
%(after_notes)s
%(example)s
"""
def _rvs(self, a, size=None, random_state=None):
u = random_state.uniform(size=size)
gm = gamma.rvs(a, size=size, random_state=random_state)
return gm * np.where(u >= 0.5, 1, -1)
def _pdf(self, x, a):
# dgamma.pdf(x, a) = 1 / (2*gamma(a)) * abs(x)**(a-1) * exp(-abs(x))
ax = abs(x)
return 1.0/(2*sc.gamma(a))*ax**(a-1.0) * np.exp(-ax)
def _logpdf(self, x, a):
ax = abs(x)
return sc.xlogy(a - 1.0, ax) - ax - np.log(2) - sc.gammaln(a)
def _cdf(self, x, a):
fac = 0.5*sc.gammainc(a, abs(x))
return np.where(x > 0, 0.5 + fac, 0.5 - fac)
def _sf(self, x, a):
fac = 0.5*sc.gammainc(a, abs(x))
return np.where(x > 0, 0.5-fac, 0.5+fac)
def _ppf(self, q, a):
fac = sc.gammainccinv(a, 1-abs(2*q-1))
return np.where(q > 0.5, fac, -fac)
def _stats(self, a):
mu2 = a*(a+1.0)
return 0.0, mu2, 0.0, (a+2.0)*(a+3.0)/mu2-3.0
dgamma = dgamma_gen(name='dgamma')
class dweibull_gen(rv_continuous):
r"""A double Weibull continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `dweibull` is given by
.. math::
f(x, c) = c / 2 |x|^{c-1} \exp(-|x|^c)
for a real number :math:`x` and :math:`c > 0`.
`dweibull` takes ``c`` as a shape parameter for :math:`c`.
%(after_notes)s
%(example)s
"""
def _rvs(self, c, size=None, random_state=None):
u = random_state.uniform(size=size)
w = weibull_min.rvs(c, size=size, random_state=random_state)
return w * (np.where(u >= 0.5, 1, -1))
def _pdf(self, x, c):
# dweibull.pdf(x, c) = c / 2 * abs(x)**(c-1) * exp(-abs(x)**c)
ax = abs(x)
Px = c / 2.0 * ax**(c-1.0) * np.exp(-ax**c)
return Px
def _logpdf(self, x, c):
ax = abs(x)
return np.log(c) - np.log(2.0) + sc.xlogy(c - 1.0, ax) - ax**c
def _cdf(self, x, c):
Cx1 = 0.5 * np.exp(-abs(x)**c)
return np.where(x > 0, 1 - Cx1, Cx1)
def _ppf(self, q, c):
fac = 2. * np.where(q <= 0.5, q, 1. - q)
fac = np.power(-np.log(fac), 1.0 / c)
return np.where(q > 0.5, fac, -fac)
def _munp(self, n, c):
return (1 - (n % 2)) * sc.gamma(1.0 + 1.0 * n / c)
# since we know that all odd moments are zeros, return them at once.
# returning Nones from _stats makes the public stats call _munp
# so overall we're saving one or two gamma function evaluations here.
def _stats(self, c):
return 0, None, 0, None
dweibull = dweibull_gen(name='dweibull')
## Exponential (gamma distributed with a=1.0, loc=loc and scale=scale)
class expon_gen(rv_continuous):
r"""An exponential continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `expon` is:
.. math::
f(x) = \exp(-x)
for :math:`x \ge 0`.
%(after_notes)s
A common parameterization for `expon` is in terms of the rate parameter
``lambda``, such that ``pdf = lambda * exp(-lambda * x)``. This
parameterization corresponds to using ``scale = 1 / lambda``.
%(example)s
"""
def _rvs(self, size=None, random_state=None):
return random_state.standard_exponential(size)
def _pdf(self, x):
# expon.pdf(x) = exp(-x)
return np.exp(-x)
def _logpdf(self, x):
return -x
def _cdf(self, x):
return -sc.expm1(-x)
def _ppf(self, q):
return -sc.log1p(-q)
def _sf(self, x):
return np.exp(-x)
def _logsf(self, x):
return -x
def _isf(self, q):
return -np.log(q)
def _stats(self):
return 1.0, 1.0, 2.0, 6.0
def _entropy(self):
return 1.0
@replace_notes_in_docstring(rv_continuous, notes="""\
This function uses explicit formulas for the maximum likelihood
estimation of the exponential distribution parameters, so the
`optimizer`, `loc` and `scale` keyword arguments are ignored.\n\n""")
def fit(self, data, *args, **kwds):
if len(args) > 0:
raise TypeError("Too many arguments.")
floc = kwds.pop('floc', None)
fscale = kwds.pop('fscale', None)
_remove_optimizer_parameters(kwds)
if floc is not None and fscale is not None:
# This check is for consistency with `rv_continuous.fit`.
raise ValueError("All parameters fixed. There is nothing to "
"optimize.")
data = np.asarray(data)
if not np.isfinite(data).all():
raise RuntimeError("The data contains non-finite values.")
data_min = data.min()
if floc is None:
# ML estimate of the location is the minimum of the data.
loc = data_min
else:
loc = floc
if data_min < loc:
# There are values that are less than the specified loc.
raise FitDataError("expon", lower=floc, upper=np.inf)
if fscale is None:
# ML estimate of the scale is the shifted mean.
scale = data.mean() - loc
else:
scale = fscale
# We expect the return values to be floating point, so ensure it
# by explicitly converting to float.
return float(loc), float(scale)
expon = expon_gen(a=0.0, name='expon')
## Exponentially Modified Normal (exponential distribution
## convolved with a Normal).
## This is called an exponentially modified gaussian on wikipedia
class exponnorm_gen(rv_continuous):
r"""An exponentially modified Normal continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `exponnorm` is:
.. math::
f(x, K) = \frac{1}{2K} \exp\left(\frac{1}{2 K^2} - x / K \right)
\text{erfc}\left(-\frac{x - 1/K}{\sqrt{2}}\right)
where :math:`x` is a real number and :math:`K > 0`.
It can be thought of as the sum of a standard normal random variable
and an independent exponentially distributed random variable with rate
``1/K``.
%(after_notes)s
An alternative parameterization of this distribution (for example, in
`Wikipedia <https://en.wikipedia.org/wiki/Exponentially_modified_Gaussian_distribution>`_)
involves three parameters, :math:`\mu`, :math:`\lambda` and
:math:`\sigma`.
In the present parameterization this corresponds to having ``loc`` and
``scale`` equal to :math:`\mu` and :math:`\sigma`, respectively, and
shape parameter :math:`K = 1/(\sigma\lambda)`.
.. versionadded:: 0.16.0
%(example)s
"""
def _rvs(self, K, size=None, random_state=None):
expval = random_state.standard_exponential(size) * K
gval = random_state.standard_normal(size)
return expval + gval
def _pdf(self, x, K):
# exponnorm.pdf(x, K) =
# 1/(2*K) exp(1/(2 * K**2)) exp(-x / K) * erfc-(x - 1/K) / sqrt(2))
invK = 1.0 / K
exparg = 0.5 * invK**2 - invK * x
# Avoid overflows; setting np.exp(exparg) to the max float works
# all right here
expval = _lazywhere(exparg < _LOGXMAX, (exparg,), np.exp, _XMAX)
return 0.5 * invK * (expval * sc.erfc(-(x - invK) / np.sqrt(2)))
def _logpdf(self, x, K):
invK = 1.0 / K
exparg = 0.5 * invK**2 - invK * x
return exparg + np.log(0.5 * invK * sc.erfc(-(x - invK) / np.sqrt(2)))
def _cdf(self, x, K):
invK = 1.0 / K
expval = invK * (0.5 * invK - x)
return _norm_cdf(x) - np.exp(expval) * _norm_cdf(x - invK)
def _sf(self, x, K):
invK = 1.0 / K
expval = invK * (0.5 * invK - x)
return _norm_cdf(-x) + np.exp(expval) * _norm_cdf(x - invK)
def _stats(self, K):
K2 = K * K
opK2 = 1.0 + K2
skw = 2 * K**3 * opK2**(-1.5)
krt = 6.0 * K2 * K2 * opK2**(-2)
return K, opK2, skw, krt
exponnorm = exponnorm_gen(name='exponnorm')
class exponweib_gen(rv_continuous):
r"""An exponentiated Weibull continuous random variable.
%(before_notes)s
See Also
--------
weibull_min, numpy.random.RandomState.weibull
Notes
-----
The probability density function for `exponweib` is:
.. math::
f(x, a, c) = a c [1-\exp(-x^c)]^{a-1} \exp(-x^c) x^{c-1}
and its cumulative distribution function is:
.. math::
F(x, a, c) = [1-\exp(-x^c)]^a
for :math:`x > 0`, :math:`a > 0`, :math:`c > 0`.
`exponweib` takes :math:`a` and :math:`c` as shape parameters:
* :math:`a` is the exponentiation parameter,
with the special case :math:`a=1` corresponding to the
(non-exponentiated) Weibull distribution `weibull_min`.
* :math:`c` is the shape parameter of the non-exponentiated Weibull law.
%(after_notes)s
References
----------
https://en.wikipedia.org/wiki/Exponentiated_Weibull_distribution
%(example)s
"""
def _pdf(self, x, a, c):
# exponweib.pdf(x, a, c) =
# a * c * (1-exp(-x**c))**(a-1) * exp(-x**c)*x**(c-1)
return np.exp(self._logpdf(x, a, c))
def _logpdf(self, x, a, c):
negxc = -x**c
exm1c = -sc.expm1(negxc)
logp = (np.log(a) + np.log(c) + sc.xlogy(a - 1.0, exm1c) +
negxc + sc.xlogy(c - 1.0, x))
return logp
def _cdf(self, x, a, c):
exm1c = -sc.expm1(-x**c)
return exm1c**a
def _ppf(self, q, a, c):
return (-sc.log1p(-q**(1.0/a)))**np.asarray(1.0/c)
exponweib = exponweib_gen(a=0.0, name='exponweib')
class exponpow_gen(rv_continuous):
r"""An exponential power continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `exponpow` is:
.. math::
f(x, b) = b x^{b-1} \exp(1 + x^b - \exp(x^b))
for :math:`x \ge 0`, :math:`b > 0`. Note that this is a different
distribution from the exponential power distribution that is also known
under the names "generalized normal" or "generalized Gaussian".
`exponpow` takes ``b`` as a shape parameter for :math:`b`.
%(after_notes)s
References
----------
http://www.math.wm.edu/~leemis/chart/UDR/PDFs/Exponentialpower.pdf
%(example)s
"""
def _pdf(self, x, b):
# exponpow.pdf(x, b) = b * x**(b-1) * exp(1 + x**b - exp(x**b))
return np.exp(self._logpdf(x, b))
def _logpdf(self, x, b):
xb = x**b
f = 1 + np.log(b) + sc.xlogy(b - 1.0, x) + xb - np.exp(xb)
return f
def _cdf(self, x, b):
return -sc.expm1(-sc.expm1(x**b))
def _sf(self, x, b):
return np.exp(-sc.expm1(x**b))
def _isf(self, x, b):
return (sc.log1p(-np.log(x)))**(1./b)
def _ppf(self, q, b):
return pow(sc.log1p(-sc.log1p(-q)), 1.0/b)
exponpow = exponpow_gen(a=0.0, name='exponpow')
class fatiguelife_gen(rv_continuous):
r"""A fatigue-life (Birnbaum-Saunders) continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `fatiguelife` is:
.. math::
f(x, c) = \frac{x+1}{2c\sqrt{2\pi x^3}} \exp(-\frac{(x-1)^2}{2x c^2})
for :math:`x >= 0` and :math:`c > 0`.
`fatiguelife` takes ``c`` as a shape parameter for :math:`c`.
%(after_notes)s
References
----------
.. [1] "Birnbaum-Saunders distribution",
https://en.wikipedia.org/wiki/Birnbaum-Saunders_distribution
%(example)s
"""
_support_mask = rv_continuous._open_support_mask
def _rvs(self, c, size=None, random_state=None):
z = random_state.standard_normal(size)
x = 0.5*c*z
x2 = x*x
t = 1.0 + 2*x2 + 2*x*np.sqrt(1 + x2)
return t
def _pdf(self, x, c):
# fatiguelife.pdf(x, c) =
# (x+1) / (2*c*sqrt(2*pi*x**3)) * exp(-(x-1)**2/(2*x*c**2))
return np.exp(self._logpdf(x, c))
def _logpdf(self, x, c):
return (np.log(x+1) - (x-1)**2 / (2.0*x*c**2) - np.log(2*c) -
0.5*(np.log(2*np.pi) + 3*np.log(x)))
def _cdf(self, x, c):
return _norm_cdf(1.0 / c * (np.sqrt(x) - 1.0/np.sqrt(x)))
def _ppf(self, q, c):
tmp = c*sc.ndtri(q)
return 0.25 * (tmp + np.sqrt(tmp**2 + 4))**2
def _stats(self, c):
# NB: the formula for kurtosis in wikipedia seems to have an error:
# it's 40, not 41. At least it disagrees with the one from Wolfram
# Alpha. And the latter one, below, passes the tests, while the wiki
# one doesn't So far I didn't have the guts to actually check the
# coefficients from the expressions for the raw moments.
c2 = c*c
mu = c2 / 2.0 + 1.0
den = 5.0 * c2 + 4.0
mu2 = c2*den / 4.0
g1 = 4 * c * (11*c2 + 6.0) / np.power(den, 1.5)
g2 = 6 * c2 * (93*c2 + 40.0) / den**2.0
return mu, mu2, g1, g2
fatiguelife = fatiguelife_gen(a=0.0, name='fatiguelife')
class foldcauchy_gen(rv_continuous):
r"""A folded Cauchy continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `foldcauchy` is:
.. math::
f(x, c) = \frac{1}{\pi (1+(x-c)^2)} + \frac{1}{\pi (1+(x+c)^2)}
for :math:`x \ge 0`.
`foldcauchy` takes ``c`` as a shape parameter for :math:`c`.
%(example)s
"""
def _rvs(self, c, size=None, random_state=None):
return abs(cauchy.rvs(loc=c, size=size,
random_state=random_state))
def _pdf(self, x, c):
# foldcauchy.pdf(x, c) = 1/(pi*(1+(x-c)**2)) + 1/(pi*(1+(x+c)**2))
return 1.0/np.pi*(1.0/(1+(x-c)**2) + 1.0/(1+(x+c)**2))
def _cdf(self, x, c):
return 1.0/np.pi*(np.arctan(x-c) + np.arctan(x+c))
def _stats(self, c):
return np.inf, np.inf, np.nan, np.nan
foldcauchy = foldcauchy_gen(a=0.0, name='foldcauchy')
class f_gen(rv_continuous):
r"""An F continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `f` is:
.. math::
f(x, df_1, df_2) = \frac{df_2^{df_2/2} df_1^{df_1/2} x^{df_1 / 2-1}}
{(df_2+df_1 x)^{(df_1+df_2)/2}
B(df_1/2, df_2/2)}
for :math:`x > 0`.
`f` takes ``dfn`` and ``dfd`` as shape parameters.
%(after_notes)s
%(example)s
"""
def _rvs(self, dfn, dfd, size=None, random_state=None):
return random_state.f(dfn, dfd, size)
def _pdf(self, x, dfn, dfd):
# df2**(df2/2) * df1**(df1/2) * x**(df1/2-1)
# F.pdf(x, df1, df2) = --------------------------------------------
# (df2+df1*x)**((df1+df2)/2) * B(df1/2, df2/2)
return np.exp(self._logpdf(x, dfn, dfd))
def _logpdf(self, x, dfn, dfd):
n = 1.0 * dfn
m = 1.0 * dfd
lPx = m/2 * np.log(m) + n/2 * np.log(n) + sc.xlogy(n/2 - 1, x)
lPx -= ((n+m)/2) * np.log(m + n*x) + sc.betaln(n/2, m/2)
return lPx
def _cdf(self, x, dfn, dfd):
return sc.fdtr(dfn, dfd, x)
def _sf(self, x, dfn, dfd):
return sc.fdtrc(dfn, dfd, x)
def _ppf(self, q, dfn, dfd):
return sc.fdtri(dfn, dfd, q)
def _stats(self, dfn, dfd):
v1, v2 = 1. * dfn, 1. * dfd
v2_2, v2_4, v2_6, v2_8 = v2 - 2., v2 - 4., v2 - 6., v2 - 8.
mu = _lazywhere(
v2 > 2, (v2, v2_2),
lambda v2, v2_2: v2 / v2_2,
np.inf)
mu2 = _lazywhere(
v2 > 4, (v1, v2, v2_2, v2_4),
lambda v1, v2, v2_2, v2_4:
2 * v2 * v2 * (v1 + v2_2) / (v1 * v2_2**2 * v2_4),
np.inf)
g1 = _lazywhere(
v2 > 6, (v1, v2_2, v2_4, v2_6),
lambda v1, v2_2, v2_4, v2_6:
(2 * v1 + v2_2) / v2_6 * np.sqrt(v2_4 / (v1 * (v1 + v2_2))),
np.nan)
g1 *= np.sqrt(8.)
g2 = _lazywhere(
v2 > 8, (g1, v2_6, v2_8),
lambda g1, v2_6, v2_8: (8 + g1 * g1 * v2_6) / v2_8,
np.nan)
g2 *= 3. / 2.
return mu, mu2, g1, g2
f = f_gen(a=0.0, name='f')
## Folded Normal
## abs(Z) where (Z is normal with mu=L and std=S so that c=abs(L)/S)
##
## note: regress docs have scale parameter correct, but first parameter
## he gives is a shape parameter A = c * scale
## Half-normal is folded normal with shape-parameter c=0.
class foldnorm_gen(rv_continuous):
r"""A folded normal continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `foldnorm` is:
.. math::
f(x, c) = \sqrt{2/\pi} cosh(c x) \exp(-\frac{x^2+c^2}{2})
for :math:`c \ge 0`.
`foldnorm` takes ``c`` as a shape parameter for :math:`c`.
%(after_notes)s
%(example)s
"""
def _argcheck(self, c):
return c >= 0
def _rvs(self, c, size=None, random_state=None):
return abs(random_state.standard_normal(size) + c)
def _pdf(self, x, c):
# foldnormal.pdf(x, c) = sqrt(2/pi) * cosh(c*x) * exp(-(x**2+c**2)/2)
return _norm_pdf(x + c) + _norm_pdf(x-c)
def _cdf(self, x, c):
return _norm_cdf(x-c) + _norm_cdf(x+c) - 1.0
def _stats(self, c):
# Regina C. Elandt, Technometrics 3, 551 (1961)
# https://www.jstor.org/stable/1266561
#
c2 = c*c
expfac = np.exp(-0.5*c2) / np.sqrt(2.*np.pi)
mu = 2.*expfac + c * sc.erf(c/np.sqrt(2))
mu2 = c2 + 1 - mu*mu
g1 = 2. * (mu*mu*mu - c2*mu - expfac)
g1 /= np.power(mu2, 1.5)
g2 = c2 * (c2 + 6.) + 3 + 8.*expfac*mu
g2 += (2. * (c2 - 3.) - 3. * mu**2) * mu**2
g2 = g2 / mu2**2.0 - 3.
return mu, mu2, g1, g2
foldnorm = foldnorm_gen(a=0.0, name='foldnorm')
class weibull_min_gen(rv_continuous):
r"""Weibull minimum continuous random variable.
The Weibull Minimum Extreme Value distribution, from extreme value theory
(Fisher-Gnedenko theorem), is also often simply called the Weibull
distribution. It arises as the limiting distribution of the rescaled
minimum of iid random variables.
%(before_notes)s
See Also
--------
weibull_max, numpy.random.RandomState.weibull, exponweib
Notes
-----
The probability density function for `weibull_min` is:
.. math::
f(x, c) = c x^{c-1} \exp(-x^c)
for :math:`x > 0`, :math:`c > 0`.
`weibull_min` takes ``c`` as a shape parameter for :math:`c`.
(named :math:`k` in Wikipedia article and :math:`a` in
``numpy.random.weibull``). Special shape values are :math:`c=1` and
:math:`c=2` where Weibull distribution reduces to the `expon` and
`rayleigh` distributions respectively.
%(after_notes)s
References
----------
https://en.wikipedia.org/wiki/Weibull_distribution
https://en.wikipedia.org/wiki/Fisher-Tippett-Gnedenko_theorem
%(example)s
"""
def _pdf(self, x, c):
# frechet_r.pdf(x, c) = c * x**(c-1) * exp(-x**c)
return c*pow(x, c-1)*np.exp(-pow(x, c))
def _logpdf(self, x, c):
return np.log(c) + sc.xlogy(c - 1, x) - pow(x, c)
def _cdf(self, x, c):
return -sc.expm1(-pow(x, c))
def _sf(self, x, c):
return np.exp(-pow(x, c))
def _logsf(self, x, c):
return -pow(x, c)
def _ppf(self, q, c):
return pow(-sc.log1p(-q), 1.0/c)
def _munp(self, n, c):
return sc.gamma(1.0+n*1.0/c)
def _entropy(self, c):
return -_EULER / c - np.log(c) + _EULER + 1
weibull_min = weibull_min_gen(a=0.0, name='weibull_min')
class weibull_max_gen(rv_continuous):
r"""Weibull maximum continuous random variable.
The Weibull Maximum Extreme Value distribution, from extreme value theory
(Fisher-Gnedenko theorem), is the limiting distribution of rescaled
maximum of iid random variables. This is the distribution of -X
if X is from the `weibull_min` function.
%(before_notes)s
See Also
--------
weibull_min
Notes
-----
The probability density function for `weibull_max` is:
.. math::
f(x, c) = c (-x)^{c-1} \exp(-(-x)^c)
for :math:`x < 0`, :math:`c > 0`.
`weibull_max` takes ``c`` as a shape parameter for :math:`c`.
%(after_notes)s
References
----------
https://en.wikipedia.org/wiki/Weibull_distribution
https://en.wikipedia.org/wiki/Fisher-Tippett-Gnedenko_theorem
%(example)s
"""
def _pdf(self, x, c):
# frechet_l.pdf(x, c) = c * (-x)**(c-1) * exp(-(-x)**c)
return c*pow(-x, c-1)*np.exp(-pow(-x, c))
def _logpdf(self, x, c):
return np.log(c) + sc.xlogy(c-1, -x) - pow(-x, c)
def _cdf(self, x, c):
return np.exp(-pow(-x, c))
def _logcdf(self, x, c):
return -pow(-x, c)
def _sf(self, x, c):
return -sc.expm1(-pow(-x, c))
def _ppf(self, q, c):
return -pow(-np.log(q), 1.0/c)
def _munp(self, n, c):
val = sc.gamma(1.0+n*1.0/c)
if int(n) % 2:
sgn = -1
else:
sgn = 1
return sgn * val
def _entropy(self, c):
return -_EULER / c - np.log(c) + _EULER + 1
weibull_max = weibull_max_gen(b=0.0, name='weibull_max')
# Public methods to be deprecated in frechet_r and frechet_l:
# ['__call__', 'cdf', 'entropy', 'expect', 'fit', 'fit_loc_scale', 'freeze',
# 'interval', 'isf', 'logcdf', 'logpdf', 'logsf', 'mean', 'median', 'moment',
# 'nnlf', 'pdf', 'ppf', 'rvs', 'sf', 'stats', 'std', 'var']
_frechet_r_deprec_msg = """\
The distribution `frechet_r` is a synonym for `weibull_min`; this historical
usage is deprecated because of possible confusion with the (quite different)
Frechet distribution. To preserve the existing behavior of the program, use
`scipy.stats.weibull_min`. For the Frechet distribution (i.e. the Type II
extreme value distribution), use `scipy.stats.invweibull`."""
class frechet_r_gen(weibull_min_gen):
"""A Frechet right (or Weibull minimum) continuous random variable.
%(before_notes)s
See Also
--------
weibull_min : The same distribution as `frechet_r`.
Notes
-----
%(after_notes)s
%(example)s
"""
@np.deprecate(old_name='frechet_r', message=_frechet_r_deprec_msg)
def __call__(self, *args, **kwargs):
return weibull_min_gen.__call__(self, *args, **kwargs)
@np.deprecate(old_name='frechet_r', message=_frechet_r_deprec_msg)
def cdf(self, *args, **kwargs):
return weibull_min_gen.cdf(self, *args, **kwargs)
@np.deprecate(old_name='frechet_r', message=_frechet_r_deprec_msg)
def entropy(self, *args, **kwargs):
return weibull_min_gen.entropy(self, *args, **kwargs)
@np.deprecate(old_name='frechet_r', message=_frechet_r_deprec_msg)
def expect(self, *args, **kwargs):
return weibull_min_gen.expect(self, *args, **kwargs)
@np.deprecate(old_name='frechet_r', message=_frechet_r_deprec_msg)
def fit(self, *args, **kwargs):
return weibull_min_gen.fit(self, *args, **kwargs)
@np.deprecate(old_name='frechet_r', message=_frechet_r_deprec_msg)
def fit_loc_scale(self, *args, **kwargs):
return weibull_min_gen.fit_loc_scale(self, *args, **kwargs)
@np.deprecate(old_name='frechet_r', message=_frechet_r_deprec_msg)
def freeze(self, *args, **kwargs):
return weibull_min_gen.freeze(self, *args, **kwargs)
@np.deprecate(old_name='frechet_r', message=_frechet_r_deprec_msg)
def interval(self, *args, **kwargs):
return weibull_min_gen.interval(self, *args, **kwargs)
@np.deprecate(old_name='frechet_r', message=_frechet_r_deprec_msg)
def isf(self, *args, **kwargs):
return weibull_min_gen.isf(self, *args, **kwargs)
@np.deprecate(old_name='frechet_r', message=_frechet_r_deprec_msg)
def logcdf(self, *args, **kwargs):
return weibull_min_gen.logcdf(self, *args, **kwargs)
@np.deprecate(old_name='frechet_r', message=_frechet_r_deprec_msg)
def logpdf(self, *args, **kwargs):
return weibull_min_gen.logpdf(self, *args, **kwargs)
@np.deprecate(old_name='frechet_r', message=_frechet_r_deprec_msg)
def logsf(self, *args, **kwargs):
return weibull_min_gen.logsf(self, *args, **kwargs)
@np.deprecate(old_name='frechet_r', message=_frechet_r_deprec_msg)
def mean(self, *args, **kwargs):
return weibull_min_gen.mean(self, *args, **kwargs)
@np.deprecate(old_name='frechet_r', message=_frechet_r_deprec_msg)
def median(self, *args, **kwargs):
return weibull_min_gen.median(self, *args, **kwargs)
@np.deprecate(old_name='frechet_r', message=_frechet_r_deprec_msg)
def moment(self, *args, **kwargs):
return weibull_min_gen.moment(self, *args, **kwargs)
@np.deprecate(old_name='frechet_r', message=_frechet_r_deprec_msg)
def nnlf(self, *args, **kwargs):
return weibull_min_gen.nnlf(self, *args, **kwargs)
@np.deprecate(old_name='frechet_r', message=_frechet_r_deprec_msg)
def pdf(self, *args, **kwargs):
return weibull_min_gen.pdf(self, *args, **kwargs)
@np.deprecate(old_name='frechet_r', message=_frechet_r_deprec_msg)
def ppf(self, *args, **kwargs):
return weibull_min_gen.ppf(self, *args, **kwargs)
@np.deprecate(old_name='frechet_r', message=_frechet_r_deprec_msg)
def rvs(self, *args, **kwargs):
return weibull_min_gen.rvs(self, *args, **kwargs)
@np.deprecate(old_name='frechet_r', message=_frechet_r_deprec_msg)
def sf(self, *args, **kwargs):
return weibull_min_gen.sf(self, *args, **kwargs)
@np.deprecate(old_name='frechet_r', message=_frechet_r_deprec_msg)
def stats(self, *args, **kwargs):
return weibull_min_gen.stats(self, *args, **kwargs)
@np.deprecate(old_name='frechet_r', message=_frechet_r_deprec_msg)
def std(self, *args, **kwargs):
return weibull_min_gen.std(self, *args, **kwargs)
@np.deprecate(old_name='frechet_r', message=_frechet_r_deprec_msg)
def var(self, *args, **kwargs):
return weibull_min_gen.var(self, *args, **kwargs)
frechet_r = frechet_r_gen(a=0.0, name='frechet_r')
_frechet_l_deprec_msg = """\
The distribution `frechet_l` is a synonym for `weibull_max`; this historical
usage is deprecated because of possible confusion with the (quite different)
Frechet distribution. To preserve the existing behavior of the program, use
`scipy.stats.weibull_max`. For the Frechet distribution (i.e. the Type II
extreme value distribution), use `scipy.stats.invweibull`."""
class frechet_l_gen(weibull_max_gen):
"""A Frechet left (or Weibull maximum) continuous random variable.
%(before_notes)s
See Also
--------
weibull_max : The same distribution as `frechet_l`.
Notes
-----
%(after_notes)s
%(example)s
"""
@np.deprecate(old_name='frechet_l', message=_frechet_l_deprec_msg)
def __call__(self, *args, **kwargs):
return weibull_max_gen.__call__(self, *args, **kwargs)
@np.deprecate(old_name='frechet_l', message=_frechet_l_deprec_msg)
def cdf(self, *args, **kwargs):
return weibull_max_gen.cdf(self, *args, **kwargs)
@np.deprecate(old_name='frechet_l', message=_frechet_l_deprec_msg)
def entropy(self, *args, **kwargs):
return weibull_max_gen.entropy(self, *args, **kwargs)
@np.deprecate(old_name='frechet_l', message=_frechet_l_deprec_msg)
def expect(self, *args, **kwargs):
return weibull_max_gen.expect(self, *args, **kwargs)
@np.deprecate(old_name='frechet_l', message=_frechet_l_deprec_msg)
def fit(self, *args, **kwargs):
return weibull_max_gen.fit(self, *args, **kwargs)
@np.deprecate(old_name='frechet_l', message=_frechet_l_deprec_msg)
def fit_loc_scale(self, *args, **kwargs):
return weibull_max_gen.fit_loc_scale(self, *args, **kwargs)
@np.deprecate(old_name='frechet_l', message=_frechet_l_deprec_msg)
def freeze(self, *args, **kwargs):
return weibull_max_gen.freeze(self, *args, **kwargs)
@np.deprecate(old_name='frechet_l', message=_frechet_l_deprec_msg)
def interval(self, *args, **kwargs):
return weibull_max_gen.interval(self, *args, **kwargs)
@np.deprecate(old_name='frechet_l', message=_frechet_l_deprec_msg)
def isf(self, *args, **kwargs):
return weibull_max_gen.isf(self, *args, **kwargs)
@np.deprecate(old_name='frechet_l', message=_frechet_l_deprec_msg)
def logcdf(self, *args, **kwargs):
return weibull_max_gen.logcdf(self, *args, **kwargs)
@np.deprecate(old_name='frechet_l', message=_frechet_l_deprec_msg)
def logpdf(self, *args, **kwargs):
return weibull_max_gen.logpdf(self, *args, **kwargs)
@np.deprecate(old_name='frechet_l', message=_frechet_l_deprec_msg)
def logsf(self, *args, **kwargs):
return weibull_max_gen.logsf(self, *args, **kwargs)
@np.deprecate(old_name='frechet_l', message=_frechet_l_deprec_msg)
def mean(self, *args, **kwargs):
return weibull_max_gen.mean(self, *args, **kwargs)
@np.deprecate(old_name='frechet_l', message=_frechet_l_deprec_msg)
def median(self, *args, **kwargs):
return weibull_max_gen.median(self, *args, **kwargs)
@np.deprecate(old_name='frechet_l', message=_frechet_l_deprec_msg)
def moment(self, *args, **kwargs):
return weibull_max_gen.moment(self, *args, **kwargs)
@np.deprecate(old_name='frechet_l', message=_frechet_l_deprec_msg)
def nnlf(self, *args, **kwargs):
return weibull_max_gen.nnlf(self, *args, **kwargs)
@np.deprecate(old_name='frechet_l', message=_frechet_l_deprec_msg)
def pdf(self, *args, **kwargs):
return weibull_max_gen.pdf(self, *args, **kwargs)
@np.deprecate(old_name='frechet_l', message=_frechet_l_deprec_msg)
def ppf(self, *args, **kwargs):
return weibull_max_gen.ppf(self, *args, **kwargs)
@np.deprecate(old_name='frechet_l', message=_frechet_l_deprec_msg)
def rvs(self, *args, **kwargs):
return weibull_max_gen.rvs(self, *args, **kwargs)
@np.deprecate(old_name='frechet_l', message=_frechet_l_deprec_msg)
def sf(self, *args, **kwargs):
return weibull_max_gen.sf(self, *args, **kwargs)
@np.deprecate(old_name='frechet_l', message=_frechet_l_deprec_msg)
def stats(self, *args, **kwargs):
return weibull_max_gen.stats(self, *args, **kwargs)
@np.deprecate(old_name='frechet_l', message=_frechet_l_deprec_msg)
def std(self, *args, **kwargs):
return weibull_max_gen.std(self, *args, **kwargs)
@np.deprecate(old_name='frechet_l', message=_frechet_l_deprec_msg)
def var(self, *args, **kwargs):
return weibull_max_gen.var(self, *args, **kwargs)
frechet_l = frechet_l_gen(b=0.0, name='frechet_l')
class genlogistic_gen(rv_continuous):
r"""A generalized logistic continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `genlogistic` is:
.. math::
f(x, c) = c \frac{\exp(-x)}
{(1 + \exp(-x))^{c+1}}
for :math:`x >= 0`, :math:`c > 0`.
`genlogistic` takes ``c`` as a shape parameter for :math:`c`.
%(after_notes)s
%(example)s
"""
def _pdf(self, x, c):
# genlogistic.pdf(x, c) = c * exp(-x) / (1 + exp(-x))**(c+1)
return np.exp(self._logpdf(x, c))
def _logpdf(self, x, c):
return np.log(c) - x - (c+1.0)*sc.log1p(np.exp(-x))
def _cdf(self, x, c):
Cx = (1+np.exp(-x))**(-c)
return Cx
def _ppf(self, q, c):
vals = -np.log(pow(q, -1.0/c)-1)
return vals
def _stats(self, c):
mu = _EULER + sc.psi(c)
mu2 = np.pi*np.pi/6.0 + sc.zeta(2, c)
g1 = -2*sc.zeta(3, c) + 2*_ZETA3
g1 /= np.power(mu2, 1.5)
g2 = np.pi**4/15.0 + 6*sc.zeta(4, c)
g2 /= mu2**2.0
return mu, mu2, g1, g2
genlogistic = genlogistic_gen(name='genlogistic')
class genpareto_gen(rv_continuous):
r"""A generalized Pareto continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `genpareto` is:
.. math::
f(x, c) = (1 + c x)^{-1 - 1/c}
defined for :math:`x \ge 0` if :math:`c \ge 0`, and for
:math:`0 \le x \le -1/c` if :math:`c < 0`.
`genpareto` takes ``c`` as a shape parameter for :math:`c`.
For :math:`c=0`, `genpareto` reduces to the exponential
distribution, `expon`:
.. math::
f(x, 0) = \exp(-x)
For :math:`c=-1`, `genpareto` is uniform on ``[0, 1]``:
.. math::
f(x, -1) = 1
%(after_notes)s
%(example)s
"""
def _argcheck(self, c):
return np.isfinite(c)
def _get_support(self, c):
c = np.asarray(c)
b = _lazywhere(c < 0, (c,),
lambda c: -1. / c,
np.inf)
a = np.where(c >= 0, self.a, self.a)
return a, b
def _pdf(self, x, c):
# genpareto.pdf(x, c) = (1 + c * x)**(-1 - 1/c)
return np.exp(self._logpdf(x, c))
def _logpdf(self, x, c):
return _lazywhere((x == x) & (c != 0), (x, c),
lambda x, c: -sc.xlog1py(c + 1., c*x) / c,
-x)
def _cdf(self, x, c):
return -sc.inv_boxcox1p(-x, -c)
def _sf(self, x, c):
return sc.inv_boxcox(-x, -c)
def _logsf(self, x, c):
return _lazywhere((x == x) & (c != 0), (x, c),
lambda x, c: -sc.log1p(c*x) / c,
-x)
def _ppf(self, q, c):
return -sc.boxcox1p(-q, -c)
def _isf(self, q, c):
return -sc.boxcox(q, -c)
def _stats(self, c, moments='mv'):
if 'm' not in moments:
m = None
else:
m = _lazywhere(c < 1, (c,),
lambda xi: 1/(1 - xi),
np.inf)
if 'v' not in moments:
v = None
else:
v = _lazywhere(c < 1/2, (c,),
lambda xi: 1 / (1 - xi)**2 / (1 - 2*xi),
np.nan)
if 's' not in moments:
s = None
else:
s = _lazywhere(c < 1/3, (c,),
lambda xi: 2 * (1 + xi) * np.sqrt(1 - 2*xi) /
(1 - 3*xi),
np.nan)
if 'k' not in moments:
k = None
else:
k = _lazywhere(c < 1/4, (c,),
lambda xi: 3 * (1 - 2*xi) * (2*xi**2 + xi + 3) /
(1 - 3*xi) / (1 - 4*xi) - 3,
np.nan)
return m, v, s, k
def _munp(self, n, c):
def __munp(n, c):
val = 0.0
k = np.arange(0, n + 1)
for ki, cnk in zip(k, sc.comb(n, k)):
val = val + cnk * (-1) ** ki / (1.0 - c * ki)
return np.where(c * n < 1, val * (-1.0 / c) ** n, np.inf)
return _lazywhere(c != 0, (c,),
lambda c: __munp(n, c),
sc.gamma(n + 1))
def _entropy(self, c):
return 1. + c
genpareto = genpareto_gen(a=0.0, name='genpareto')
class genexpon_gen(rv_continuous):
r"""A generalized exponential continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `genexpon` is:
.. math::
f(x, a, b, c) = (a + b (1 - \exp(-c x)))
\exp(-a x - b x + \frac{b}{c} (1-\exp(-c x)))
for :math:`x \ge 0`, :math:`a, b, c > 0`.
`genexpon` takes :math:`a`, :math:`b` and :math:`c` as shape parameters.
%(after_notes)s
References
----------
H.K. Ryu, "An Extension of Marshall and Olkin's Bivariate Exponential
Distribution", Journal of the American Statistical Association, 1993.
N. Balakrishnan, "The Exponential Distribution: Theory, Methods and
Applications", Asit P. Basu.
%(example)s
"""
def _pdf(self, x, a, b, c):
# genexpon.pdf(x, a, b, c) = (a + b * (1 - exp(-c*x))) * \
# exp(-a*x - b*x + b/c * (1-exp(-c*x)))
return (a + b*(-sc.expm1(-c*x)))*np.exp((-a-b)*x +
b*(-sc.expm1(-c*x))/c)
def _cdf(self, x, a, b, c):
return -sc.expm1((-a-b)*x + b*(-sc.expm1(-c*x))/c)
def _logpdf(self, x, a, b, c):
return np.log(a+b*(-sc.expm1(-c*x))) + (-a-b)*x+b*(-sc.expm1(-c*x))/c
genexpon = genexpon_gen(a=0.0, name='genexpon')
class genextreme_gen(rv_continuous):
r"""A generalized extreme value continuous random variable.
%(before_notes)s
See Also
--------
gumbel_r
Notes
-----
For :math:`c=0`, `genextreme` is equal to `gumbel_r`.
The probability density function for `genextreme` is:
.. math::
f(x, c) = \begin{cases}
\exp(-\exp(-x)) \exp(-x) &\text{for } c = 0\\
\exp(-(1-c x)^{1/c}) (1-c x)^{1/c-1} &\text{for }
x \le 1/c, c > 0
\end{cases}
Note that several sources and software packages use the opposite
convention for the sign of the shape parameter :math:`c`.
`genextreme` takes ``c`` as a shape parameter for :math:`c`.
%(after_notes)s
%(example)s
"""
def _argcheck(self, c):
return np.where(abs(c) == np.inf, 0, 1)
def _get_support(self, c):
_b = np.where(c > 0, 1.0 / np.maximum(c, _XMIN), np.inf)
_a = np.where(c < 0, 1.0 / np.minimum(c, -_XMIN), -np.inf)
return _a, _b
def _loglogcdf(self, x, c):
return _lazywhere((x == x) & (c != 0), (x, c),
lambda x, c: sc.log1p(-c*x)/c, -x)
def _pdf(self, x, c):
# genextreme.pdf(x, c) =
# exp(-exp(-x))*exp(-x), for c==0
# exp(-(1-c*x)**(1/c))*(1-c*x)**(1/c-1), for x \le 1/c, c > 0
return np.exp(self._logpdf(x, c))
def _logpdf(self, x, c):
cx = _lazywhere((x == x) & (c != 0), (x, c), lambda x, c: c*x, 0.0)
logex2 = sc.log1p(-cx)
logpex2 = self._loglogcdf(x, c)
pex2 = np.exp(logpex2)
# Handle special cases
np.putmask(logpex2, (c == 0) & (x == -np.inf), 0.0)
logpdf = np.where((cx == 1) | (cx == -np.inf),
-np.inf,
-pex2+logpex2-logex2)
np.putmask(logpdf, (c == 1) & (x == 1), 0.0)
return logpdf
def _logcdf(self, x, c):
return -np.exp(self._loglogcdf(x, c))
def _cdf(self, x, c):
return np.exp(self._logcdf(x, c))
def _sf(self, x, c):
return -sc.expm1(self._logcdf(x, c))
def _ppf(self, q, c):
x = -np.log(-np.log(q))
return _lazywhere((x == x) & (c != 0), (x, c),
lambda x, c: -sc.expm1(-c * x) / c, x)
def _isf(self, q, c):
x = -np.log(-sc.log1p(-q))
return _lazywhere((x == x) & (c != 0), (x, c),
lambda x, c: -sc.expm1(-c * x) / c, x)
def _stats(self, c):
g = lambda n: sc.gamma(n*c + 1)
g1 = g(1)
g2 = g(2)
g3 = g(3)
g4 = g(4)
g2mg12 = np.where(abs(c) < 1e-7, (c*np.pi)**2.0/6.0, g2-g1**2.0)
gam2k = np.where(abs(c) < 1e-7, np.pi**2.0/6.0,
sc.expm1(sc.gammaln(2.0*c+1.0)-2*sc.gammaln(c + 1.0))/c**2.0)
eps = 1e-14
gamk = np.where(abs(c) < eps, -_EULER, sc.expm1(sc.gammaln(c + 1))/c)
m = np.where(c < -1.0, np.nan, -gamk)
v = np.where(c < -0.5, np.nan, g1**2.0*gam2k)
# skewness
sk1 = _lazywhere(c >= -1./3,
(c, g1, g2, g3, g2mg12),
lambda c, g1, g2, g3, g2gm12:
np.sign(c)*(-g3 + (g2 + 2*g2mg12)*g1)/g2mg12**1.5,
fillvalue=np.nan)
sk = np.where(abs(c) <= eps**0.29, 12*np.sqrt(6)*_ZETA3/np.pi**3, sk1)
# kurtosis
ku1 = _lazywhere(c >= -1./4,
(g1, g2, g3, g4, g2mg12),
lambda g1, g2, g3, g4, g2mg12:
(g4 + (-4*g3 + 3*(g2 + g2mg12)*g1)*g1)/g2mg12**2,
fillvalue=np.nan)
ku = np.where(abs(c) <= (eps)**0.23, 12.0/5.0, ku1-3.0)
return m, v, sk, ku
def _fitstart(self, data):
# This is better than the default shape of (1,).
g = _skew(data)
if g < 0:
a = 0.5
else:
a = -0.5
return super(genextreme_gen, self)._fitstart(data, args=(a,))
def _munp(self, n, c):
k = np.arange(0, n+1)
vals = 1.0/c**n * np.sum(
sc.comb(n, k) * (-1)**k * sc.gamma(c*k + 1),
axis=0)
return np.where(c*n > -1, vals, np.inf)
def _entropy(self, c):
return _EULER*(1 - c) + 1
genextreme = genextreme_gen(name='genextreme')
def _digammainv(y):
# Inverse of the digamma function (real positive arguments only).
# This function is used in the `fit` method of `gamma_gen`.
# The function uses either optimize.fsolve or optimize.newton
# to solve `sc.digamma(x) - y = 0`. There is probably room for
# improvement, but currently it works over a wide range of y:
# >>> y = 64*np.random.randn(1000000)
# >>> y.min(), y.max()
# (-311.43592651416662, 351.77388222276869)
# x = [_digammainv(t) for t in y]
# np.abs(sc.digamma(x) - y).max()
# 1.1368683772161603e-13
#
_em = 0.5772156649015328606065120
func = lambda x: sc.digamma(x) - y
if y > -0.125:
x0 = np.exp(y) + 0.5
if y < 10:
# Some experimentation shows that newton reliably converges
# must faster than fsolve in this y range. For larger y,
# newton sometimes fails to converge.
value = optimize.newton(func, x0, tol=1e-10)
return value
elif y > -3:
x0 = np.exp(y/2.332) + 0.08661
else:
x0 = 1.0 / (-y - _em)
value, info, ier, mesg = optimize.fsolve(func, x0, xtol=1e-11,
full_output=True)
if ier != 1:
raise RuntimeError("_digammainv: fsolve failed, y = %r" % y)
return value[0]
## Gamma (Use MATLAB and MATHEMATICA (b=theta=scale, a=alpha=shape) definition)
## gamma(a, loc, scale) with a an integer is the Erlang distribution
## gamma(1, loc, scale) is the Exponential distribution
## gamma(df/2, 0, 2) is the chi2 distribution with df degrees of freedom.
class gamma_gen(rv_continuous):
r"""A gamma continuous random variable.
%(before_notes)s
See Also
--------
erlang, expon
Notes
-----
The probability density function for `gamma` is:
.. math::
f(x, a) = \frac{x^{a-1} \exp(-x)}{\Gamma(a)}
for :math:`x \ge 0`, :math:`a > 0`. Here :math:`\Gamma(a)` refers to the
gamma function.
`gamma` takes ``a`` as a shape parameter for :math:`a`.
When :math:`a` is an integer, `gamma` reduces to the Erlang
distribution, and when :math:`a=1` to the exponential distribution.
%(after_notes)s
%(example)s
"""
def _rvs(self, a, size=None, random_state=None):
return random_state.standard_gamma(a, size)
def _pdf(self, x, a):
# gamma.pdf(x, a) = x**(a-1) * exp(-x) / gamma(a)
return np.exp(self._logpdf(x, a))
def _logpdf(self, x, a):
return sc.xlogy(a-1.0, x) - x - sc.gammaln(a)
def _cdf(self, x, a):
return sc.gammainc(a, x)
def _sf(self, x, a):
return sc.gammaincc(a, x)
def _ppf(self, q, a):
return sc.gammaincinv(a, q)
def _stats(self, a):
return a, a, 2.0/np.sqrt(a), 6.0/a
def _entropy(self, a):
return sc.psi(a)*(1-a) + a + sc.gammaln(a)
def _fitstart(self, data):
# The skewness of the gamma distribution is `4 / np.sqrt(a)`.
# We invert that to estimate the shape `a` using the skewness
# of the data. The formula is regularized with 1e-8 in the
# denominator to allow for degenerate data where the skewness
# is close to 0.
a = 4 / (1e-8 + _skew(data)**2)
return super(gamma_gen, self)._fitstart(data, args=(a,))
@extend_notes_in_docstring(rv_continuous, notes="""\
When the location is fixed by using the argument `floc`, this
function uses explicit formulas or solves a simpler numerical
problem than the full ML optimization problem. So in that case,
the `optimizer`, `loc` and `scale` arguments are ignored.\n\n""")
def fit(self, data, *args, **kwds):
floc = kwds.get('floc', None)
if floc is None:
# loc is not fixed. Use the default fit method.
return super(gamma_gen, self).fit(data, *args, **kwds)
# We already have this value, so just pop it from kwds.
kwds.pop('floc', None)
f0 = _get_fixed_fit_value(kwds, ['f0', 'fa', 'fix_a'])
fscale = kwds.pop('fscale', None)
_remove_optimizer_parameters(kwds)
# Special case: loc is fixed.
if f0 is not None and fscale is not None:
# This check is for consistency with `rv_continuous.fit`.
# Without this check, this function would just return the
# parameters that were given.
raise ValueError("All parameters fixed. There is nothing to "
"optimize.")
# Fixed location is handled by shifting the data.
data = np.asarray(data)
if not np.isfinite(data).all():
raise RuntimeError("The data contains non-finite values.")
if np.any(data <= floc):
raise FitDataError("gamma", lower=floc, upper=np.inf)
if floc != 0:
# Don't do the subtraction in-place, because `data` might be a
# view of the input array.
data = data - floc
xbar = data.mean()
# Three cases to handle:
# * shape and scale both free
# * shape fixed, scale free
# * shape free, scale fixed
if fscale is None:
# scale is free
if f0 is not None:
# shape is fixed
a = f0
else:
# shape and scale are both free.
# The MLE for the shape parameter `a` is the solution to:
# np.log(a) - sc.digamma(a) - np.log(xbar) +
# np.log(data).mean() = 0
s = np.log(xbar) - np.log(data).mean()
func = lambda a: np.log(a) - sc.digamma(a) - s
aest = (3-s + np.sqrt((s-3)**2 + 24*s)) / (12*s)
xa = aest*(1-0.4)
xb = aest*(1+0.4)
a = optimize.brentq(func, xa, xb, disp=0)
# The MLE for the scale parameter is just the data mean
# divided by the shape parameter.
scale = xbar / a
else:
# scale is fixed, shape is free
# The MLE for the shape parameter `a` is the solution to:
# sc.digamma(a) - np.log(data).mean() + np.log(fscale) = 0
c = np.log(data).mean() - np.log(fscale)
a = _digammainv(c)
scale = fscale
return a, floc, scale
gamma = gamma_gen(a=0.0, name='gamma')
class erlang_gen(gamma_gen):
"""An Erlang continuous random variable.
%(before_notes)s
See Also
--------
gamma
Notes
-----
The Erlang distribution is a special case of the Gamma distribution, with
the shape parameter `a` an integer. Note that this restriction is not
enforced by `erlang`. It will, however, generate a warning the first time
a non-integer value is used for the shape parameter.
Refer to `gamma` for examples.
"""
def _argcheck(self, a):
allint = np.all(np.floor(a) == a)
if not allint:
# An Erlang distribution shouldn't really have a non-integer
# shape parameter, so warn the user.
warnings.warn(
'The shape parameter of the erlang distribution '
'has been given a non-integer value %r.' % (a,),
RuntimeWarning)
return a > 0
def _fitstart(self, data):
# Override gamma_gen_fitstart so that an integer initial value is
# used. (Also regularize the division, to avoid issues when
# _skew(data) is 0 or close to 0.)
a = int(4.0 / (1e-8 + _skew(data)**2))
return super(gamma_gen, self)._fitstart(data, args=(a,))
# Trivial override of the fit method, so we can monkey-patch its
# docstring.
def fit(self, data, *args, **kwds):
return super(erlang_gen, self).fit(data, *args, **kwds)
if fit.__doc__:
fit.__doc__ = (rv_continuous.fit.__doc__ +
"""
Notes
-----
The Erlang distribution is generally defined to have integer values
for the shape parameter. This is not enforced by the `erlang` class.
When fitting the distribution, it will generally return a non-integer
value for the shape parameter. By using the keyword argument
`f0=<integer>`, the fit method can be constrained to fit the data to
a specific integer shape parameter.
""")
erlang = erlang_gen(a=0.0, name='erlang')
class gengamma_gen(rv_continuous):
r"""A generalized gamma continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `gengamma` is:
.. math::
f(x, a, c) = \frac{|c| x^{c a-1} \exp(-x^c)}{\Gamma(a)}
for :math:`x \ge 0`, :math:`a > 0`, and :math:`c \ne 0`.
:math:`\Gamma` is the gamma function (`scipy.special.gamma`).
`gengamma` takes :math:`a` and :math:`c` as shape parameters.
%(after_notes)s
%(example)s
"""
def _argcheck(self, a, c):
return (a > 0) & (c != 0)
def _pdf(self, x, a, c):
# gengamma.pdf(x, a, c) = abs(c) * x**(c*a-1) * exp(-x**c) / gamma(a)
return np.exp(self._logpdf(x, a, c))
def _logpdf(self, x, a, c):
return np.log(abs(c)) + sc.xlogy(c*a - 1, x) - x**c - sc.gammaln(a)
def _cdf(self, x, a, c):
xc = x**c
val1 = sc.gammainc(a, xc)
val2 = sc.gammaincc(a, xc)
return np.where(c > 0, val1, val2)
def _sf(self, x, a, c):
xc = x**c
val1 = sc.gammainc(a, xc)
val2 = sc.gammaincc(a, xc)
return np.where(c > 0, val2, val1)
def _ppf(self, q, a, c):
val1 = sc.gammaincinv(a, q)
val2 = sc.gammainccinv(a, q)
return np.where(c > 0, val1, val2)**(1.0/c)
def _isf(self, q, a, c):
val1 = sc.gammaincinv(a, q)
val2 = sc.gammainccinv(a, q)
return np.where(c > 0, val2, val1)**(1.0/c)
def _munp(self, n, a, c):
# Pochhammer symbol: sc.pocha,n) = gamma(a+n)/gamma(a)
return sc.poch(a, n*1.0/c)
def _entropy(self, a, c):
val = sc.psi(a)
return a*(1-val) + 1.0/c*val + sc.gammaln(a) - np.log(abs(c))
gengamma = gengamma_gen(a=0.0, name='gengamma')
class genhalflogistic_gen(rv_continuous):
r"""A generalized half-logistic continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `genhalflogistic` is:
.. math::
f(x, c) = \frac{2 (1 - c x)^{1/(c-1)}}{[1 + (1 - c x)^{1/c}]^2}
for :math:`0 \le x \le 1/c`, and :math:`c > 0`.
`genhalflogistic` takes ``c`` as a shape parameter for :math:`c`.
%(after_notes)s
%(example)s
"""
def _argcheck(self, c):
return c > 0
def _get_support(self, c):
return self.a, 1.0/c
def _pdf(self, x, c):
# genhalflogistic.pdf(x, c) =
# 2 * (1-c*x)**(1/c-1) / (1+(1-c*x)**(1/c))**2
limit = 1.0/c
tmp = np.asarray(1-c*x)
tmp0 = tmp**(limit-1)
tmp2 = tmp0*tmp
return 2*tmp0 / (1+tmp2)**2
def _cdf(self, x, c):
limit = 1.0/c
tmp = np.asarray(1-c*x)
tmp2 = tmp**(limit)
return (1.0-tmp2) / (1+tmp2)
def _ppf(self, q, c):
return 1.0/c*(1-((1.0-q)/(1.0+q))**c)
def _entropy(self, c):
return 2 - (2*c+1)*np.log(2)
genhalflogistic = genhalflogistic_gen(a=0.0, name='genhalflogistic')
class gompertz_gen(rv_continuous):
r"""A Gompertz (or truncated Gumbel) continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `gompertz` is:
.. math::
f(x, c) = c \exp(x) \exp(-c (e^x-1))
for :math:`x \ge 0`, :math:`c > 0`.
`gompertz` takes ``c`` as a shape parameter for :math:`c`.
%(after_notes)s
%(example)s
"""
def _pdf(self, x, c):
# gompertz.pdf(x, c) = c * exp(x) * exp(-c*(exp(x)-1))
return np.exp(self._logpdf(x, c))
def _logpdf(self, x, c):
return np.log(c) + x - c * sc.expm1(x)
def _cdf(self, x, c):
return -sc.expm1(-c * sc.expm1(x))
def _ppf(self, q, c):
return sc.log1p(-1.0 / c * sc.log1p(-q))
def _entropy(self, c):
return 1.0 - np.log(c) - np.exp(c)*sc.expn(1, c)
gompertz = gompertz_gen(a=0.0, name='gompertz')
class gumbel_r_gen(rv_continuous):
r"""A right-skewed Gumbel continuous random variable.
%(before_notes)s
See Also
--------
gumbel_l, gompertz, genextreme
Notes
-----
The probability density function for `gumbel_r` is:
.. math::
f(x) = \exp(-(x + e^{-x}))
The Gumbel distribution is sometimes referred to as a type I Fisher-Tippett
distribution. It is also related to the extreme value distribution,
log-Weibull and Gompertz distributions.
%(after_notes)s
%(example)s
"""
def _pdf(self, x):
# gumbel_r.pdf(x) = exp(-(x + exp(-x)))
return np.exp(self._logpdf(x))
def _logpdf(self, x):
return -x - np.exp(-x)
def _cdf(self, x):
return np.exp(-np.exp(-x))
def _logcdf(self, x):
return -np.exp(-x)
def _ppf(self, q):
return -np.log(-np.log(q))
def _stats(self):
return _EULER, np.pi*np.pi/6.0, 12*np.sqrt(6)/np.pi**3 * _ZETA3, 12.0/5
def _entropy(self):
# https://en.wikipedia.org/wiki/Gumbel_distribution
return _EULER + 1.
gumbel_r = gumbel_r_gen(name='gumbel_r')
class gumbel_l_gen(rv_continuous):
r"""A left-skewed Gumbel continuous random variable.
%(before_notes)s
See Also
--------
gumbel_r, gompertz, genextreme
Notes
-----
The probability density function for `gumbel_l` is:
.. math::
f(x) = \exp(x - e^x)
The Gumbel distribution is sometimes referred to as a type I Fisher-Tippett
distribution. It is also related to the extreme value distribution,
log-Weibull and Gompertz distributions.
%(after_notes)s
%(example)s
"""
def _pdf(self, x):
# gumbel_l.pdf(x) = exp(x - exp(x))
return np.exp(self._logpdf(x))
def _logpdf(self, x):
return x - np.exp(x)
def _cdf(self, x):
return -sc.expm1(-np.exp(x))
def _ppf(self, q):
return np.log(-sc.log1p(-q))
def _logsf(self, x):
return -np.exp(x)
def _sf(self, x):
return np.exp(-np.exp(x))
def _isf(self, x):
return np.log(-np.log(x))
def _stats(self):
return -_EULER, np.pi*np.pi/6.0, \
-12*np.sqrt(6)/np.pi**3 * _ZETA3, 12.0/5
def _entropy(self):
return _EULER + 1.
gumbel_l = gumbel_l_gen(name='gumbel_l')
class halfcauchy_gen(rv_continuous):
r"""A Half-Cauchy continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `halfcauchy` is:
.. math::
f(x) = \frac{2}{\pi (1 + x^2)}
for :math:`x \ge 0`.
%(after_notes)s
%(example)s
"""
def _pdf(self, x):
# halfcauchy.pdf(x) = 2 / (pi * (1 + x**2))
return 2.0/np.pi/(1.0+x*x)
def _logpdf(self, x):
return np.log(2.0/np.pi) - sc.log1p(x*x)
def _cdf(self, x):
return 2.0/np.pi*np.arctan(x)
def _ppf(self, q):
return np.tan(np.pi/2*q)
def _stats(self):
return np.inf, np.inf, np.nan, np.nan
def _entropy(self):
return np.log(2*np.pi)
halfcauchy = halfcauchy_gen(a=0.0, name='halfcauchy')
class halflogistic_gen(rv_continuous):
r"""A half-logistic continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `halflogistic` is:
.. math::
f(x) = \frac{ 2 e^{-x} }{ (1+e^{-x})^2 }
= \frac{1}{2} \text{sech}(x/2)^2
for :math:`x \ge 0`.
%(after_notes)s
%(example)s
"""
def _pdf(self, x):
# halflogistic.pdf(x) = 2 * exp(-x) / (1+exp(-x))**2
# = 1/2 * sech(x/2)**2
return np.exp(self._logpdf(x))
def _logpdf(self, x):
return np.log(2) - x - 2. * sc.log1p(np.exp(-x))
def _cdf(self, x):
return np.tanh(x/2.0)
def _ppf(self, q):
return 2*np.arctanh(q)
def _munp(self, n):
if n == 1:
return 2*np.log(2)
if n == 2:
return np.pi*np.pi/3.0
if n == 3:
return 9*_ZETA3
if n == 4:
return 7*np.pi**4 / 15.0
return 2*(1-pow(2.0, 1-n))*sc.gamma(n+1)*sc.zeta(n, 1)
def _entropy(self):
return 2-np.log(2)
halflogistic = halflogistic_gen(a=0.0, name='halflogistic')
class halfnorm_gen(rv_continuous):
r"""A half-normal continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `halfnorm` is:
.. math::
f(x) = \sqrt{2/\pi} \exp(-x^2 / 2)
for :math:`x >= 0`.
`halfnorm` is a special case of `chi` with ``df=1``.
%(after_notes)s
%(example)s
"""
def _rvs(self, size=None, random_state=None):
return abs(random_state.standard_normal(size=size))
def _pdf(self, x):
# halfnorm.pdf(x) = sqrt(2/pi) * exp(-x**2/2)
return np.sqrt(2.0/np.pi)*np.exp(-x*x/2.0)
def _logpdf(self, x):
return 0.5 * np.log(2.0/np.pi) - x*x/2.0
def _cdf(self, x):
return _norm_cdf(x)*2-1.0
def _ppf(self, q):
return sc.ndtri((1+q)/2.0)
def _stats(self):
return (np.sqrt(2.0/np.pi),
1-2.0/np.pi,
np.sqrt(2)*(4-np.pi)/(np.pi-2)**1.5,
8*(np.pi-3)/(np.pi-2)**2)
def _entropy(self):
return 0.5*np.log(np.pi/2.0)+0.5
halfnorm = halfnorm_gen(a=0.0, name='halfnorm')
class hypsecant_gen(rv_continuous):
r"""A hyperbolic secant continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `hypsecant` is:
.. math::
f(x) = \frac{1}{\pi} \text{sech}(x)
for a real number :math:`x`.
%(after_notes)s
%(example)s
"""
def _pdf(self, x):
# hypsecant.pdf(x) = 1/pi * sech(x)
return 1.0/(np.pi*np.cosh(x))
def _cdf(self, x):
return 2.0/np.pi*np.arctan(np.exp(x))
def _ppf(self, q):
return np.log(np.tan(np.pi*q/2.0))
def _stats(self):
return 0, np.pi*np.pi/4, 0, 2
def _entropy(self):
return np.log(2*np.pi)
hypsecant = hypsecant_gen(name='hypsecant')
class gausshyper_gen(rv_continuous):
r"""A Gauss hypergeometric continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `gausshyper` is:
.. math::
f(x, a, b, c, z) = C x^{a-1} (1-x)^{b-1} (1+zx)^{-c}
for :math:`0 \le x \le 1`, :math:`a > 0`, :math:`b > 0`, and
:math:`C = \frac{1}{B(a, b) F[2, 1](c, a; a+b; -z)}`.
:math:`F[2, 1]` is the Gauss hypergeometric function
`scipy.special.hyp2f1`.
`gausshyper` takes :math:`a`, :math:`b`, :math:`c` and :math:`z` as shape
parameters.
%(after_notes)s
%(example)s
"""
def _argcheck(self, a, b, c, z):
return (a > 0) & (b > 0) & (c == c) & (z == z)
def _pdf(self, x, a, b, c, z):
# gausshyper.pdf(x, a, b, c, z) =
# C * x**(a-1) * (1-x)**(b-1) * (1+z*x)**(-c)
Cinv = sc.gamma(a)*sc.gamma(b)/sc.gamma(a+b)*sc.hyp2f1(c, a, a+b, -z)
return 1.0/Cinv * x**(a-1.0) * (1.0-x)**(b-1.0) / (1.0+z*x)**c
def _munp(self, n, a, b, c, z):
fac = sc.beta(n+a, b) / sc.beta(a, b)
num = sc.hyp2f1(c, a+n, a+b+n, -z)
den = sc.hyp2f1(c, a, a+b, -z)
return fac*num / den
gausshyper = gausshyper_gen(a=0.0, b=1.0, name='gausshyper')
class invgamma_gen(rv_continuous):
r"""An inverted gamma continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `invgamma` is:
.. math::
f(x, a) = \frac{x^{-a-1}}{\Gamma(a)} \exp(-\frac{1}{x})
for :math:`x >= 0`, :math:`a > 0`. :math:`\Gamma` is the gamma function
(`scipy.special.gamma`).
`invgamma` takes ``a`` as a shape parameter for :math:`a`.
`invgamma` is a special case of `gengamma` with ``c=-1``.
%(after_notes)s
%(example)s
"""
_support_mask = rv_continuous._open_support_mask
def _pdf(self, x, a):
# invgamma.pdf(x, a) = x**(-a-1) / gamma(a) * exp(-1/x)
return np.exp(self._logpdf(x, a))
def _logpdf(self, x, a):
return -(a+1) * np.log(x) - sc.gammaln(a) - 1.0/x
def _cdf(self, x, a):
return sc.gammaincc(a, 1.0 / x)
def _ppf(self, q, a):
return 1.0 / sc.gammainccinv(a, q)
def _sf(self, x, a):
return sc.gammainc(a, 1.0 / x)
def _isf(self, q, a):
return 1.0 / sc.gammaincinv(a, q)
def _stats(self, a, moments='mvsk'):
m1 = _lazywhere(a > 1, (a,), lambda x: 1. / (x - 1.), np.inf)
m2 = _lazywhere(a > 2, (a,), lambda x: 1. / (x - 1.)**2 / (x - 2.),
np.inf)
g1, g2 = None, None
if 's' in moments:
g1 = _lazywhere(
a > 3, (a,),
lambda x: 4. * np.sqrt(x - 2.) / (x - 3.), np.nan)
if 'k' in moments:
g2 = _lazywhere(
a > 4, (a,),
lambda x: 6. * (5. * x - 11.) / (x - 3.) / (x - 4.), np.nan)
return m1, m2, g1, g2
def _entropy(self, a):
return a - (a+1.0) * sc.psi(a) + sc.gammaln(a)
invgamma = invgamma_gen(a=0.0, name='invgamma')
# scale is gamma from DATAPLOT and B from Regress
class invgauss_gen(rv_continuous):
r"""An inverse Gaussian continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `invgauss` is:
.. math::
f(x, \mu) = \frac{1}{\sqrt{2 \pi x^3}}
\exp(-\frac{(x-\mu)^2}{2 x \mu^2})
for :math:`x >= 0` and :math:`\mu > 0`.
`invgauss` takes ``mu`` as a shape parameter for :math:`\mu`.
%(after_notes)s
When :math:`\mu` is too small, evaluating the cumulative distribution
function will be inaccurate due to ``cdf(mu -> 0) = inf * 0``.
NaNs are returned for :math:`\mu \le 0.0028`.
%(example)s
"""
_support_mask = rv_continuous._open_support_mask
def _rvs(self, mu, size=None, random_state=None):
return random_state.wald(mu, 1.0, size=size)
def _pdf(self, x, mu):
# invgauss.pdf(x, mu) =
# 1 / sqrt(2*pi*x**3) * exp(-(x-mu)**2/(2*x*mu**2))
return 1.0/np.sqrt(2*np.pi*x**3.0)*np.exp(-1.0/(2*x)*((x-mu)/mu)**2)
def _logpdf(self, x, mu):
return -0.5*np.log(2*np.pi) - 1.5*np.log(x) - ((x-mu)/mu)**2/(2*x)
def _cdf(self, x, mu):
fac = np.sqrt(1.0/x)
# Numerical accuracy for small `mu` is bad. See #869.
C1 = _norm_cdf(fac*(x-mu)/mu)
C1 += np.exp(1.0/mu) * _norm_cdf(-fac*(x+mu)/mu) * np.exp(1.0/mu)
return C1
def _stats(self, mu):
return mu, mu**3.0, 3*np.sqrt(mu), 15*mu
invgauss = invgauss_gen(a=0.0, name='invgauss')
class geninvgauss_gen(rv_continuous):
r"""A Generalized Inverse Gaussian continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `geninvgauss` is:
.. math::
f(x, p, b) = x^{p-1} \exp(-b (x + 1/x) / 2) / (2 K_p(b))
where `x > 0`, and the parameters `p, b` satisfy `b > 0` ([1]_).
:math:`K_p` is the modified Bessel function of second kind of order `p`
(`scipy.special.kv`).
%(after_notes)s
The inverse Gaussian distribution `stats.invgauss(mu)` is a special case of
`geninvgauss` with `p = -1/2`, `b = 1 / mu` and `scale = mu`.
Generating random variates is challenging for this distribution. The
implementation is based on [2]_.
References
----------
.. [1] O. Barndorff-Nielsen, P. Blaesild, C. Halgreen, "First hitting time
models for the generalized inverse gaussian distribution",
Stochastic Processes and their Applications 7, pp. 49--54, 1978.
.. [2] W. Hoermann and J. Leydold, "Generating generalized inverse Gaussian
random variates", Statistics and Computing, 24(4), p. 547--557, 2014.
%(example)s
"""
def _argcheck(self, p, b):
return (p == p) & (b > 0)
def _logpdf(self, x, p, b):
# kve instead of kv works better for large values of b
# warn if kve produces infinite values and replace by nan
# otherwise c = -inf and the results are often incorrect
@np.vectorize
def logpdf_single(x, p, b):
return _stats.geninvgauss_logpdf(x, p, b)
z = logpdf_single(x, p, b)
if np.isnan(z).any():
msg = ("Infinite values encountered in scipy.special.kve(p, b). "
"Values replaced by NaN to avoid incorrect results.")
warnings.warn(msg, RuntimeWarning)
return z
def _pdf(self, x, p, b):
# relying on logpdf avoids overflow of x**(p-1) for large x and p
return np.exp(self._logpdf(x, p, b))
def _cdf(self, x, *args):
_a, _b = self._get_support(*args)
@np.vectorize
def _cdf_single(x, *args):
p, b = args
user_data = np.array([p, b], float).ctypes.data_as(ctypes.c_void_p)
llc = LowLevelCallable.from_cython(_stats, '_geninvgauss_pdf', user_data)
return integrate.quad(llc, _a, x)[0]
return _cdf_single(x, *args)
def _logquasipdf(self, x, p, b):
# log of the quasi-density (w/o normalizing constant) used in _rvs
return _lazywhere(x > 0, (x, p, b),
lambda x, p, b: (p - 1)*np.log(x) - b*(x + 1/x)/2,
-np.inf)
def _rvs(self, p, b, size=None, random_state=None):
# if p and b are scalar, use _rvs_scalar, otherwise need to create
# output by iterating over parameters
if np.isscalar(p) and np.isscalar(b):
out = self._rvs_scalar(p, b, size, random_state)
elif p.size == 1 and b.size == 1:
out = self._rvs_scalar(p.item(), b.item(), size, random_state)
else:
# When this method is called, size will be a (possibly empty)
# tuple of integers. It will not be None; if `size=None` is passed
# to `rvs()`, size will be the empty tuple ().
p, b = np.broadcast_arrays(p, b)
# p and b now have the same shape.
# `shp` is the shape of the blocks of random variates that are
# generated for each combination of parameters associated with
# broadcasting p and b.
# bc is a tuple the same lenth as size. The values
# in bc are bools. If bc[j] is True, it means that
# entire axis is filled in for a given combination of the
# broadcast arguments.
shp, bc = _check_shape(p.shape, size)
# `numsamples` is the total number of variates to be generated
# for each combination of the input arguments.
numsamples = int(np.prod(shp))
# `out` is the array to be returned. It is filled in in the
# loop below.
out = np.empty(size)
it = np.nditer([p, b],
flags=['multi_index'],
op_flags=[['readonly'], ['readonly']])
while not it.finished:
# Convert the iterator's multi_index into an index into the
# `out` array where the call to _rvs_scalar() will be stored.
# Where bc is True, we use a full slice; otherwise we use the
# index value from it.multi_index. len(it.multi_index) might
# be less than len(bc), and in that case we want to align these
# two sequences to the right, so the loop variable j runs from
# -len(size) to 0. This doesn't cause an IndexError, as
# bc[j] will be True in those cases where it.multi_index[j]
# would cause an IndexError.
idx = tuple((it.multi_index[j] if not bc[j] else slice(None))
for j in range(-len(size), 0))
out[idx] = self._rvs_scalar(it[0], it[1], numsamples, random_state).reshape(shp)
it.iternext()
if size == ():
out = out.item()
return out
def _rvs_scalar(self, p, b, numsamples, random_state):
# following [2], the quasi-pdf is used instead of the pdf for the
# generation of rvs
invert_res = False
if not(numsamples):
numsamples = 1
if p < 0:
# note: if X is geninvgauss(p, b), then 1/X is geninvgauss(-p, b)
p = -p
invert_res = True
m = self._mode(p, b)
# determine method to be used following [2]
ratio_unif = True
if p >= 1 or b > 1:
# ratio of uniforms with mode shift below
mode_shift = True
elif b >= min(0.5, 2 * np.sqrt(1 - p) / 3):
# ratio of uniforms without mode shift below
mode_shift = False
else:
# new algorithm in [2]
ratio_unif = False
# prepare sampling of rvs
size1d = tuple(np.atleast_1d(numsamples))
N = np.prod(size1d) # number of rvs needed, reshape upon return
x = np.zeros(N)
simulated = 0
if ratio_unif:
# use ratio of uniforms method
if mode_shift:
a2 = -2 * (p + 1) / b - m
a1 = 2 * m * (p - 1) / b - 1
# find roots of x**3 + a2*x**2 + a1*x + m (Cardano's formula)
p1 = a1 - a2**2 / 3
q1 = 2 * a2**3 / 27 - a2 * a1 / 3 + m
phi = np.arccos(-q1 * np.sqrt(-27 / p1**3) / 2)
s1 = -np.sqrt(-4 * p1 / 3)
root1 = s1 * np.cos(phi / 3 + np.pi / 3) - a2 / 3
root2 = -s1 * np.cos(phi / 3) - a2 / 3
# root3 = s1 * np.cos(phi / 3 - np.pi / 3) - a2 / 3
# if g is the quasipdf, rescale: g(x) / g(m) which we can write
# as exp(log(g(x)) - log(g(m))). This is important
# since for large values of p and b, g cannot be evaluated.
# denote the rescaled quasipdf by h
lm = self._logquasipdf(m, p, b)
d1 = self._logquasipdf(root1, p, b) - lm
d2 = self._logquasipdf(root2, p, b) - lm
# compute the bounding rectangle w.r.t. h. Note that
# np.exp(0.5*d1) = np.sqrt(g(root1)/g(m)) = np.sqrt(h(root1))
vmin = (root1 - m) * np.exp(0.5 * d1)
vmax = (root2 - m) * np.exp(0.5 * d2)
umax = 1 # umax = sqrt(h(m)) = 1
logqpdf = lambda x: self._logquasipdf(x, p, b) - lm
c = m
else:
# ratio of uniforms without mode shift
# compute np.sqrt(quasipdf(m))
umax = np.exp(0.5*self._logquasipdf(m, p, b))
xplus = ((1 + p) + np.sqrt((1 + p)**2 + b**2))/b
vmin = 0
# compute xplus * np.sqrt(quasipdf(xplus))
vmax = xplus * np.exp(0.5 * self._logquasipdf(xplus, p, b))
c = 0
logqpdf = lambda x: self._logquasipdf(x, p, b)
if vmin >= vmax:
raise ValueError("vmin must be smaller than vmax.")
if umax <= 0:
raise ValueError("umax must be positive.")
i = 1
while simulated < N:
k = N - simulated
# simulate uniform rvs on [0, umax] and [vmin, vmax]
u = umax * random_state.uniform(size=k)
v = random_state.uniform(size=k)
v = vmin + (vmax - vmin) * v
rvs = v / u + c
# rewrite acceptance condition u**2 <= pdf(rvs) by taking logs
accept = (2*np.log(u) <= logqpdf(rvs))
num_accept = np.sum(accept)
if num_accept > 0:
x[simulated:(simulated + num_accept)] = rvs[accept]
simulated += num_accept
if (simulated == 0) and (i*N >= 50000):
msg = ("Not a single random variate could be generated "
"in {} attempts. Sampling does not appear to "
"work for the provided parameters.".format(i*N))
raise RuntimeError(msg)
i += 1
else:
# use new algorithm in [2]
x0 = b / (1 - p)
xs = np.max((x0, 2 / b))
k1 = np.exp(self._logquasipdf(m, p, b))
A1 = k1 * x0
if x0 < 2 / b:
k2 = np.exp(-b)
if p > 0:
A2 = k2 * ((2 / b)**p - x0**p) / p
else:
A2 = k2 * np.log(2 / b**2)
else:
k2, A2 = 0, 0
k3 = xs**(p - 1)
A3 = 2 * k3 * np.exp(-xs * b / 2) / b
A = A1 + A2 + A3
# [2]: rejection constant is < 2.73; so expected runtime is finite
while simulated < N:
k = N - simulated
h, rvs = np.zeros(k), np.zeros(k)
# simulate uniform rvs on [x1, x2] and [0, y2]
u = random_state.uniform(size=k)
v = A * random_state.uniform(size=k)
cond1 = v <= A1
cond2 = np.logical_not(cond1) & (v <= A1 + A2)
cond3 = np.logical_not(cond1 | cond2)
# subdomain (0, x0)
rvs[cond1] = x0 * v[cond1] / A1
h[cond1] = k1
# subdomain (x0, 2 / b)
if p > 0:
rvs[cond2] = (x0**p + (v[cond2] - A1) * p / k2)**(1 / p)
else:
rvs[cond2] = b * np.exp((v[cond2] - A1) * np.exp(b))
h[cond2] = k2 * rvs[cond2]**(p - 1)
# subdomain (xs, infinity)
z = np.exp(-xs * b / 2) - b * (v[cond3] - A1 - A2) / (2 * k3)
rvs[cond3] = -2 / b * np.log(z)
h[cond3] = k3 * np.exp(-rvs[cond3] * b / 2)
# apply rejection method
accept = (np.log(u * h) <= self._logquasipdf(rvs, p, b))
num_accept = sum(accept)
if num_accept > 0:
x[simulated:(simulated + num_accept)] = rvs[accept]
simulated += num_accept
rvs = np.reshape(x, size1d)
if invert_res:
rvs = 1 / rvs
return rvs
def _mode(self, p, b):
# distinguish cases to avoid catastrophic cancellation (see [2])
if p < 1:
return b / (np.sqrt((p - 1)**2 + b**2) + 1 - p)
else:
return (np.sqrt((1 - p)**2 + b**2) - (1 - p)) / b
def _munp(self, n, p, b):
num = sc.kve(p + n, b)
denom = sc.kve(p, b)
inf_vals = np.isinf(num) | np.isinf(denom)
if inf_vals.any():
msg = ("Infinite values encountered in the moment calculation "
"involving scipy.special.kve. Values replaced by NaN to "
"avoid incorrect results.")
warnings.warn(msg, RuntimeWarning)
m = np.full_like(num, np.nan, dtype=np.double)
m[~inf_vals] = num[~inf_vals] / denom[~inf_vals]
else:
m = num / denom
return m
geninvgauss = geninvgauss_gen(a=0.0, name="geninvgauss")
class norminvgauss_gen(rv_continuous):
r"""A Normal Inverse Gaussian continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `norminvgauss` is:
.. math::
f(x, a, b) = \frac{a \, K_1(a \sqrt{1 + x^2})}{\pi \sqrt{1 + x^2}} \,
\exp(\sqrt{a^2 - b^2} + b x)
where :math:`x` is a real number, the parameter :math:`a` is the tail
heaviness and :math:`b` is the asymmetry parameter satisfying
:math:`a > 0` and :math:`|b| <= a`.
:math:`K_1` is the modified Bessel function of second kind
(`scipy.special.k1`).
%(after_notes)s
A normal inverse Gaussian random variable `Y` with parameters `a` and `b`
can be expressed as a normal mean-variance mixture:
`Y = b * V + sqrt(V) * X` where `X` is `norm(0,1)` and `V` is
`invgauss(mu=1/sqrt(a**2 - b**2))`. This representation is used
to generate random variates.
References
----------
O. Barndorff-Nielsen, "Hyperbolic Distributions and Distributions on
Hyperbolae", Scandinavian Journal of Statistics, Vol. 5(3),
pp. 151-157, 1978.
O. Barndorff-Nielsen, "Normal Inverse Gaussian Distributions and Stochastic
Volatility Modelling", Scandinavian Journal of Statistics, Vol. 24,
pp. 1-13, 1997.
%(example)s
"""
_support_mask = rv_continuous._open_support_mask
def _argcheck(self, a, b):
return (a > 0) & (np.absolute(b) < a)
def _pdf(self, x, a, b):
gamma = np.sqrt(a**2 - b**2)
fac1 = a / np.pi * np.exp(gamma)
sq = np.hypot(1, x) # reduce overflows
return fac1 * sc.k1e(a * sq) * np.exp(b*x - a*sq) / sq
def _rvs(self, a, b, size=None, random_state=None):
# note: X = b * V + sqrt(V) * X is norminvgaus(a,b) if X is standard
# normal and V is invgauss(mu=1/sqrt(a**2 - b**2))
gamma = np.sqrt(a**2 - b**2)
ig = invgauss.rvs(mu=1/gamma, size=size, random_state=random_state)
return b * ig + np.sqrt(ig) * norm.rvs(size=size, random_state=random_state)
def _stats(self, a, b):
gamma = np.sqrt(a**2 - b**2)
mean = b / gamma
variance = a**2 / gamma**3
skewness = 3.0 * b / (a * np.sqrt(gamma))
kurtosis = 3.0 * (1 + 4 * b**2 / a**2) / gamma
return mean, variance, skewness, kurtosis
norminvgauss = norminvgauss_gen(name="norminvgauss")
class invweibull_gen(rv_continuous):
u"""An inverted Weibull continuous random variable.
This distribution is also known as the Fréchet distribution or the
type II extreme value distribution.
%(before_notes)s
Notes
-----
The probability density function for `invweibull` is:
.. math::
f(x, c) = c x^{-c-1} \\exp(-x^{-c})
for :math:`x > 0`, :math:`c > 0`.
`invweibull` takes ``c`` as a shape parameter for :math:`c`.
%(after_notes)s
References
----------
F.R.S. de Gusmao, E.M.M Ortega and G.M. Cordeiro, "The generalized inverse
Weibull distribution", Stat. Papers, vol. 52, pp. 591-619, 2011.
%(example)s
"""
_support_mask = rv_continuous._open_support_mask
def _pdf(self, x, c):
# invweibull.pdf(x, c) = c * x**(-c-1) * exp(-x**(-c))
xc1 = np.power(x, -c - 1.0)
xc2 = np.power(x, -c)
xc2 = np.exp(-xc2)
return c * xc1 * xc2
def _cdf(self, x, c):
xc1 = np.power(x, -c)
return np.exp(-xc1)
def _ppf(self, q, c):
return np.power(-np.log(q), -1.0/c)
def _munp(self, n, c):
return sc.gamma(1 - n / c)
def _entropy(self, c):
return 1+_EULER + _EULER / c - np.log(c)
invweibull = invweibull_gen(a=0, name='invweibull')
class johnsonsb_gen(rv_continuous):
r"""A Johnson SB continuous random variable.
%(before_notes)s
See Also
--------
johnsonsu
Notes
-----
The probability density function for `johnsonsb` is:
.. math::
f(x, a, b) = \frac{b}{x(1-x)} \phi(a + b \log \frac{x}{1-x} )
for :math:`0 <= x < =1` and :math:`a, b > 0`, and :math:`\phi` is the normal
pdf.
`johnsonsb` takes :math:`a` and :math:`b` as shape parameters.
%(after_notes)s
%(example)s
"""
_support_mask = rv_continuous._open_support_mask
def _argcheck(self, a, b):
return (b > 0) & (a == a)
def _pdf(self, x, a, b):
# johnsonsb.pdf(x, a, b) = b / (x*(1-x)) * phi(a + b * log(x/(1-x)))
trm = _norm_pdf(a + b*np.log(x/(1.0-x)))
return b*1.0/(x*(1-x))*trm
def _cdf(self, x, a, b):
return _norm_cdf(a + b*np.log(x/(1.0-x)))
def _ppf(self, q, a, b):
return 1.0 / (1 + np.exp(-1.0 / b * (_norm_ppf(q) - a)))
johnsonsb = johnsonsb_gen(a=0.0, b=1.0, name='johnsonsb')
class johnsonsu_gen(rv_continuous):
r"""A Johnson SU continuous random variable.
%(before_notes)s
See Also
--------
johnsonsb
Notes
-----
The probability density function for `johnsonsu` is:
.. math::
f(x, a, b) = \frac{b}{\sqrt{x^2 + 1}}
\phi(a + b \log(x + \sqrt{x^2 + 1}))
for all :math:`x, a, b > 0`, and :math:`\phi` is the normal pdf.
`johnsonsu` takes :math:`a` and :math:`b` as shape parameters.
%(after_notes)s
%(example)s
"""
def _argcheck(self, a, b):
return (b > 0) & (a == a)
def _pdf(self, x, a, b):
# johnsonsu.pdf(x, a, b) = b / sqrt(x**2 + 1) *
# phi(a + b * log(x + sqrt(x**2 + 1)))
x2 = x*x
trm = _norm_pdf(a + b * np.log(x + np.sqrt(x2+1)))
return b*1.0/np.sqrt(x2+1.0)*trm
def _cdf(self, x, a, b):
return _norm_cdf(a + b * np.log(x + np.sqrt(x*x + 1)))
def _ppf(self, q, a, b):
return np.sinh((_norm_ppf(q) - a) / b)
johnsonsu = johnsonsu_gen(name='johnsonsu')
class laplace_gen(rv_continuous):
r"""A Laplace continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `laplace` is
.. math::
f(x) = \frac{1}{2} \exp(-|x|)
for a real number :math:`x`.
%(after_notes)s
%(example)s
"""
def _rvs(self, size=None, random_state=None):
return random_state.laplace(0, 1, size=size)
def _pdf(self, x):
# laplace.pdf(x) = 1/2 * exp(-abs(x))
return 0.5*np.exp(-abs(x))
def _cdf(self, x):
return np.where(x > 0, 1.0-0.5*np.exp(-x), 0.5*np.exp(x))
def _ppf(self, q):
return np.where(q > 0.5, -np.log(2*(1-q)), np.log(2*q))
def _stats(self):
return 0, 2, 0, 3
def _entropy(self):
return np.log(2)+1
@replace_notes_in_docstring(rv_continuous, notes="""\
This function uses explicit formulas for the maximum likelihood
estimation of the Laplace distribution parameters, so the keyword
arguments `loc`, `scale`, and `optimizer` are ignored.\n\n""")
def fit(self, data, *args, **kwds):
floc = kwds.pop('floc', None)
fscale = kwds.pop('fscale', None)
_check_fit_input_parameters(data, args,
kwds, fixed_param=(floc, fscale))
# MLE for the laplace distribution
if floc is None:
loc = np.median(data)
else:
loc = floc
if fscale is None:
scale = (np.sum(np.abs(data - loc))) / len(data)
else:
scale = fscale
# Source: Statistical Distributions, 3rd Edition. Evans, Hastings,
# and Peacock (2000), Page 124
return loc, scale
laplace = laplace_gen(name='laplace')
def _check_fit_input_parameters(data, args, kwds, fixed_param):
if len(args) > 0:
raise TypeError("Too many arguments.")
_remove_optimizer_parameters(kwds)
if None not in fixed_param:
# This check is for consistency with `rv_continuous.fit`.
# Without this check, this function would just return the
# parameters that were given.
raise RuntimeError("All parameters fixed. There is nothing to "
"optimize.")
data = np.asarray(data)
if not np.isfinite(data).all():
raise RuntimeError("The data contains non-finite values.")
class levy_gen(rv_continuous):
r"""A Levy continuous random variable.
%(before_notes)s
See Also
--------
levy_stable, levy_l
Notes
-----
The probability density function for `levy` is:
.. math::
f(x) = \frac{1}{\sqrt{2\pi x^3}} \exp\left(-\frac{1}{2x}\right)
for :math:`x >= 0`.
This is the same as the Levy-stable distribution with :math:`a=1/2` and
:math:`b=1`.
%(after_notes)s
%(example)s
"""
_support_mask = rv_continuous._open_support_mask
def _pdf(self, x):
# levy.pdf(x) = 1 / (x * sqrt(2*pi*x)) * exp(-1/(2*x))
return 1 / np.sqrt(2*np.pi*x) / x * np.exp(-1/(2*x))
def _cdf(self, x):
# Equivalent to 2*norm.sf(np.sqrt(1/x))
return sc.erfc(np.sqrt(0.5 / x))
def _ppf(self, q):
# Equivalent to 1.0/(norm.isf(q/2)**2) or 0.5/(erfcinv(q)**2)
val = -sc.ndtri(q/2)
return 1.0 / (val * val)
def _stats(self):
return np.inf, np.inf, np.nan, np.nan
levy = levy_gen(a=0.0, name="levy")
class levy_l_gen(rv_continuous):
r"""A left-skewed Levy continuous random variable.
%(before_notes)s
See Also
--------
levy, levy_stable
Notes
-----
The probability density function for `levy_l` is:
.. math::
f(x) = \frac{1}{|x| \sqrt{2\pi |x|}} \exp{ \left(-\frac{1}{2|x|} \right)}
for :math:`x <= 0`.
This is the same as the Levy-stable distribution with :math:`a=1/2` and
:math:`b=-1`.
%(after_notes)s
%(example)s
"""
_support_mask = rv_continuous._open_support_mask
def _pdf(self, x):
# levy_l.pdf(x) = 1 / (abs(x) * sqrt(2*pi*abs(x))) * exp(-1/(2*abs(x)))
ax = abs(x)
return 1/np.sqrt(2*np.pi*ax)/ax*np.exp(-1/(2*ax))
def _cdf(self, x):
ax = abs(x)
return 2 * _norm_cdf(1 / np.sqrt(ax)) - 1
def _ppf(self, q):
val = _norm_ppf((q + 1.0) / 2)
return -1.0 / (val * val)
def _stats(self):
return np.inf, np.inf, np.nan, np.nan
levy_l = levy_l_gen(b=0.0, name="levy_l")
class levy_stable_gen(rv_continuous):
r"""A Levy-stable continuous random variable.
%(before_notes)s
See Also
--------
levy, levy_l
Notes
-----
The distribution for `levy_stable` has characteristic function:
.. math::
\varphi(t, \alpha, \beta, c, \mu) =
e^{it\mu -|ct|^{\alpha}(1-i\beta \operatorname{sign}(t)\Phi(\alpha, t))}
where:
.. math::
\Phi = \begin{cases}
\tan \left({\frac {\pi \alpha }{2}}\right)&\alpha \neq 1\\
-{\frac {2}{\pi }}\log |t|&\alpha =1
\end{cases}
The probability density function for `levy_stable` is:
.. math::
f(x) = \frac{1}{2\pi}\int_{-\infty}^\infty \varphi(t)e^{-ixt}\,dt
where :math:`-\infty < t < \infty`. This integral does not have a known closed form.
For evaluation of pdf we use either Zolotarev :math:`S_0` parameterization with integration,
direct integration of standard parameterization of characteristic function or FFT of
characteristic function. If set to other than None and if number of points is greater than
``levy_stable.pdf_fft_min_points_threshold`` (defaults to None) we use FFT otherwise we use one
of the other methods.
The default method is 'best' which uses Zolotarev's method if alpha = 1 and integration of
characteristic function otherwise. The default method can be changed by setting
``levy_stable.pdf_default_method`` to either 'zolotarev', 'quadrature' or 'best'.
To increase accuracy of FFT calculation one can specify ``levy_stable.pdf_fft_grid_spacing``
(defaults to 0.001) and ``pdf_fft_n_points_two_power`` (defaults to a value that covers the
input range * 4). Setting ``pdf_fft_n_points_two_power`` to 16 should be sufficiently accurate
in most cases at the expense of CPU time.
For evaluation of cdf we use Zolatarev :math:`S_0` parameterization with integration or integral of
the pdf FFT interpolated spline. The settings affecting FFT calculation are the same as
for pdf calculation. Setting the threshold to ``None`` (default) will disable FFT. For cdf
calculations the Zolatarev method is superior in accuracy, so FFT is disabled by default.
Fitting estimate uses quantile estimation method in [MC]. MLE estimation of parameters in
fit method uses this quantile estimate initially. Note that MLE doesn't always converge if
using FFT for pdf calculations; so it's best that ``pdf_fft_min_points_threshold`` is left unset.
.. warning::
For pdf calculations implementation of Zolatarev is unstable for values where alpha = 1 and
beta != 0. In this case the quadrature method is recommended. FFT calculation is also
considered experimental.
For cdf calculations FFT calculation is considered experimental. Use Zolatarev's method
instead (default).
%(after_notes)s
References
----------
.. [MC] McCulloch, J., 1986. Simple consistent estimators of stable distribution parameters.
Communications in Statistics - Simulation and Computation 15, 11091136.
.. [MS] Mittnik, S.T. Rachev, T. Doganoglu, D. Chenyao, 1999. Maximum likelihood estimation
of stable Paretian models, Mathematical and Computer Modelling, Volume 29, Issue 10,
1999, Pages 275-293.
.. [BS] Borak, S., Hardle, W., Rafal, W. 2005. Stable distributions, Economic Risk.
%(example)s
"""
def _rvs(self, alpha, beta, size=None, random_state=None):
def alpha1func(alpha, beta, TH, aTH, bTH, cosTH, tanTH, W):
return (2/np.pi*(np.pi/2 + bTH)*tanTH -
beta*np.log((np.pi/2*W*cosTH)/(np.pi/2 + bTH)))
def beta0func(alpha, beta, TH, aTH, bTH, cosTH, tanTH, W):
return (W/(cosTH/np.tan(aTH) + np.sin(TH)) *
((np.cos(aTH) + np.sin(aTH)*tanTH)/W)**(1.0/alpha))
def otherwise(alpha, beta, TH, aTH, bTH, cosTH, tanTH, W):
# alpha is not 1 and beta is not 0
val0 = beta*np.tan(np.pi*alpha/2)
th0 = np.arctan(val0)/alpha
val3 = W/(cosTH/np.tan(alpha*(th0 + TH)) + np.sin(TH))
res3 = val3*((np.cos(aTH) + np.sin(aTH)*tanTH -
val0*(np.sin(aTH) - np.cos(aTH)*tanTH))/W)**(1.0/alpha)
return res3
def alphanot1func(alpha, beta, TH, aTH, bTH, cosTH, tanTH, W):
res = _lazywhere(beta == 0,
(alpha, beta, TH, aTH, bTH, cosTH, tanTH, W),
beta0func, f2=otherwise)
return res
alpha = np.broadcast_to(alpha, size)
beta = np.broadcast_to(beta, size)
TH = uniform.rvs(loc=-np.pi/2.0, scale=np.pi, size=size,
random_state=random_state)
W = expon.rvs(size=size, random_state=random_state)
aTH = alpha*TH
bTH = beta*TH
cosTH = np.cos(TH)
tanTH = np.tan(TH)
res = _lazywhere(alpha == 1,
(alpha, beta, TH, aTH, bTH, cosTH, tanTH, W),
alpha1func, f2=alphanot1func)
return res
def _argcheck(self, alpha, beta):
return (alpha > 0) & (alpha <= 2) & (beta <= 1) & (beta >= -1)
@staticmethod
def _cf(t, alpha, beta):
Phi = lambda alpha, t: np.tan(np.pi*alpha/2) if alpha != 1 else -2.0*np.log(np.abs(t))/np.pi
return np.exp(-(np.abs(t)**alpha)*(1-1j*beta*np.sign(t)*Phi(alpha, t)))
@staticmethod
def _pdf_from_cf_with_fft(cf, h=0.01, q=9):
"""Calculates pdf from cf using fft. Using region around 0 with N=2**q points
separated by distance h. As suggested by [MS].
"""
N = 2**q
n = np.arange(1,N+1)
density = ((-1)**(n-1-N/2))*np.fft.fft(((-1)**(n-1))*cf(2*np.pi*(n-1-N/2)/h/N))/h/N
x = (n-1-N/2)*h
return (x, density)
@staticmethod
def _pdf_single_value_best(x, alpha, beta):
if alpha != 1. or (alpha == 1. and beta == 0.):
return levy_stable_gen._pdf_single_value_zolotarev(x, alpha, beta)
else:
return levy_stable_gen._pdf_single_value_cf_integrate(x, alpha, beta)
@staticmethod
def _pdf_single_value_cf_integrate(x, alpha, beta):
cf = lambda t: levy_stable_gen._cf(t, alpha, beta)
return integrate.quad(lambda t: np.real(np.exp(-1j*t*x)*cf(t)), -np.inf, np.inf, limit=1000)[0]/np.pi/2
@staticmethod
def _pdf_single_value_zolotarev(x, alpha, beta):
"""Calculate pdf using Zolotarev's methods as detailed in [BS].
"""
zeta = -beta*np.tan(np.pi*alpha/2.)
if alpha != 1:
x0 = x + zeta # convert to S_0 parameterization
xi = np.arctan(-zeta)/alpha
def V(theta):
return np.cos(alpha*xi)**(1/(alpha-1)) * \
(np.cos(theta)/np.sin(alpha*(xi+theta)))**(alpha/(alpha-1)) * \
(np.cos(alpha*xi+(alpha-1)*theta)/np.cos(theta))
if x0 > zeta:
def g(theta):
return (V(theta) *
np.real(np.complex128(x0-zeta)**(alpha/(alpha-1))))
def f(theta):
return g(theta) * np.exp(-g(theta))
# spare calculating integral on null set
# use isclose as macos has fp differences
if np.isclose(-xi, np.pi/2, rtol=1e-014, atol=1e-014):
return 0.
with np.errstate(all="ignore"):
intg_max = optimize.minimize_scalar(lambda theta: -f(theta), bounds=[-xi, np.pi/2])
intg_kwargs = {}
# windows quadpack less forgiving with points out of bounds
if intg_max.success and not np.isnan(intg_max.fun)\
and intg_max.x > -xi and intg_max.x < np.pi/2:
intg_kwargs["points"] = [intg_max.x]
intg = integrate.quad(f, -xi, np.pi/2, **intg_kwargs)[0]
return alpha * intg / np.pi / np.abs(alpha-1) / (x0-zeta)
elif x0 == zeta:
return sc.gamma(1+1/alpha)*np.cos(xi)/np.pi/((1+zeta**2)**(1/alpha/2))
else:
return levy_stable_gen._pdf_single_value_zolotarev(-x, alpha, -beta)
else:
# since location zero, no need to reposition x for S_0 parameterization
xi = np.pi/2
if beta != 0:
warnings.warn('Density calculation unstable for alpha=1 and beta!=0.' +
' Use quadrature method instead.', RuntimeWarning)
def V(theta):
expr_1 = np.pi/2+beta*theta
return 2. * expr_1 * np.exp(expr_1*np.tan(theta)/beta) / np.cos(theta) / np.pi
def g(theta):
return np.exp(-np.pi * x / 2. / beta) * V(theta)
def f(theta):
return g(theta) * np.exp(-g(theta))
with np.errstate(all="ignore"):
intg_max = optimize.minimize_scalar(lambda theta: -f(theta), bounds=[-np.pi/2, np.pi/2])
intg = integrate.fixed_quad(f, -np.pi/2, intg_max.x)[0] + integrate.fixed_quad(f, intg_max.x, np.pi/2)[0]
return intg / np.abs(beta) / 2.
else:
return 1/(1+x**2)/np.pi
@staticmethod
def _cdf_single_value_zolotarev(x, alpha, beta):
"""Calculate cdf using Zolotarev's methods as detailed in [BS].
"""
zeta = -beta*np.tan(np.pi*alpha/2.)
if alpha != 1:
x0 = x + zeta # convert to S_0 parameterization
xi = np.arctan(-zeta)/alpha
def V(theta):
return np.cos(alpha*xi)**(1/(alpha-1)) * \
(np.cos(theta)/np.sin(alpha*(xi+theta)))**(alpha/(alpha-1)) * \
(np.cos(alpha*xi+(alpha-1)*theta)/np.cos(theta))
if x0 > zeta:
c_1 = 1 if alpha > 1 else .5 - xi/np.pi
def f(theta):
z = np.complex128(x0 - zeta)
return np.exp(-V(theta) * np.real(z**(alpha/(alpha-1))))
with np.errstate(all="ignore"):
# spare calculating integral on null set
# use isclose as macos has fp differences
if np.isclose(-xi, np.pi/2, rtol=1e-014, atol=1e-014):
intg = 0
else:
intg = integrate.quad(f, -xi, np.pi/2)[0]
return c_1 + np.sign(1-alpha) * intg / np.pi
elif x0 == zeta:
return .5 - xi/np.pi
else:
return 1 - levy_stable_gen._cdf_single_value_zolotarev(-x, alpha, -beta)
else:
# since location zero, no need to reposition x for S_0 parameterization
xi = np.pi/2
if beta > 0:
def V(theta):
expr_1 = np.pi/2+beta*theta
return 2. * expr_1 * np.exp(expr_1*np.tan(theta)/beta) / np.cos(theta) / np.pi
with np.errstate(all="ignore"):
expr_1 = np.exp(-np.pi*x/beta/2.)
int_1 = integrate.quad(lambda theta: np.exp(-expr_1 * V(theta)), -np.pi/2, np.pi/2)[0]
return int_1 / np.pi
elif beta == 0:
return .5 + np.arctan(x)/np.pi
else:
return 1 - levy_stable_gen._cdf_single_value_zolotarev(-x, 1, -beta)
def _pdf(self, x, alpha, beta):
x = np.asarray(x).reshape(1, -1)[0,:]
x, alpha, beta = np.broadcast_arrays(x, alpha, beta)
data_in = np.dstack((x, alpha, beta))[0]
data_out = np.empty(shape=(len(data_in),1))
pdf_default_method_name = getattr(self, 'pdf_default_method', 'best')
if pdf_default_method_name == 'best':
pdf_single_value_method = levy_stable_gen._pdf_single_value_best
elif pdf_default_method_name == 'zolotarev':
pdf_single_value_method = levy_stable_gen._pdf_single_value_zolotarev
else:
pdf_single_value_method = levy_stable_gen._pdf_single_value_cf_integrate
fft_min_points_threshold = getattr(self, 'pdf_fft_min_points_threshold', None)
fft_grid_spacing = getattr(self, 'pdf_fft_grid_spacing', 0.001)
fft_n_points_two_power = getattr(self, 'pdf_fft_n_points_two_power', None)
# group data in unique arrays of alpha, beta pairs
uniq_param_pairs = np.vstack(list({tuple(row) for row in
data_in[:, 1:]}))
for pair in uniq_param_pairs:
data_mask = np.all(data_in[:,1:] == pair, axis=-1)
data_subset = data_in[data_mask]
if fft_min_points_threshold is None or len(data_subset) < fft_min_points_threshold:
data_out[data_mask] = np.array([pdf_single_value_method(_x, _alpha, _beta)
for _x, _alpha, _beta in data_subset]).reshape(len(data_subset), 1)
else:
warnings.warn('Density calculations experimental for FFT method.' +
' Use combination of zolatarev and quadrature methods instead.', RuntimeWarning)
_alpha, _beta = pair
_x = data_subset[:,(0,)]
# need enough points to "cover" _x for interpolation
h = fft_grid_spacing
q = np.ceil(np.log(2*np.max(np.abs(_x))/h)/np.log(2)) + 2 if fft_n_points_two_power is None else int(fft_n_points_two_power)
density_x, density = levy_stable_gen._pdf_from_cf_with_fft(lambda t: levy_stable_gen._cf(t, _alpha, _beta), h=h, q=q)
f = interpolate.interp1d(density_x, np.real(density))
data_out[data_mask] = f(_x)
return data_out.T[0]
def _cdf(self, x, alpha, beta):
x = np.asarray(x).reshape(1, -1)[0,:]
x, alpha, beta = np.broadcast_arrays(x, alpha, beta)
data_in = np.dstack((x, alpha, beta))[0]
data_out = np.empty(shape=(len(data_in),1))
fft_min_points_threshold = getattr(self, 'pdf_fft_min_points_threshold', None)
fft_grid_spacing = getattr(self, 'pdf_fft_grid_spacing', 0.001)
fft_n_points_two_power = getattr(self, 'pdf_fft_n_points_two_power', None)
# group data in unique arrays of alpha, beta pairs
uniq_param_pairs = np.vstack(
list({tuple(row) for row in data_in[:,1:]}))
for pair in uniq_param_pairs:
data_mask = np.all(data_in[:,1:] == pair, axis=-1)
data_subset = data_in[data_mask]
if fft_min_points_threshold is None or len(data_subset) < fft_min_points_threshold:
data_out[data_mask] = np.array([levy_stable._cdf_single_value_zolotarev(_x, _alpha, _beta)
for _x, _alpha, _beta in data_subset]).reshape(len(data_subset), 1)
else:
warnings.warn(u'FFT method is considered experimental for ' +
u'cumulative distribution function ' +
u'evaluations. Use Zolotarev’s method instead).',
RuntimeWarning)
_alpha, _beta = pair
_x = data_subset[:,(0,)]
# need enough points to "cover" _x for interpolation
h = fft_grid_spacing
q = 16 if fft_n_points_two_power is None else int(fft_n_points_two_power)
density_x, density = levy_stable_gen._pdf_from_cf_with_fft(lambda t: levy_stable_gen._cf(t, _alpha, _beta), h=h, q=q)
f = interpolate.InterpolatedUnivariateSpline(density_x, np.real(density))
data_out[data_mask] = np.array([f.integral(self.a, x_1) for x_1 in _x]).reshape(data_out[data_mask].shape)
return data_out.T[0]
def _fitstart(self, data):
# We follow McCullock 1986 method - Simple Consistent Estimators
# of Stable Distribution Parameters
# Table III and IV
nu_alpha_range = [2.439, 2.5, 2.6, 2.7, 2.8, 3, 3.2, 3.5, 4, 5, 6, 8, 10, 15, 25]
nu_beta_range = [0, 0.1, 0.2, 0.3, 0.5, 0.7, 1]
# table III - alpha = psi_1(nu_alpha, nu_beta)
alpha_table = [
[2.000, 2.000, 2.000, 2.000, 2.000, 2.000, 2.000],
[1.916, 1.924, 1.924, 1.924, 1.924, 1.924, 1.924],
[1.808, 1.813, 1.829, 1.829, 1.829, 1.829, 1.829],
[1.729, 1.730, 1.737, 1.745, 1.745, 1.745, 1.745],
[1.664, 1.663, 1.663, 1.668, 1.676, 1.676, 1.676],
[1.563, 1.560, 1.553, 1.548, 1.547, 1.547, 1.547],
[1.484, 1.480, 1.471, 1.460, 1.448, 1.438, 1.438],
[1.391, 1.386, 1.378, 1.364, 1.337, 1.318, 1.318],
[1.279, 1.273, 1.266, 1.250, 1.210, 1.184, 1.150],
[1.128, 1.121, 1.114, 1.101, 1.067, 1.027, 0.973],
[1.029, 1.021, 1.014, 1.004, 0.974, 0.935, 0.874],
[0.896, 0.892, 0.884, 0.883, 0.855, 0.823, 0.769],
[0.818, 0.812, 0.806, 0.801, 0.780, 0.756, 0.691],
[0.698, 0.695, 0.692, 0.689, 0.676, 0.656, 0.597],
[0.593, 0.590, 0.588, 0.586, 0.579, 0.563, 0.513]]
# table IV - beta = psi_2(nu_alpha, nu_beta)
beta_table = [
[0, 2.160, 1.000, 1.000, 1.000, 1.000, 1.000],
[0, 1.592, 3.390, 1.000, 1.000, 1.000, 1.000],
[0, 0.759, 1.800, 1.000, 1.000, 1.000, 1.000],
[0, 0.482, 1.048, 1.694, 1.000, 1.000, 1.000],
[0, 0.360, 0.760, 1.232, 2.229, 1.000, 1.000],
[0, 0.253, 0.518, 0.823, 1.575, 1.000, 1.000],
[0, 0.203, 0.410, 0.632, 1.244, 1.906, 1.000],
[0, 0.165, 0.332, 0.499, 0.943, 1.560, 1.000],
[0, 0.136, 0.271, 0.404, 0.689, 1.230, 2.195],
[0, 0.109, 0.216, 0.323, 0.539, 0.827, 1.917],
[0, 0.096, 0.190, 0.284, 0.472, 0.693, 1.759],
[0, 0.082, 0.163, 0.243, 0.412, 0.601, 1.596],
[0, 0.074, 0.147, 0.220, 0.377, 0.546, 1.482],
[0, 0.064, 0.128, 0.191, 0.330, 0.478, 1.362],
[0, 0.056, 0.112, 0.167, 0.285, 0.428, 1.274]]
# Table V and VII
alpha_range = [2, 1.9, 1.8, 1.7, 1.6, 1.5, 1.4, 1.3, 1.2, 1.1, 1, 0.9, 0.8, 0.7, 0.6, 0.5]
beta_range = [0, 0.25, 0.5, 0.75, 1]
# Table V - nu_c = psi_3(alpha, beta)
nu_c_table = [
[1.908, 1.908, 1.908, 1.908, 1.908],
[1.914, 1.915, 1.916, 1.918, 1.921],
[1.921, 1.922, 1.927, 1.936, 1.947],
[1.927, 1.930, 1.943, 1.961, 1.987],
[1.933, 1.940, 1.962, 1.997, 2.043],
[1.939, 1.952, 1.988, 2.045, 2.116],
[1.946, 1.967, 2.022, 2.106, 2.211],
[1.955, 1.984, 2.067, 2.188, 2.333],
[1.965, 2.007, 2.125, 2.294, 2.491],
[1.980, 2.040, 2.205, 2.435, 2.696],
[2.000, 2.085, 2.311, 2.624, 2.973],
[2.040, 2.149, 2.461, 2.886, 3.356],
[2.098, 2.244, 2.676, 3.265, 3.912],
[2.189, 2.392, 3.004, 3.844, 4.775],
[2.337, 2.634, 3.542, 4.808, 6.247],
[2.588, 3.073, 4.534, 6.636, 9.144]]
# Table VII - nu_zeta = psi_5(alpha, beta)
nu_zeta_table = [
[0, 0.000, 0.000, 0.000, 0.000],
[0, -0.017, -0.032, -0.049, -0.064],
[0, -0.030, -0.061, -0.092, -0.123],
[0, -0.043, -0.088, -0.132, -0.179],
[0, -0.056, -0.111, -0.170, -0.232],
[0, -0.066, -0.134, -0.206, -0.283],
[0, -0.075, -0.154, -0.241, -0.335],
[0, -0.084, -0.173, -0.276, -0.390],
[0, -0.090, -0.192, -0.310, -0.447],
[0, -0.095, -0.208, -0.346, -0.508],
[0, -0.098, -0.223, -0.380, -0.576],
[0, -0.099, -0.237, -0.424, -0.652],
[0, -0.096, -0.250, -0.469, -0.742],
[0, -0.089, -0.262, -0.520, -0.853],
[0, -0.078, -0.272, -0.581, -0.997],
[0, -0.061, -0.279, -0.659, -1.198]]
psi_1 = interpolate.interp2d(nu_beta_range, nu_alpha_range, alpha_table, kind='linear')
psi_2 = interpolate.interp2d(nu_beta_range, nu_alpha_range, beta_table, kind='linear')
psi_2_1 = lambda nu_beta, nu_alpha: psi_2(nu_beta, nu_alpha) if nu_beta > 0 else -psi_2(-nu_beta, nu_alpha)
phi_3 = interpolate.interp2d(beta_range, alpha_range, nu_c_table, kind='linear')
phi_3_1 = lambda beta, alpha: phi_3(beta, alpha) if beta > 0 else phi_3(-beta, alpha)
phi_5 = interpolate.interp2d(beta_range, alpha_range, nu_zeta_table, kind='linear')
phi_5_1 = lambda beta, alpha: phi_5(beta, alpha) if beta > 0 else -phi_5(-beta, alpha)
# quantiles
p05 = np.percentile(data, 5)
p50 = np.percentile(data, 50)
p95 = np.percentile(data, 95)
p25 = np.percentile(data, 25)
p75 = np.percentile(data, 75)
nu_alpha = (p95 - p05)/(p75 - p25)
nu_beta = (p95 + p05 - 2*p50)/(p95 - p05)
if nu_alpha >= 2.439:
alpha = np.clip(psi_1(nu_beta, nu_alpha)[0], np.finfo(float).eps, 2.)
beta = np.clip(psi_2_1(nu_beta, nu_alpha)[0], -1., 1.)
else:
alpha = 2.0
beta = np.sign(nu_beta)
c = (p75 - p25) / phi_3_1(beta, alpha)[0]
zeta = p50 + c*phi_5_1(beta, alpha)[0]
delta = np.clip(zeta-beta*c*np.tan(np.pi*alpha/2.) if alpha == 1. else zeta, np.finfo(float).eps, np.inf)
return (alpha, beta, delta, c)
def _stats(self, alpha, beta):
mu = 0 if alpha > 1 else np.nan
mu2 = 2 if alpha == 2 else np.inf
g1 = 0. if alpha == 2. else np.NaN
g2 = 0. if alpha == 2. else np.NaN
return mu, mu2, g1, g2
levy_stable = levy_stable_gen(name='levy_stable')
class logistic_gen(rv_continuous):
r"""A logistic (or Sech-squared) continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `logistic` is:
.. math::
f(x) = \frac{\exp(-x)}
{(1+\exp(-x))^2}
`logistic` is a special case of `genlogistic` with ``c=1``.
%(after_notes)s
%(example)s
"""
def _rvs(self, size=None, random_state=None):
return random_state.logistic(size=size)
def _pdf(self, x):
# logistic.pdf(x) = exp(-x) / (1+exp(-x))**2
return np.exp(self._logpdf(x))
def _logpdf(self, x):
return -x - 2. * sc.log1p(np.exp(-x))
def _cdf(self, x):
return sc.expit(x)
def _ppf(self, q):
return sc.logit(q)
def _sf(self, x):
return sc.expit(-x)
def _isf(self, q):
return -sc.logit(q)
def _stats(self):
return 0, np.pi*np.pi/3.0, 0, 6.0/5.0
def _entropy(self):
# https://en.wikipedia.org/wiki/Logistic_distribution
return 2.0
logistic = logistic_gen(name='logistic')
class loggamma_gen(rv_continuous):
r"""A log gamma continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `loggamma` is:
.. math::
f(x, c) = \frac{\exp(c x - \exp(x))}
{\Gamma(c)}
for all :math:`x, c > 0`. Here, :math:`\Gamma` is the
gamma function (`scipy.special.gamma`).
`loggamma` takes ``c`` as a shape parameter for :math:`c`.
%(after_notes)s
%(example)s
"""
def _rvs(self, c, size=None, random_state=None):
return np.log(random_state.gamma(c, size=size))
def _pdf(self, x, c):
# loggamma.pdf(x, c) = exp(c*x-exp(x)) / gamma(c)
return np.exp(c*x-np.exp(x)-sc.gammaln(c))
def _cdf(self, x, c):
return sc.gammainc(c, np.exp(x))
def _ppf(self, q, c):
return np.log(sc.gammaincinv(c, q))
def _stats(self, c):
# See, for example, "A Statistical Study of Log-Gamma Distribution", by
# Ping Shing Chan (thesis, McMaster University, 1993).
mean = sc.digamma(c)
var = sc.polygamma(1, c)
skewness = sc.polygamma(2, c) / np.power(var, 1.5)
excess_kurtosis = sc.polygamma(3, c) / (var*var)
return mean, var, skewness, excess_kurtosis
loggamma = loggamma_gen(name='loggamma')
class loglaplace_gen(rv_continuous):
r"""A log-Laplace continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `loglaplace` is:
.. math::
f(x, c) = \begin{cases}\frac{c}{2} x^{ c-1} &\text{for } 0 < x < 1\\
\frac{c}{2} x^{-c-1} &\text{for } x \ge 1
\end{cases}
for :math:`c > 0`.
`loglaplace` takes ``c`` as a shape parameter for :math:`c`.
%(after_notes)s
References
----------
T.J. Kozubowski and K. Podgorski, "A log-Laplace growth rate model",
The Mathematical Scientist, vol. 28, pp. 49-60, 2003.
%(example)s
"""
def _pdf(self, x, c):
# loglaplace.pdf(x, c) = c / 2 * x**(c-1), for 0 < x < 1
# = c / 2 * x**(-c-1), for x >= 1
cd2 = c/2.0
c = np.where(x < 1, c, -c)
return cd2*x**(c-1)
def _cdf(self, x, c):
return np.where(x < 1, 0.5*x**c, 1-0.5*x**(-c))
def _ppf(self, q, c):
return np.where(q < 0.5, (2.0*q)**(1.0/c), (2*(1.0-q))**(-1.0/c))
def _munp(self, n, c):
return c**2 / (c**2 - n**2)
def _entropy(self, c):
return np.log(2.0/c) + 1.0
loglaplace = loglaplace_gen(a=0.0, name='loglaplace')
def _lognorm_logpdf(x, s):
return _lazywhere(x != 0, (x, s),
lambda x, s: -np.log(x)**2 / (2*s**2) - np.log(s*x*np.sqrt(2*np.pi)),
-np.inf)
class lognorm_gen(rv_continuous):
r"""A lognormal continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `lognorm` is:
.. math::
f(x, s) = \frac{1}{s x \sqrt{2\pi}}
\exp\left(-\frac{\log^2(x)}{2s^2}\right)
for :math:`x > 0`, :math:`s > 0`.
`lognorm` takes ``s`` as a shape parameter for :math:`s`.
%(after_notes)s
A common parametrization for a lognormal random variable ``Y`` is in
terms of the mean, ``mu``, and standard deviation, ``sigma``, of the
unique normally distributed random variable ``X`` such that exp(X) = Y.
This parametrization corresponds to setting ``s = sigma`` and ``scale =
exp(mu)``.
%(example)s
"""
_support_mask = rv_continuous._open_support_mask
def _rvs(self, s, size=None, random_state=None):
return np.exp(s * random_state.standard_normal(size))
def _pdf(self, x, s):
# lognorm.pdf(x, s) = 1 / (s*x*sqrt(2*pi)) * exp(-1/2*(log(x)/s)**2)
return np.exp(self._logpdf(x, s))
def _logpdf(self, x, s):
return _lognorm_logpdf(x, s)
def _cdf(self, x, s):
return _norm_cdf(np.log(x) / s)
def _logcdf(self, x, s):
return _norm_logcdf(np.log(x) / s)
def _ppf(self, q, s):
return np.exp(s * _norm_ppf(q))
def _sf(self, x, s):
return _norm_sf(np.log(x) / s)
def _logsf(self, x, s):
return _norm_logsf(np.log(x) / s)
def _stats(self, s):
p = np.exp(s*s)
mu = np.sqrt(p)
mu2 = p*(p-1)
g1 = np.sqrt((p-1))*(2+p)
g2 = np.polyval([1, 2, 3, 0, -6.0], p)
return mu, mu2, g1, g2
def _entropy(self, s):
return 0.5 * (1 + np.log(2*np.pi) + 2 * np.log(s))
@extend_notes_in_docstring(rv_continuous, notes="""\
When the location parameter is fixed by using the `floc` argument,
this function uses explicit formulas for the maximum likelihood
estimation of the log-normal shape and scale parameters, so the
`optimizer`, `loc` and `scale` keyword arguments are ignored.\n\n""")
def fit(self, data, *args, **kwds):
floc = kwds.get('floc', None)
if floc is None:
# loc is not fixed. Use the default fit method.
return super(lognorm_gen, self).fit(data, *args, **kwds)
f0 = (kwds.get('f0', None) or kwds.get('fs', None) or
kwds.get('fix_s', None))
fscale = kwds.get('fscale', None)
if len(args) > 1:
raise TypeError("Too many input arguments.")
for name in ['f0', 'fs', 'fix_s', 'floc', 'fscale', 'loc', 'scale',
'optimizer']:
kwds.pop(name, None)
if kwds:
raise TypeError("Unknown arguments: %s." % kwds)
# Special case: loc is fixed. Use the maximum likelihood formulas
# instead of the numerical solver.
if f0 is not None and fscale is not None:
# This check is for consistency with `rv_continuous.fit`.
raise ValueError("All parameters fixed. There is nothing to "
"optimize.")
data = np.asarray(data)
if not np.isfinite(data).all():
raise RuntimeError("The data contains non-finite values.")
floc = float(floc)
if floc != 0:
# Shifting the data by floc. Don't do the subtraction in-place,
# because `data` might be a view of the input array.
data = data - floc
if np.any(data <= 0):
raise FitDataError("lognorm", lower=floc, upper=np.inf)
lndata = np.log(data)
# Three cases to handle:
# * shape and scale both free
# * shape fixed, scale free
# * shape free, scale fixed
if fscale is None:
# scale is free.
scale = np.exp(lndata.mean())
if f0 is None:
# shape is free.
shape = lndata.std()
else:
# shape is fixed.
shape = float(f0)
else:
# scale is fixed, shape is free
scale = float(fscale)
shape = np.sqrt(((lndata - np.log(scale))**2).mean())
return shape, floc, scale
lognorm = lognorm_gen(a=0.0, name='lognorm')
class gilbrat_gen(rv_continuous):
r"""A Gilbrat continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `gilbrat` is:
.. math::
f(x) = \frac{1}{x \sqrt{2\pi}} \exp(-\frac{1}{2} (\log(x))^2)
`gilbrat` is a special case of `lognorm` with ``s=1``.
%(after_notes)s
%(example)s
"""
_support_mask = rv_continuous._open_support_mask
def _rvs(self, size=None, random_state=None):
return np.exp(random_state.standard_normal(size))
def _pdf(self, x):
# gilbrat.pdf(x) = 1/(x*sqrt(2*pi)) * exp(-1/2*(log(x))**2)
return np.exp(self._logpdf(x))
def _logpdf(self, x):
return _lognorm_logpdf(x, 1.0)
def _cdf(self, x):
return _norm_cdf(np.log(x))
def _ppf(self, q):
return np.exp(_norm_ppf(q))
def _stats(self):
p = np.e
mu = np.sqrt(p)
mu2 = p * (p - 1)
g1 = np.sqrt((p - 1)) * (2 + p)
g2 = np.polyval([1, 2, 3, 0, -6.0], p)
return mu, mu2, g1, g2
def _entropy(self):
return 0.5 * np.log(2 * np.pi) + 0.5
gilbrat = gilbrat_gen(a=0.0, name='gilbrat')
class maxwell_gen(rv_continuous):
r"""A Maxwell continuous random variable.
%(before_notes)s
Notes
-----
A special case of a `chi` distribution, with ``df=3``, ``loc=0.0``,
and given ``scale = a``, where ``a`` is the parameter used in the
Mathworld description [1]_.
The probability density function for `maxwell` is:
.. math::
f(x) = \sqrt{2/\pi}x^2 \exp(-x^2/2)
for :math:`x >= 0`.
%(after_notes)s
References
----------
.. [1] http://mathworld.wolfram.com/MaxwellDistribution.html
%(example)s
"""
def _rvs(self, size=None, random_state=None):
return chi.rvs(3.0, size=size, random_state=random_state)
def _pdf(self, x):
# maxwell.pdf(x) = sqrt(2/pi)x**2 * exp(-x**2/2)
return _SQRT_2_OVER_PI*x*x*np.exp(-x*x/2.0)
def _logpdf(self, x):
return _LOG_SQRT_2_OVER_PI + 2*np.log(x) - 0.5*x*x
def _cdf(self, x):
return sc.gammainc(1.5, x*x/2.0)
def _ppf(self, q):
return np.sqrt(2*sc.gammaincinv(1.5, q))
def _stats(self):
val = 3*np.pi-8
return (2*np.sqrt(2.0/np.pi),
3-8/np.pi,
np.sqrt(2)*(32-10*np.pi)/val**1.5,
(-12*np.pi*np.pi + 160*np.pi - 384) / val**2.0)
def _entropy(self):
return _EULER + 0.5*np.log(2*np.pi)-0.5
maxwell = maxwell_gen(a=0.0, name='maxwell')
class mielke_gen(rv_continuous):
r"""A Mielke Beta-Kappa / Dagum continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `mielke` is:
.. math::
f(x, k, s) = \frac{k x^{k-1}}{(1+x^s)^{1+k/s}}
for :math:`x > 0` and :math:`k, s > 0`. The distribution is sometimes
called Dagum distribution ([2]_). It was already defined in [3]_, called
a Burr Type III distribution (`burr` with parameters ``c=s`` and
``d=k/s``).
`mielke` takes ``k`` and ``s`` as shape parameters.
%(after_notes)s
References
----------
.. [1] Mielke, P.W., 1973 "Another Family of Distributions for Describing
and Analyzing Precipitation Data." J. Appl. Meteor., 12, 275-280
.. [2] Dagum, C., 1977 "A new model for personal income distribution."
Economie Appliquee, 33, 327-367.
.. [3] Burr, I. W. "Cumulative frequency functions", Annals of
Mathematical Statistics, 13(2), pp 215-232 (1942).
%(example)s
"""
def _argcheck(self, k, s):
return (k > 0) & (s > 0)
def _pdf(self, x, k, s):
return k*x**(k-1.0) / (1.0+x**s)**(1.0+k*1.0/s)
def _logpdf(self, x, k, s):
return np.log(k) + np.log(x)*(k-1.0) - np.log1p(x**s)*(1.0+k*1.0/s)
def _cdf(self, x, k, s):
return x**k / (1.0+x**s)**(k*1.0/s)
def _ppf(self, q, k, s):
qsk = pow(q, s*1.0/k)
return pow(qsk/(1.0-qsk), 1.0/s)
def _munp(self, n, k, s):
def nth_moment(n, k, s):
# n-th moment is defined for -k < n < s
return sc.gamma((k+n)/s)*sc.gamma(1-n/s)/sc.gamma(k/s)
return _lazywhere(n < s, (n, k, s), nth_moment, np.inf)
mielke = mielke_gen(a=0.0, name='mielke')
class kappa4_gen(rv_continuous):
r"""Kappa 4 parameter distribution.
%(before_notes)s
Notes
-----
The probability density function for kappa4 is:
.. math::
f(x, h, k) = (1 - k x)^{1/k - 1} (1 - h (1 - k x)^{1/k})^{1/h-1}
if :math:`h` and :math:`k` are not equal to 0.
If :math:`h` or :math:`k` are zero then the pdf can be simplified:
h = 0 and k != 0::
kappa4.pdf(x, h, k) = (1.0 - k*x)**(1.0/k - 1.0)*
exp(-(1.0 - k*x)**(1.0/k))
h != 0 and k = 0::
kappa4.pdf(x, h, k) = exp(-x)*(1.0 - h*exp(-x))**(1.0/h - 1.0)
h = 0 and k = 0::
kappa4.pdf(x, h, k) = exp(-x)*exp(-exp(-x))
kappa4 takes :math:`h` and :math:`k` as shape parameters.
The kappa4 distribution returns other distributions when certain
:math:`h` and :math:`k` values are used.
+------+-------------+----------------+------------------+
| h | k=0.0 | k=1.0 | -inf<=k<=inf |
+======+=============+================+==================+
| -1.0 | Logistic | | Generalized |
| | | | Logistic(1) |
| | | | |
| | logistic(x) | | |
+------+-------------+----------------+------------------+
| 0.0 | Gumbel | Reverse | Generalized |
| | | Exponential(2) | Extreme Value |
| | | | |
| | gumbel_r(x) | | genextreme(x, k) |
+------+-------------+----------------+------------------+
| 1.0 | Exponential | Uniform | Generalized |
| | | | Pareto |
| | | | |
| | expon(x) | uniform(x) | genpareto(x, -k) |
+------+-------------+----------------+------------------+
(1) There are at least five generalized logistic distributions.
Four are described here:
https://en.wikipedia.org/wiki/Generalized_logistic_distribution
The "fifth" one is the one kappa4 should match which currently
isn't implemented in scipy:
https://en.wikipedia.org/wiki/Talk:Generalized_logistic_distribution
https://www.mathwave.com/help/easyfit/html/analyses/distributions/gen_logistic.html
(2) This distribution is currently not in scipy.
References
----------
J.C. Finney, "Optimization of a Skewed Logistic Distribution With Respect
to the Kolmogorov-Smirnov Test", A Dissertation Submitted to the Graduate
Faculty of the Louisiana State University and Agricultural and Mechanical
College, (August, 2004),
https://digitalcommons.lsu.edu/gradschool_dissertations/3672
J.R.M. Hosking, "The four-parameter kappa distribution". IBM J. Res.
Develop. 38 (3), 25 1-258 (1994).
B. Kumphon, A. Kaew-Man, P. Seenoi, "A Rainfall Distribution for the Lampao
Site in the Chi River Basin, Thailand", Journal of Water Resource and
Protection, vol. 4, 866-869, (2012).
https://doi.org/10.4236/jwarp.2012.410101
C. Winchester, "On Estimation of the Four-Parameter Kappa Distribution", A
Thesis Submitted to Dalhousie University, Halifax, Nova Scotia, (March
2000).
http://www.nlc-bnc.ca/obj/s4/f2/dsk2/ftp01/MQ57336.pdf
%(after_notes)s
%(example)s
"""
def _argcheck(self, h, k):
return h == h
def _get_support(self, h, k):
condlist = [np.logical_and(h > 0, k > 0),
np.logical_and(h > 0, k == 0),
np.logical_and(h > 0, k < 0),
np.logical_and(h <= 0, k > 0),
np.logical_and(h <= 0, k == 0),
np.logical_and(h <= 0, k < 0)]
def f0(h, k):
return (1.0 - float_power(h, -k))/k
def f1(h, k):
return np.log(h)
def f3(h, k):
a = np.empty(np.shape(h))
a[:] = -np.inf
return a
def f5(h, k):
return 1.0/k
_a = _lazyselect(condlist,
[f0, f1, f0, f3, f3, f5],
[h, k],
default=np.nan)
def f0(h, k):
return 1.0/k
def f1(h, k):
a = np.empty(np.shape(h))
a[:] = np.inf
return a
_b = _lazyselect(condlist,
[f0, f1, f1, f0, f1, f1],
[h, k],
default=np.nan)
return _a, _b
def _pdf(self, x, h, k):
# kappa4.pdf(x, h, k) = (1.0 - k*x)**(1.0/k - 1.0)*
# (1.0 - h*(1.0 - k*x)**(1.0/k))**(1.0/h-1)
return np.exp(self._logpdf(x, h, k))
def _logpdf(self, x, h, k):
condlist = [np.logical_and(h != 0, k != 0),
np.logical_and(h == 0, k != 0),
np.logical_and(h != 0, k == 0),
np.logical_and(h == 0, k == 0)]
def f0(x, h, k):
'''pdf = (1.0 - k*x)**(1.0/k - 1.0)*(
1.0 - h*(1.0 - k*x)**(1.0/k))**(1.0/h-1.0)
logpdf = ...
'''
return (sc.xlog1py(1.0/k - 1.0, -k*x) +
sc.xlog1py(1.0/h - 1.0, -h*(1.0 - k*x)**(1.0/k)))
def f1(x, h, k):
'''pdf = (1.0 - k*x)**(1.0/k - 1.0)*np.exp(-(
1.0 - k*x)**(1.0/k))
logpdf = ...
'''
return sc.xlog1py(1.0/k - 1.0, -k*x) - (1.0 - k*x)**(1.0/k)
def f2(x, h, k):
'''pdf = np.exp(-x)*(1.0 - h*np.exp(-x))**(1.0/h - 1.0)
logpdf = ...
'''
return -x + sc.xlog1py(1.0/h - 1.0, -h*np.exp(-x))
def f3(x, h, k):
'''pdf = np.exp(-x-np.exp(-x))
logpdf = ...
'''
return -x - np.exp(-x)
return _lazyselect(condlist,
[f0, f1, f2, f3],
[x, h, k],
default=np.nan)
def _cdf(self, x, h, k):
return np.exp(self._logcdf(x, h, k))
def _logcdf(self, x, h, k):
condlist = [np.logical_and(h != 0, k != 0),
np.logical_and(h == 0, k != 0),
np.logical_and(h != 0, k == 0),
np.logical_and(h == 0, k == 0)]
def f0(x, h, k):
'''cdf = (1.0 - h*(1.0 - k*x)**(1.0/k))**(1.0/h)
logcdf = ...
'''
return (1.0/h)*sc.log1p(-h*(1.0 - k*x)**(1.0/k))
def f1(x, h, k):
'''cdf = np.exp(-(1.0 - k*x)**(1.0/k))
logcdf = ...
'''
return -(1.0 - k*x)**(1.0/k)
def f2(x, h, k):
'''cdf = (1.0 - h*np.exp(-x))**(1.0/h)
logcdf = ...
'''
return (1.0/h)*sc.log1p(-h*np.exp(-x))
def f3(x, h, k):
'''cdf = np.exp(-np.exp(-x))
logcdf = ...
'''
return -np.exp(-x)
return _lazyselect(condlist,
[f0, f1, f2, f3],
[x, h, k],
default=np.nan)
def _ppf(self, q, h, k):
condlist = [np.logical_and(h != 0, k != 0),
np.logical_and(h == 0, k != 0),
np.logical_and(h != 0, k == 0),
np.logical_and(h == 0, k == 0)]
def f0(q, h, k):
return 1.0/k*(1.0 - ((1.0 - (q**h))/h)**k)
def f1(q, h, k):
return 1.0/k*(1.0 - (-np.log(q))**k)
def f2(q, h, k):
'''ppf = -np.log((1.0 - (q**h))/h)
'''
return -sc.log1p(-(q**h)) + np.log(h)
def f3(q, h, k):
return -np.log(-np.log(q))
return _lazyselect(condlist,
[f0, f1, f2, f3],
[q, h, k],
default=np.nan)
def _stats(self, h, k):
if h >= 0 and k >= 0:
maxr = 5
elif h < 0 and k >= 0:
maxr = int(-1.0/h*k)
elif k < 0:
maxr = int(-1.0/k)
else:
maxr = 5
outputs = [None if r < maxr else np.nan for r in range(1, 5)]
return outputs[:]
kappa4 = kappa4_gen(name='kappa4')
class kappa3_gen(rv_continuous):
r"""Kappa 3 parameter distribution.
%(before_notes)s
Notes
-----
The probability density function for `kappa3` is:
.. math::
f(x, a) = a (a + x^a)^{-(a + 1)/a}
for :math:`x > 0` and :math:`a > 0`.
`kappa3` takes ``a`` as a shape parameter for :math:`a`.
References
----------
P.W. Mielke and E.S. Johnson, "Three-Parameter Kappa Distribution Maximum
Likelihood and Likelihood Ratio Tests", Methods in Weather Research,
701-707, (September, 1973),
https://doi.org/10.1175/1520-0493(1973)101<0701:TKDMLE>2.3.CO;2
B. Kumphon, "Maximum Entropy and Maximum Likelihood Estimation for the
Three-Parameter Kappa Distribution", Open Journal of Statistics, vol 2,
415-419 (2012), https://doi.org/10.4236/ojs.2012.24050
%(after_notes)s
%(example)s
"""
def _argcheck(self, a):
return a > 0
def _pdf(self, x, a):
# kappa3.pdf(x, a) = a*(a + x**a)**(-(a + 1)/a), for x > 0
return a*(a + x**a)**(-1.0/a-1)
def _cdf(self, x, a):
return x*(a + x**a)**(-1.0/a)
def _ppf(self, q, a):
return (a/(q**-a - 1.0))**(1.0/a)
def _stats(self, a):
outputs = [None if i < a else np.nan for i in range(1, 5)]
return outputs[:]
kappa3 = kappa3_gen(a=0.0, name='kappa3')
class moyal_gen(rv_continuous):
r"""A Moyal continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `moyal` is:
.. math::
f(x) = \exp(-(x + \exp(-x))/2) / \sqrt{2\pi}
for a real number :math:`x`.
%(after_notes)s
This distribution has utility in high-energy physics and radiation
detection. It describes the energy loss of a charged relativistic
particle due to ionization of the medium [1]_. It also provides an
approximation for the Landau distribution. For an in depth description
see [2]_. For additional description, see [3]_.
References
----------
.. [1] J.E. Moyal, "XXX. Theory of ionization fluctuations",
The London, Edinburgh, and Dublin Philosophical Magazine
and Journal of Science, vol 46, 263-280, (1955).
:doi:`10.1080/14786440308521076` (gated)
.. [2] G. Cordeiro et al., "The beta Moyal: a useful skew distribution",
International Journal of Research and Reviews in Applied Sciences,
vol 10, 171-192, (2012).
http://www.arpapress.com/Volumes/Vol10Issue2/IJRRAS_10_2_02.pdf
.. [3] C. Walck, "Handbook on Statistical Distributions for
Experimentalists; International Report SUF-PFY/96-01", Chapter 26,
University of Stockholm: Stockholm, Sweden, (2007).
http://www.stat.rice.edu/~dobelman/textfiles/DistributionsHandbook.pdf
.. versionadded:: 1.1.0
%(example)s
"""
def _rvs(self, size=None, random_state=None):
u1 = gamma.rvs(a = 0.5, scale = 2, size=size, random_state=random_state)
return -np.log(u1)
def _pdf(self, x):
return np.exp(-0.5 * (x + np.exp(-x))) / np.sqrt(2*np.pi)
def _cdf(self, x):
return sc.erfc(np.exp(-0.5 * x) / np.sqrt(2))
def _sf(self, x):
return sc.erf(np.exp(-0.5 * x) / np.sqrt(2))
def _ppf(self, x):
return -np.log(2 * sc.erfcinv(x)**2)
def _stats(self):
mu = np.log(2) + np.euler_gamma
mu2 = np.pi**2 / 2
g1 = 28 * np.sqrt(2) * sc.zeta(3) / np.pi**3
g2 = 4.
return mu, mu2, g1, g2
def _munp(self, n):
if n == 1.0:
return np.log(2) + np.euler_gamma
elif n == 2.0:
return np.pi**2 / 2 + (np.log(2) + np.euler_gamma)**2
elif n == 3.0:
tmp1 = 1.5 * np.pi**2 * (np.log(2)+np.euler_gamma)
tmp2 = (np.log(2)+np.euler_gamma)**3
tmp3 = 14 * sc.zeta(3)
return tmp1 + tmp2 + tmp3
elif n == 4.0:
tmp1 = 4 * 14 * sc.zeta(3) * (np.log(2) + np.euler_gamma)
tmp2 = 3 * np.pi**2 * (np.log(2) + np.euler_gamma)**2
tmp3 = (np.log(2) + np.euler_gamma)**4
tmp4 = 7 * np.pi**4 / 4
return tmp1 + tmp2 + tmp3 + tmp4
else:
# return generic for higher moments
# return rv_continuous._mom1_sc(self, n, b)
return self._mom1_sc(n)
moyal = moyal_gen(name="moyal")
class nakagami_gen(rv_continuous):
r"""A Nakagami continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `nakagami` is:
.. math::
f(x, \nu) = \frac{2 \nu^\nu}{\Gamma(\nu)} x^{2\nu-1} \exp(-\nu x^2)
for :math:`x >= 0`, :math:`\nu > 0`.
`nakagami` takes ``nu`` as a shape parameter for :math:`\nu`.
%(after_notes)s
%(example)s
"""
def _pdf(self, x, nu):
# nakagami.pdf(x, nu) = 2 * nu**nu / gamma(nu) *
# x**(2*nu-1) * exp(-nu*x**2)
return 2*nu**nu/sc.gamma(nu)*(x**(2*nu-1.0))*np.exp(-nu*x*x)
def _cdf(self, x, nu):
return sc.gammainc(nu, nu*x*x)
def _ppf(self, q, nu):
return np.sqrt(1.0/nu*sc.gammaincinv(nu, q))
def _stats(self, nu):
mu = sc.gamma(nu+0.5)/sc.gamma(nu)/np.sqrt(nu)
mu2 = 1.0-mu*mu
g1 = mu * (1 - 4*nu*mu2) / 2.0 / nu / np.power(mu2, 1.5)
g2 = -6*mu**4*nu + (8*nu-2)*mu**2-2*nu + 1
g2 /= nu*mu2**2.0
return mu, mu2, g1, g2
nakagami = nakagami_gen(a=0.0, name="nakagami")
class ncx2_gen(rv_continuous):
r"""A non-central chi-squared continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `ncx2` is:
.. math::
f(x, k, \lambda) = \frac{1}{2} \exp(-(\lambda+x)/2)
(x/\lambda)^{(k-2)/4} I_{(k-2)/2}(\sqrt{\lambda x})
for :math:`x >= 0` and :math:`k, \lambda > 0`. :math:`k` specifies the
degrees of freedom (denoted ``df`` in the implementation) and
:math:`\lambda` is the non-centrality parameter (denoted ``nc`` in the
implementation). :math:`I_\nu` denotes the modified Bessel function of
first order of degree :math:`\nu` (`scipy.special.iv`).
`ncx2` takes ``df`` and ``nc`` as shape parameters.
%(after_notes)s
%(example)s
"""
def _argcheck(self, df, nc):
return (df > 0) & (nc >= 0)
def _rvs(self, df, nc, size=None, random_state=None):
return random_state.noncentral_chisquare(df, nc, size)
def _logpdf(self, x, df, nc):
cond = np.ones_like(x, dtype=bool) & (nc != 0)
return _lazywhere(cond, (x, df, nc), f=_ncx2_log_pdf, f2=chi2.logpdf)
def _pdf(self, x, df, nc):
# ncx2.pdf(x, df, nc) = exp(-(nc+x)/2) * 1/2 * (x/nc)**((df-2)/4)
# * I[(df-2)/2](sqrt(nc*x))
cond = np.ones_like(x, dtype=bool) & (nc != 0)
return _lazywhere(cond, (x, df, nc), f=_ncx2_pdf, f2=chi2.pdf)
def _cdf(self, x, df, nc):
cond = np.ones_like(x, dtype=bool) & (nc != 0)
return _lazywhere(cond, (x, df, nc), f=_ncx2_cdf, f2=chi2.cdf)
def _ppf(self, q, df, nc):
cond = np.ones_like(q, dtype=bool) & (nc != 0)
return _lazywhere(cond, (q, df, nc), f=sc.chndtrix, f2=chi2.ppf)
def _stats(self, df, nc):
val = df + 2.0*nc
return (df + nc,
2*val,
np.sqrt(8)*(val+nc)/val**1.5,
12.0*(val+2*nc)/val**2.0)
ncx2 = ncx2_gen(a=0.0, name='ncx2')
class ncf_gen(rv_continuous):
r"""A non-central F distribution continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `ncf` is:
.. math::
f(x, n_1, n_2, \lambda) =
\exp\left(\frac{\lambda}{2} +
\lambda n_1 \frac{x}{2(n_1 x + n_2)}
\right)
n_1^{n_1/2} n_2^{n_2/2} x^{n_1/2 - 1} \\
(n_2 + n_1 x)^{-(n_1 + n_2)/2}
\gamma(n_1/2) \gamma(1 + n_2/2) \\
\frac{L^{\frac{n_1}{2}-1}_{n_2/2}
\left(-\lambda n_1 \frac{x}{2(n_1 x + n_2)}\right)}
{B(n_1/2, n_2/2)
\gamma\left(\frac{n_1 + n_2}{2}\right)}
for :math:`n_1, n_2 > 0`, :math:`\lambda\geq 0`. Here :math:`n_1` is the
degrees of freedom in the numerator, :math:`n_2` the degrees of freedom in
the denominator, :math:`\lambda` the non-centrality parameter,
:math:`\gamma` is the logarithm of the Gamma function, :math:`L_n^k` is a
generalized Laguerre polynomial and :math:`B` is the beta function.
`ncf` takes ``df1``, ``df2`` and ``nc`` as shape parameters. If ``nc=0``,
the distribution becomes equivalent to the Fisher distribution.
%(after_notes)s
See Also
--------
scipy.stats.f : Fisher distribution
%(example)s
"""
def _argcheck(self, df1, df2, nc):
return (df1 > 0) & (df2 > 0) & (nc >= 0)
def _rvs(self, dfn, dfd, nc, size=None, random_state=None):
return random_state.noncentral_f(dfn, dfd, nc, size)
def _pdf_skip(self, x, dfn, dfd, nc):
# ncf.pdf(x, df1, df2, nc) = exp(nc/2 + nc*df1*x/(2*(df1*x+df2))) *
# df1**(df1/2) * df2**(df2/2) * x**(df1/2-1) *
# (df2+df1*x)**(-(df1+df2)/2) *
# gamma(df1/2)*gamma(1+df2/2) *
# L^{v1/2-1}^{v2/2}(-nc*v1*x/(2*(v1*x+v2))) /
# (B(v1/2, v2/2) * gamma((v1+v2)/2))
n1, n2 = dfn, dfd
term = -nc/2+nc*n1*x/(2*(n2+n1*x)) + sc.gammaln(n1/2.)+sc.gammaln(1+n2/2.)
term -= sc.gammaln((n1+n2)/2.0)
Px = np.exp(term)
Px *= n1**(n1/2) * n2**(n2/2) * x**(n1/2-1)
Px *= (n2+n1*x)**(-(n1+n2)/2)
Px *= sc.assoc_laguerre(-nc*n1*x/(2.0*(n2+n1*x)), n2/2, n1/2-1)
Px /= sc.beta(n1/2, n2/2)
# This function does not have a return. Drop it for now, the generic
# function seems to work OK.
def _cdf(self, x, dfn, dfd, nc):
return sc.ncfdtr(dfn, dfd, nc, x)
def _ppf(self, q, dfn, dfd, nc):
return sc.ncfdtri(dfn, dfd, nc, q)
def _munp(self, n, dfn, dfd, nc):
val = (dfn * 1.0/dfd)**n
term = sc.gammaln(n+0.5*dfn) + sc.gammaln(0.5*dfd-n) - sc.gammaln(dfd*0.5)
val *= np.exp(-nc / 2.0+term)
val *= sc.hyp1f1(n+0.5*dfn, 0.5*dfn, 0.5*nc)
return val
def _stats(self, dfn, dfd, nc):
# Note: the rv_continuous class ensures that dfn > 0 when this function
# is called, so we don't have to check for division by zero with dfn
# in the following.
mu_num = dfd * (dfn + nc)
mu_den = dfn * (dfd - 2)
mu = np.full_like(mu_num, dtype=np.float64, fill_value=np.inf)
np.true_divide(mu_num, mu_den, where=dfd > 2, out=mu)
mu2_num = 2*((dfn + nc)**2 + (dfn + 2*nc)*(dfd - 2))*(dfd/dfn)**2
mu2_den = (dfd - 2)**2 * (dfd - 4)
mu2 = np.full_like(mu2_num, dtype=np.float64, fill_value=np.inf)
np.true_divide(mu2_num, mu2_den, where=dfd > 4, out=mu2)
return mu, mu2, None, None
ncf = ncf_gen(a=0.0, name='ncf')
class t_gen(rv_continuous):
r"""A Student's t continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `t` is:
.. math::
f(x, \nu) = \frac{\Gamma((\nu+1)/2)}
{\sqrt{\pi \nu} \Gamma(\nu/2)}
(1+x^2/\nu)^{-(\nu+1)/2}
where :math:`x` is a real number and the degrees of freedom parameter
:math:`\nu` (denoted ``df`` in the implementation) satisfies
:math:`\nu > 0`. :math:`\Gamma` is the gamma function
(`scipy.special.gamma`).
%(after_notes)s
%(example)s
"""
def _argcheck(self, df):
return df > 0
def _rvs(self, df, size=None, random_state=None):
return random_state.standard_t(df, size=size)
def _pdf(self, x, df):
# gamma((df+1)/2)
# t.pdf(x, df) = ---------------------------------------------------
# sqrt(pi*df) * gamma(df/2) * (1+x**2/df)**((df+1)/2)
r = np.asarray(df*1.0)
Px = np.exp(sc.gammaln((r+1)/2)-sc.gammaln(r/2))
Px /= np.sqrt(r*np.pi)*(1+(x**2)/r)**((r+1)/2)
return Px
def _logpdf(self, x, df):
r = df*1.0
lPx = sc.gammaln((r+1)/2)-sc.gammaln(r/2)
lPx -= 0.5*np.log(r*np.pi) + (r+1)/2*np.log(1+(x**2)/r)
return lPx
def _cdf(self, x, df):
return sc.stdtr(df, x)
def _sf(self, x, df):
return sc.stdtr(df, -x)
def _ppf(self, q, df):
return sc.stdtrit(df, q)
def _isf(self, q, df):
return -sc.stdtrit(df, q)
def _stats(self, df):
mu = np.where(df > 1, 0.0, np.inf)
mu2 = _lazywhere(df > 2, (df,),
lambda df: df / (df-2.0),
np.inf)
mu2 = np.where(df <= 1, np.nan, mu2)
g1 = np.where(df > 3, 0.0, np.nan)
g2 = _lazywhere(df > 4, (df,),
lambda df: 6.0 / (df-4.0),
np.inf)
g2 = np.where(df <= 2, np.nan, g2)
return mu, mu2, g1, g2
t = t_gen(name='t')
class nct_gen(rv_continuous):
r"""A non-central Student's t continuous random variable.
%(before_notes)s
Notes
-----
If :math:`Y` is a standard normal random variable and :math:`V` is
an independent chi-square random variable (`chi2`) with :math:`k` degrees
of freedom, then
.. math::
X = \frac{Y + c}{\sqrt{V/k}}
has a non-central Student's t distribution on the real line.
The degrees of freedom parameter :math:`k` (denoted ``df`` in the
implementation) satisfies :math:`k > 0` and the noncentrality parameter
:math:`c` (denoted ``nc`` in the implementation) is a real number.
%(after_notes)s
%(example)s
"""
def _argcheck(self, df, nc):
return (df > 0) & (nc == nc)
def _rvs(self, df, nc, size=None, random_state=None):
n = norm.rvs(loc=nc, size=size, random_state=random_state)
c2 = chi2.rvs(df, size=size, random_state=random_state)
return n * np.sqrt(df) / np.sqrt(c2)
def _pdf(self, x, df, nc):
n = df*1.0
nc = nc*1.0
x2 = x*x
ncx2 = nc*nc*x2
fac1 = n + x2
trm1 = n/2.*np.log(n) + sc.gammaln(n+1)
trm1 -= n*np.log(2)+nc*nc/2.+(n/2.)*np.log(fac1)+sc.gammaln(n/2.)
Px = np.exp(trm1)
valF = ncx2 / (2*fac1)
trm1 = np.sqrt(2)*nc*x*sc.hyp1f1(n/2+1, 1.5, valF)
trm1 /= np.asarray(fac1*sc.gamma((n+1)/2))
trm2 = sc.hyp1f1((n+1)/2, 0.5, valF)
trm2 /= np.asarray(np.sqrt(fac1)*sc.gamma(n/2+1))
Px *= trm1+trm2
return Px
def _cdf(self, x, df, nc):
return sc.nctdtr(df, nc, x)
def _ppf(self, q, df, nc):
return sc.nctdtrit(df, nc, q)
def _stats(self, df, nc, moments='mv'):
#
# See D. Hogben, R.S. Pinkham, and M.B. Wilk,
# 'The moments of the non-central t-distribution'
# Biometrika 48, p. 465 (2961).
# e.g. https://www.jstor.org/stable/2332772 (gated)
#
mu, mu2, g1, g2 = None, None, None, None
gfac = sc.gamma(df/2.-0.5) / sc.gamma(df/2.)
c11 = np.sqrt(df/2.) * gfac
c20 = df / (df-2.)
c22 = c20 - c11*c11
mu = np.where(df > 1, nc*c11, np.inf)
mu2 = np.where(df > 2, c22*nc*nc + c20, np.inf)
if 's' in moments:
c33t = df * (7.-2.*df) / (df-2.) / (df-3.) + 2.*c11*c11
c31t = 3.*df / (df-2.) / (df-3.)
mu3 = (c33t*nc*nc + c31t) * c11*nc
g1 = np.where(df > 3, mu3 / np.power(mu2, 1.5), np.nan)
# kurtosis
if 'k' in moments:
c44 = df*df / (df-2.) / (df-4.)
c44 -= c11*c11 * 2.*df*(5.-df) / (df-2.) / (df-3.)
c44 -= 3.*c11**4
c42 = df / (df-4.) - c11*c11 * (df-1.) / (df-3.)
c42 *= 6.*df / (df-2.)
c40 = 3.*df*df / (df-2.) / (df-4.)
mu4 = c44 * nc**4 + c42*nc**2 + c40
g2 = np.where(df > 4, mu4/mu2**2 - 3., np.nan)
return mu, mu2, g1, g2
nct = nct_gen(name="nct")
class pareto_gen(rv_continuous):
r"""A Pareto continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `pareto` is:
.. math::
f(x, b) = \frac{b}{x^{b+1}}
for :math:`x \ge 1`, :math:`b > 0`.
`pareto` takes ``b`` as a shape parameter for :math:`b`.
%(after_notes)s
%(example)s
"""
def _pdf(self, x, b):
# pareto.pdf(x, b) = b / x**(b+1)
return b * x**(-b-1)
def _cdf(self, x, b):
return 1 - x**(-b)
def _ppf(self, q, b):
return pow(1-q, -1.0/b)
def _sf(self, x, b):
return x**(-b)
def _stats(self, b, moments='mv'):
mu, mu2, g1, g2 = None, None, None, None
if 'm' in moments:
mask = b > 1
bt = np.extract(mask, b)
mu = valarray(np.shape(b), value=np.inf)
np.place(mu, mask, bt / (bt-1.0))
if 'v' in moments:
mask = b > 2
bt = np.extract(mask, b)
mu2 = valarray(np.shape(b), value=np.inf)
np.place(mu2, mask, bt / (bt-2.0) / (bt-1.0)**2)
if 's' in moments:
mask = b > 3
bt = np.extract(mask, b)
g1 = valarray(np.shape(b), value=np.nan)
vals = 2 * (bt + 1.0) * np.sqrt(bt - 2.0) / ((bt - 3.0) * np.sqrt(bt))
np.place(g1, mask, vals)
if 'k' in moments:
mask = b > 4
bt = np.extract(mask, b)
g2 = valarray(np.shape(b), value=np.nan)
vals = (6.0*np.polyval([1.0, 1.0, -6, -2], bt) /
np.polyval([1.0, -7.0, 12.0, 0.0], bt))
np.place(g2, mask, vals)
return mu, mu2, g1, g2
def _entropy(self, c):
return 1 + 1.0/c - np.log(c)
pareto = pareto_gen(a=1.0, name="pareto")
class lomax_gen(rv_continuous):
r"""A Lomax (Pareto of the second kind) continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `lomax` is:
.. math::
f(x, c) = \frac{c}{(1+x)^{c+1}}
for :math:`x \ge 0`, :math:`c > 0`.
`lomax` takes ``c`` as a shape parameter for :math:`c`.
`lomax` is a special case of `pareto` with ``loc=-1.0``.
%(after_notes)s
%(example)s
"""
def _pdf(self, x, c):
# lomax.pdf(x, c) = c / (1+x)**(c+1)
return c*1.0/(1.0+x)**(c+1.0)
def _logpdf(self, x, c):
return np.log(c) - (c+1)*sc.log1p(x)
def _cdf(self, x, c):
return -sc.expm1(-c*sc.log1p(x))
def _sf(self, x, c):
return np.exp(-c*sc.log1p(x))
def _logsf(self, x, c):
return -c*sc.log1p(x)
def _ppf(self, q, c):
return sc.expm1(-sc.log1p(-q)/c)
def _stats(self, c):
mu, mu2, g1, g2 = pareto.stats(c, loc=-1.0, moments='mvsk')
return mu, mu2, g1, g2
def _entropy(self, c):
return 1+1.0/c-np.log(c)
lomax = lomax_gen(a=0.0, name="lomax")
class pearson3_gen(rv_continuous):
r"""A pearson type III continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `pearson3` is:
.. math::
f(x, skew) = \frac{|\beta|}{\Gamma(\alpha)}
(\beta (x - \zeta))^{\alpha - 1}
\exp(-\beta (x - \zeta))
where:
.. math::
\beta = \frac{2}{skew stddev}
\alpha = (stddev \beta)^2
\zeta = loc - \frac{\alpha}{\beta}
:math:`\Gamma` is the gamma function (`scipy.special.gamma`).
`pearson3` takes ``skew`` as a shape parameter for :math:`skew`.
%(after_notes)s
%(example)s
References
----------
R.W. Vogel and D.E. McMartin, "Probability Plot Goodness-of-Fit and
Skewness Estimation Procedures for the Pearson Type 3 Distribution", Water
Resources Research, Vol.27, 3149-3158 (1991).
L.R. Salvosa, "Tables of Pearson's Type III Function", Ann. Math. Statist.,
Vol.1, 191-198 (1930).
"Using Modern Computing Tools to Fit the Pearson Type III Distribution to
Aviation Loads Data", Office of Aviation Research (2003).
"""
def _preprocess(self, x, skew):
# The real 'loc' and 'scale' are handled in the calling pdf(...). The
# local variables 'loc' and 'scale' within pearson3._pdf are set to
# the defaults just to keep them as part of the equations for
# documentation.
loc = 0.0
scale = 1.0
# If skew is small, return _norm_pdf. The divide between pearson3
# and norm was found by brute force and is approximately a skew of
# 0.000016. No one, I hope, would actually use a skew value even
# close to this small.
norm2pearson_transition = 0.000016
ans, x, skew = np.broadcast_arrays([1.0], x, skew)
ans = ans.copy()
# mask is True where skew is small enough to use the normal approx.
mask = np.absolute(skew) < norm2pearson_transition
invmask = ~mask
beta = 2.0 / (skew[invmask] * scale)
alpha = (scale * beta)**2
zeta = loc - alpha / beta
transx = beta * (x[invmask] - zeta)
return ans, x, transx, mask, invmask, beta, alpha, zeta
def _argcheck(self, skew):
# The _argcheck function in rv_continuous only allows positive
# arguments. The skew argument for pearson3 can be zero (which I want
# to handle inside pearson3._pdf) or negative. So just return True
# for all skew args.
return np.ones(np.shape(skew), dtype=bool)
def _stats(self, skew):
_, _, _, _, _, beta, alpha, zeta = (
self._preprocess([1], skew))
m = zeta + alpha / beta
v = alpha / (beta**2)
s = 2.0 / (alpha**0.5) * np.sign(beta)
k = 6.0 / alpha
return m, v, s, k
def _pdf(self, x, skew):
# pearson3.pdf(x, skew) = abs(beta) / gamma(alpha) *
# (beta * (x - zeta))**(alpha - 1) * exp(-beta*(x - zeta))
# Do the calculation in _logpdf since helps to limit
# overflow/underflow problems
ans = np.exp(self._logpdf(x, skew))
if ans.ndim == 0:
if np.isnan(ans):
return 0.0
return ans
ans[np.isnan(ans)] = 0.0
return ans
def _logpdf(self, x, skew):
# PEARSON3 logpdf GAMMA logpdf
# np.log(abs(beta))
# + (alpha - 1)*np.log(beta*(x - zeta)) + (a - 1)*np.log(x)
# - beta*(x - zeta) - x
# - sc.gammalnalpha) - sc.gammalna)
ans, x, transx, mask, invmask, beta, alpha, _ = (
self._preprocess(x, skew))
ans[mask] = np.log(_norm_pdf(x[mask]))
ans[invmask] = np.log(abs(beta)) + gamma._logpdf(transx, alpha)
return ans
def _cdf(self, x, skew):
ans, x, transx, mask, invmask, _, alpha, _ = (
self._preprocess(x, skew))
ans[mask] = _norm_cdf(x[mask])
ans[invmask] = gamma._cdf(transx, alpha)
return ans
def _rvs(self, skew, size=None, random_state=None):
skew = np.broadcast_to(skew, size)
ans, _, _, mask, invmask, beta, alpha, zeta = (
self._preprocess([0], skew))
nsmall = mask.sum()
nbig = mask.size - nsmall
ans[mask] = random_state.standard_normal(nsmall)
ans[invmask] = random_state.standard_gamma(alpha, nbig)/beta + zeta
if size == ():
ans = ans[0]
return ans
def _ppf(self, q, skew):
ans, q, _, mask, invmask, beta, alpha, zeta = (
self._preprocess(q, skew))
ans[mask] = _norm_ppf(q[mask])
ans[invmask] = sc.gammaincinv(alpha, q[invmask])/beta + zeta
return ans
pearson3 = pearson3_gen(name="pearson3")
class powerlaw_gen(rv_continuous):
r"""A power-function continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `powerlaw` is:
.. math::
f(x, a) = a x^{a-1}
for :math:`0 \le x \le 1`, :math:`a > 0`.
`powerlaw` takes ``a`` as a shape parameter for :math:`a`.
%(after_notes)s
`powerlaw` is a special case of `beta` with ``b=1``.
%(example)s
"""
def _pdf(self, x, a):
# powerlaw.pdf(x, a) = a * x**(a-1)
return a*x**(a-1.0)
def _logpdf(self, x, a):
return np.log(a) + sc.xlogy(a - 1, x)
def _cdf(self, x, a):
return x**(a*1.0)
def _logcdf(self, x, a):
return a*np.log(x)
def _ppf(self, q, a):
return pow(q, 1.0/a)
def _stats(self, a):
return (a / (a + 1.0),
a / (a + 2.0) / (a + 1.0) ** 2,
-2.0 * ((a - 1.0) / (a + 3.0)) * np.sqrt((a + 2.0) / a),
6 * np.polyval([1, -1, -6, 2], a) / (a * (a + 3.0) * (a + 4)))
def _entropy(self, a):
return 1 - 1.0/a - np.log(a)
powerlaw = powerlaw_gen(a=0.0, b=1.0, name="powerlaw")
class powerlognorm_gen(rv_continuous):
r"""A power log-normal continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `powerlognorm` is:
.. math::
f(x, c, s) = \frac{c}{x s} \phi(\log(x)/s)
(\Phi(-\log(x)/s))^{c-1}
where :math:`\phi` is the normal pdf, and :math:`\Phi` is the normal cdf,
and :math:`x > 0`, :math:`s, c > 0`.
`powerlognorm` takes :math:`c` and :math:`s` as shape parameters.
%(after_notes)s
%(example)s
"""
_support_mask = rv_continuous._open_support_mask
def _pdf(self, x, c, s):
# powerlognorm.pdf(x, c, s) = c / (x*s) * phi(log(x)/s) *
# (Phi(-log(x)/s))**(c-1),
return (c/(x*s) * _norm_pdf(np.log(x)/s) *
pow(_norm_cdf(-np.log(x)/s), c*1.0-1.0))
def _cdf(self, x, c, s):
return 1.0 - pow(_norm_cdf(-np.log(x)/s), c*1.0)
def _ppf(self, q, c, s):
return np.exp(-s * _norm_ppf(pow(1.0 - q, 1.0 / c)))
powerlognorm = powerlognorm_gen(a=0.0, name="powerlognorm")
class powernorm_gen(rv_continuous):
r"""A power normal continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `powernorm` is:
.. math::
f(x, c) = c \phi(x) (\Phi(-x))^{c-1}
where :math:`\phi` is the normal pdf, and :math:`\Phi` is the normal cdf,
and :math:`x >= 0`, :math:`c > 0`.
`powernorm` takes ``c`` as a shape parameter for :math:`c`.
%(after_notes)s
%(example)s
"""
def _pdf(self, x, c):
# powernorm.pdf(x, c) = c * phi(x) * (Phi(-x))**(c-1)
return c*_norm_pdf(x) * (_norm_cdf(-x)**(c-1.0))
def _logpdf(self, x, c):
return np.log(c) + _norm_logpdf(x) + (c-1)*_norm_logcdf(-x)
def _cdf(self, x, c):
return 1.0-_norm_cdf(-x)**(c*1.0)
def _ppf(self, q, c):
return -_norm_ppf(pow(1.0 - q, 1.0 / c))
powernorm = powernorm_gen(name='powernorm')
class rdist_gen(rv_continuous):
r"""An R-distributed (symmetric beta) continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `rdist` is:
.. math::
f(x, c) = \frac{(1-x^2)^{c/2-1}}{B(1/2, c/2)}
for :math:`-1 \le x \le 1`, :math:`c > 0`. `rdist` is also called the
symmetric beta distribution: if B has a `beta` distribution with
parameters (c/2, c/2), then X = 2*B - 1 follows a R-distribution with
parameter c.
`rdist` takes ``c`` as a shape parameter for :math:`c`.
This distribution includes the following distribution kernels as
special cases::
c = 2: uniform
c = 3: `semicircular`
c = 4: Epanechnikov (parabolic)
c = 6: quartic (biweight)
c = 8: triweight
%(after_notes)s
%(example)s
"""
# use relation to the beta distribution for pdf, cdf, etc
def _pdf(self, x, c):
return 0.5*beta._pdf((x + 1)/2, c/2, c/2)
def _logpdf(self, x, c):
return -np.log(2) + beta._logpdf((x + 1)/2, c/2, c/2)
def _cdf(self, x, c):
return beta._cdf((x + 1)/2, c/2, c/2)
def _ppf(self, q, c):
return 2*beta._ppf(q, c/2, c/2) - 1
def _rvs(self, c, size=None, random_state=None):
return 2 * random_state.beta(c/2, c/2, size) - 1
def _munp(self, n, c):
numerator = (1 - (n % 2)) * sc.beta((n + 1.0) / 2, c / 2.0)
return numerator / sc.beta(1. / 2, c / 2.)
rdist = rdist_gen(a=-1.0, b=1.0, name="rdist")
class rayleigh_gen(rv_continuous):
r"""A Rayleigh continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `rayleigh` is:
.. math::
f(x) = x \exp(-x^2/2)
for :math:`x \ge 0`.
`rayleigh` is a special case of `chi` with ``df=2``.
%(after_notes)s
%(example)s
"""
_support_mask = rv_continuous._open_support_mask
def _rvs(self, size=None, random_state=None):
return chi.rvs(2, size=size, random_state=random_state)
def _pdf(self, r):
# rayleigh.pdf(r) = r * exp(-r**2/2)
return np.exp(self._logpdf(r))
def _logpdf(self, r):
return np.log(r) - 0.5 * r * r
def _cdf(self, r):
return -sc.expm1(-0.5 * r**2)
def _ppf(self, q):
return np.sqrt(-2 * sc.log1p(-q))
def _sf(self, r):
return np.exp(self._logsf(r))
def _logsf(self, r):
return -0.5 * r * r
def _isf(self, q):
return np.sqrt(-2 * np.log(q))
def _stats(self):
val = 4 - np.pi
return (np.sqrt(np.pi/2),
val/2,
2*(np.pi-3)*np.sqrt(np.pi)/val**1.5,
6*np.pi/val-16/val**2)
def _entropy(self):
return _EULER/2.0 + 1 - 0.5*np.log(2)
rayleigh = rayleigh_gen(a=0.0, name="rayleigh")
class reciprocal_gen(rv_continuous):
r"""A loguniform or reciprocal continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for this class is:
.. math::
f(x, a, b) = \frac{1}{x \log(b/a)}
for :math:`a \le x \le b`, :math:`b > a > 0`. This class takes
:math:`a` and :math:`b` as shape parameters. %(after_notes)s
%(example)s
This doesn't show the equal probability of ``0.01``, ``0.1`` and
``1``. This is best when the x-axis is log-scaled:
>>> import numpy as np
>>> fig, ax = plt.subplots(1, 1)
>>> ax.hist(np.log10(r))
>>> ax.set_ylabel("Frequency")
>>> ax.set_xlabel("Value of random variable")
>>> ax.xaxis.set_major_locator(plt.FixedLocator([-2, -1, 0]))
>>> ticks = ["$10^{{ {} }}$".format(i) for i in [-2, -1, 0]]
>>> ax.set_xticklabels(ticks) # doctest: +SKIP
>>> plt.show()
This random variable will be log-uniform regardless of the base chosen for
``a`` and ``b``. Let's specify with base ``2`` instead:
>>> rvs = %(name)s(2**-2, 2**0).rvs(size=1000)
Values of ``1/4``, ``1/2`` and ``1`` are equally likely with this random
variable. Here's the histogram:
>>> fig, ax = plt.subplots(1, 1)
>>> ax.hist(np.log2(rvs))
>>> ax.set_ylabel("Frequency")
>>> ax.set_xlabel("Value of random variable")
>>> ax.xaxis.set_major_locator(plt.FixedLocator([-2, -1, 0]))
>>> ticks = ["$2^{{ {} }}$".format(i) for i in [-2, -1, 0]]
>>> ax.set_xticklabels(ticks) # doctest: +SKIP
>>> plt.show()
"""
def _argcheck(self, a, b):
return (a > 0) & (b > a)
def _get_support(self, a, b):
return a, b
def _pdf(self, x, a, b):
# reciprocal.pdf(x, a, b) = 1 / (x*log(b/a))
return 1.0 / (x * np.log(b * 1.0 / a))
def _logpdf(self, x, a, b):
return -np.log(x) - np.log(np.log(b * 1.0 / a))
def _cdf(self, x, a, b):
return (np.log(x)-np.log(a)) / np.log(b * 1.0 / a)
def _ppf(self, q, a, b):
return a*pow(b*1.0/a, q)
def _munp(self, n, a, b):
return 1.0/np.log(b*1.0/a) / n * (pow(b*1.0, n) - pow(a*1.0, n))
def _entropy(self, a, b):
return 0.5*np.log(a*b)+np.log(np.log(b*1.0/a))
loguniform = reciprocal_gen(name="loguniform")
reciprocal = reciprocal_gen(name="reciprocal")
class rice_gen(rv_continuous):
r"""A Rice continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `rice` is:
.. math::
f(x, b) = x \exp(- \frac{x^2 + b^2}{2}) I_0(x b)
for :math:`x >= 0`, :math:`b > 0`. :math:`I_0` is the modified Bessel
function of order zero (`scipy.special.i0`).
`rice` takes ``b`` as a shape parameter for :math:`b`.
%(after_notes)s
The Rice distribution describes the length, :math:`r`, of a 2-D vector with
components :math:`(U+u, V+v)`, where :math:`U, V` are constant, :math:`u,
v` are independent Gaussian random variables with standard deviation
:math:`s`. Let :math:`R = \sqrt{U^2 + V^2}`. Then the pdf of :math:`r` is
``rice.pdf(x, R/s, scale=s)``.
%(example)s
"""
def _argcheck(self, b):
return b >= 0
def _rvs(self, b, size=None, random_state=None):
# https://en.wikipedia.org/wiki/Rice_distribution
t = b/np.sqrt(2) + random_state.standard_normal(size=(2,) + size)
return np.sqrt((t*t).sum(axis=0))
def _cdf(self, x, b):
return sc.chndtr(np.square(x), 2, np.square(b))
def _ppf(self, q, b):
return np.sqrt(sc.chndtrix(q, 2, np.square(b)))
def _pdf(self, x, b):
# rice.pdf(x, b) = x * exp(-(x**2+b**2)/2) * I[0](x*b)
#
# We use (x**2 + b**2)/2 = ((x-b)**2)/2 + xb.
# The factor of np.exp(-xb) is then included in the i0e function
# in place of the modified Bessel function, i0, improving
# numerical stability for large values of xb.
return x * np.exp(-(x-b)*(x-b)/2.0) * sc.i0e(x*b)
def _munp(self, n, b):
nd2 = n/2.0
n1 = 1 + nd2
b2 = b*b/2.0
return (2.0**(nd2) * np.exp(-b2) * sc.gamma(n1) *
sc.hyp1f1(n1, 1, b2))
rice = rice_gen(a=0.0, name="rice")
# FIXME: PPF does not work.
class recipinvgauss_gen(rv_continuous):
r"""A reciprocal inverse Gaussian continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `recipinvgauss` is:
.. math::
f(x, \mu) = \frac{1}{\sqrt{2\pi x}}
\exp\left(\frac{-(1-\mu x)^2}{2\mu^2x}\right)
for :math:`x \ge 0`.
`recipinvgauss` takes ``mu`` as a shape parameter for :math:`\mu`.
%(after_notes)s
%(example)s
"""
def _pdf(self, x, mu):
# recipinvgauss.pdf(x, mu) =
# 1/sqrt(2*pi*x) * exp(-(1-mu*x)**2/(2*x*mu**2))
return 1.0/np.sqrt(2*np.pi*x)*np.exp(-(1-mu*x)**2.0 / (2*x*mu**2.0))
def _logpdf(self, x, mu):
return -(1-mu*x)**2.0 / (2*x*mu**2.0) - 0.5*np.log(2*np.pi*x)
def _cdf(self, x, mu):
trm1 = 1.0/mu - x
trm2 = 1.0/mu + x
isqx = 1.0/np.sqrt(x)
return 1.0-_norm_cdf(isqx*trm1)-np.exp(2.0/mu)*_norm_cdf(-isqx*trm2)
def _rvs(self, mu, size=None, random_state=None):
return 1.0/random_state.wald(mu, 1.0, size=size)
recipinvgauss = recipinvgauss_gen(a=0.0, name='recipinvgauss')
class semicircular_gen(rv_continuous):
r"""A semicircular continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `semicircular` is:
.. math::
f(x) = \frac{2}{\pi} \sqrt{1-x^2}
for :math:`-1 \le x \le 1`.
The distribution is a special case of `rdist` with `c = 3`.
%(after_notes)s
See Also
--------
rdist
References
----------
.. [1] "Wigner semicircle distribution",
https://en.wikipedia.org/wiki/Wigner_semicircle_distribution
%(example)s
"""
def _pdf(self, x):
return 2.0/np.pi*np.sqrt(1-x*x)
def _logpdf(self, x):
return np.log(2/np.pi) + 0.5*np.log1p(-x*x)
def _cdf(self, x):
return 0.5+1.0/np.pi*(x*np.sqrt(1-x*x) + np.arcsin(x))
def _ppf(self, q):
return rdist._ppf(q, 3)
def _rvs(self, size=None, random_state=None):
# generate values uniformly distributed on the area under the pdf
# (semi-circle) by randomly generating the radius and angle
r = np.sqrt(random_state.uniform(size=size))
a = np.cos(np.pi * random_state.uniform(size=size))
return r * a
def _stats(self):
return 0, 0.25, 0, -1.0
def _entropy(self):
return 0.64472988584940017414
semicircular = semicircular_gen(a=-1.0, b=1.0, name="semicircular")
class skew_norm_gen(rv_continuous):
r"""A skew-normal random variable.
%(before_notes)s
Notes
-----
The pdf is::
skewnorm.pdf(x, a) = 2 * norm.pdf(x) * norm.cdf(a*x)
`skewnorm` takes a real number :math:`a` as a skewness parameter
When ``a = 0`` the distribution is identical to a normal distribution
(`norm`). `rvs` implements the method of [1]_.
%(after_notes)s
%(example)s
References
----------
.. [1] A. Azzalini and A. Capitanio (1999). Statistical applications of the
multivariate skew-normal distribution. J. Roy. Statist. Soc., B 61, 579-602.
https://arxiv.org/abs/0911.2093
"""
def _argcheck(self, a):
return np.isfinite(a)
def _pdf(self, x, a):
return 2.*_norm_pdf(x)*_norm_cdf(a*x)
def _cdf_single(self, x, *args):
_a, _b = self._get_support(*args)
if x <= 0:
cdf = integrate.quad(self._pdf, _a, x, args=args)[0]
else:
t1 = integrate.quad(self._pdf, _a, 0, args=args)[0]
t2 = integrate.quad(self._pdf, 0, x, args=args)[0]
cdf = t1 + t2
if cdf > 1:
# Presumably numerical noise, e.g. 1.0000000000000002
cdf = 1.0
return cdf
def _sf(self, x, a):
return self._cdf(-x, -a)
def _rvs(self, a, size=None, random_state=None):
u0 = random_state.normal(size=size)
v = random_state.normal(size=size)
d = a/np.sqrt(1 + a**2)
u1 = d*u0 + v*np.sqrt(1 - d**2)
return np.where(u0 >= 0, u1, -u1)
def _stats(self, a, moments='mvsk'):
output = [None, None, None, None]
const = np.sqrt(2/np.pi) * a/np.sqrt(1 + a**2)
if 'm' in moments:
output[0] = const
if 'v' in moments:
output[1] = 1 - const**2
if 's' in moments:
output[2] = ((4 - np.pi)/2) * (const/np.sqrt(1 - const**2))**3
if 'k' in moments:
output[3] = (2*(np.pi - 3)) * (const**4/(1 - const**2)**2)
return output
skewnorm = skew_norm_gen(name='skewnorm')
class trapz_gen(rv_continuous):
r"""A trapezoidal continuous random variable.
%(before_notes)s
Notes
-----
The trapezoidal distribution can be represented with an up-sloping line
from ``loc`` to ``(loc + c*scale)``, then constant to ``(loc + d*scale)``
and then downsloping from ``(loc + d*scale)`` to ``(loc+scale)``. This
defines the trapezoid base from ``loc`` to ``(loc+scale)`` and the flat
top from ``c`` to ``d`` proportional to the position along the base
with ``0 <= c <= d <= 1``. When ``c=d``, this is equivalent to `triang`
with the same values for `loc`, `scale` and `c`.
The method of [1]_ is used for computing moments.
`trapz` takes :math:`c` and :math:`d` as shape parameters.
%(after_notes)s
The standard form is in the range [0, 1] with c the mode.
The location parameter shifts the start to `loc`.
The scale parameter changes the width from 1 to `scale`.
%(example)s
References
----------
.. [1] Kacker, R.N. and Lawrence, J.F. (2007). Trapezoidal and triangular
distributions for Type B evaluation of standard uncertainty.
Metrologia 44, 117–127. https://doi.org/10.1088/0026-1394/44/2/003
"""
def _argcheck(self, c, d):
return (c >= 0) & (c <= 1) & (d >= 0) & (d <= 1) & (d >= c)
def _pdf(self, x, c, d):
u = 2 / (d-c+1)
return _lazyselect([x < c,
(c <= x) & (x <= d),
x > d],
[lambda x, c, d, u: u * x / c,
lambda x, c, d, u: u,
lambda x, c, d, u: u * (1-x) / (1-d)],
(x, c, d, u))
def _cdf(self, x, c, d):
return _lazyselect([x < c,
(c <= x) & (x <= d),
x > d],
[lambda x, c, d: x**2 / c / (d-c+1),
lambda x, c, d: (c + 2 * (x-c)) / (d-c+1),
lambda x, c, d: 1-((1-x) ** 2
/ (d-c+1) / (1-d))],
(x, c, d))
def _ppf(self, q, c, d):
qc, qd = self._cdf(c, c, d), self._cdf(d, c, d)
condlist = [q < qc, q <= qd, q > qd]
choicelist = [np.sqrt(q * c * (1 + d - c)),
0.5 * q * (1 + d - c) + 0.5 * c,
1 - np.sqrt((1 - q) * (d - c + 1) * (1 - d))]
return np.select(condlist, choicelist)
def _munp(self, n, c, d):
# Using the parameterization from Kacker, 2007, with
# a=bottom left, c=top left, d=top right, b=bottom right, then
# E[X^n] = h/(n+1)/(n+2) [(b^{n+2}-d^{n+2})/(b-d)
# - ((c^{n+2} - a^{n+2})/(c-a)]
# with h = 2/((b-a) - (d-c)). The corresponding parameterization
# in scipy, has a'=loc, c'=loc+c*scale, d'=loc+d*scale, b'=loc+scale,
# which for standard form reduces to a'=0, b'=1, c'=c, d'=d.
# Substituting into E[X^n] gives the bd' term as (1 - d^{n+2})/(1 - d)
# and the ac' term as c^{n-1} for the standard form. The bd' term has
# numerical difficulties near d=1, so replace (1 - d^{n+2})/(1-d)
# with expm1((n+2)*log(d))/(d-1).
# Testing with n=18 for c=(1e-30,1-eps) shows that this is stable.
# We still require an explicit test for d=1 to prevent divide by zero,
# and now a test for d=0 to prevent log(0).
ab_term = c**(n+1)
dc_term = _lazyselect(
[d == 0.0, (0.0 < d) & (d < 1.0), d == 1.0],
[lambda d: 1.0,
lambda d: np.expm1((n+2) * np.log(d)) / (d-1.0),
lambda d: n+2],
[d])
val = 2.0 / (1.0+d-c) * (dc_term - ab_term) / ((n+1) * (n+2))
return val
def _entropy(self, c, d):
# Using the parameterization from Wikipedia (van Dorp, 2003)
# with a=bottom left, c=top left, d=top right, b=bottom right
# gives a'=loc, b'=loc+c*scale, c'=loc+d*scale, d'=loc+scale,
# which for loc=0, scale=1 is a'=0, b'=c, c'=d, d'=1.
# Substituting into the entropy formula from Wikipedia gives
# the following result.
return 0.5 * (1.0-d+c) / (1.0+d-c) + np.log(0.5 * (1.0+d-c))
trapz = trapz_gen(a=0.0, b=1.0, name="trapz")
class triang_gen(rv_continuous):
r"""A triangular continuous random variable.
%(before_notes)s
Notes
-----
The triangular distribution can be represented with an up-sloping line from
``loc`` to ``(loc + c*scale)`` and then downsloping for ``(loc + c*scale)``
to ``(loc + scale)``.
`triang` takes ``c`` as a shape parameter for :math:`c`.
%(after_notes)s
The standard form is in the range [0, 1] with c the mode.
The location parameter shifts the start to `loc`.
The scale parameter changes the width from 1 to `scale`.
%(example)s
"""
def _rvs(self, c, size=None, random_state=None):
return random_state.triangular(0, c, 1, size)
def _argcheck(self, c):
return (c >= 0) & (c <= 1)
def _pdf(self, x, c):
# 0: edge case where c=0
# 1: generalised case for x < c, don't use x <= c, as it doesn't cope
# with c = 0.
# 2: generalised case for x >= c, but doesn't cope with c = 1
# 3: edge case where c=1
r = _lazyselect([c == 0,
x < c,
(x >= c) & (c != 1),
c == 1],
[lambda x, c: 2 - 2 * x,
lambda x, c: 2 * x / c,
lambda x, c: 2 * (1 - x) / (1 - c),
lambda x, c: 2 * x],
(x, c))
return r
def _cdf(self, x, c):
r = _lazyselect([c == 0,
x < c,
(x >= c) & (c != 1),
c == 1],
[lambda x, c: 2*x - x*x,
lambda x, c: x * x / c,
lambda x, c: (x*x - 2*x + c) / (c-1),
lambda x, c: x * x],
(x, c))
return r
def _ppf(self, q, c):
return np.where(q < c, np.sqrt(c * q), 1-np.sqrt((1-c) * (1-q)))
def _stats(self, c):
return ((c+1.0)/3.0,
(1.0-c+c*c)/18,
np.sqrt(2)*(2*c-1)*(c+1)*(c-2) / (5*np.power((1.0-c+c*c), 1.5)),
-3.0/5.0)
def _entropy(self, c):
return 0.5-np.log(2)
triang = triang_gen(a=0.0, b=1.0, name="triang")
class truncexpon_gen(rv_continuous):
r"""A truncated exponential continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `truncexpon` is:
.. math::
f(x, b) = \frac{\exp(-x)}{1 - \exp(-b)}
for :math:`0 <= x <= b`.
`truncexpon` takes ``b`` as a shape parameter for :math:`b`.
%(after_notes)s
%(example)s
"""
def _argcheck(self, b):
return b > 0
def _get_support(self, b):
return self.a, b
def _pdf(self, x, b):
# truncexpon.pdf(x, b) = exp(-x) / (1-exp(-b))
return np.exp(-x)/(-sc.expm1(-b))
def _logpdf(self, x, b):
return -x - np.log(-sc.expm1(-b))
def _cdf(self, x, b):
return sc.expm1(-x)/sc.expm1(-b)
def _ppf(self, q, b):
return -sc.log1p(q*sc.expm1(-b))
def _munp(self, n, b):
# wrong answer with formula, same as in continuous.pdf
# return sc.gamman+1)-sc.gammainc1+n, b)
if n == 1:
return (1-(b+1)*np.exp(-b))/(-sc.expm1(-b))
elif n == 2:
return 2*(1-0.5*(b*b+2*b+2)*np.exp(-b))/(-sc.expm1(-b))
else:
# return generic for higher moments
# return rv_continuous._mom1_sc(self, n, b)
return self._mom1_sc(n, b)
def _entropy(self, b):
eB = np.exp(b)
return np.log(eB-1)+(1+eB*(b-1.0))/(1.0-eB)
truncexpon = truncexpon_gen(a=0.0, name='truncexpon')
TRUNCNORM_TAIL_X = 30
TRUNCNORM_MAX_BRENT_ITERS = 40
def _truncnorm_get_delta_scalar(a, b):
if (a > TRUNCNORM_TAIL_X) or (b < -TRUNCNORM_TAIL_X):
return 0
if a > 0:
delta = _norm_sf(a) - _norm_sf(b)
else:
delta = _norm_cdf(b) - _norm_cdf(a)
delta = max(delta, 0)
return delta
def _truncnorm_get_delta(a, b):
if np.isscalar(a) and np.isscalar(b):
return _truncnorm_get_delta_scalar(a, b)
a, b = np.atleast_1d(a), np.atleast_1d(b)
if a.size == 1 and b.size == 1:
return _truncnorm_get_delta_scalar(a.item(), b.item())
delta = np.zeros(np.shape(a))
condinner = (a <= TRUNCNORM_TAIL_X) & (b >= -TRUNCNORM_TAIL_X)
conda = (a > 0) & condinner
condb = (a <= 0) & condinner
if np.any(conda):
np.place(delta, conda, _norm_sf(a[conda]) - _norm_sf(b[conda]))
if np.any(condb):
np.place(delta, condb, _norm_cdf(b[condb]) - _norm_cdf(a[condb]))
delta[delta < 0] = 0
return delta
def _truncnorm_get_logdelta_scalar(a, b):
if (a <= TRUNCNORM_TAIL_X) and (b >= -TRUNCNORM_TAIL_X):
if a > 0:
delta = _norm_sf(a) - _norm_sf(b)
else:
delta = _norm_cdf(b) - _norm_cdf(a)
delta = max(delta, 0)
if delta > 0:
return np.log(delta)
if b < 0 or (np.abs(a) >= np.abs(b)):
nla, nlb = _norm_logcdf(a), _norm_logcdf(b)
logdelta = nlb + np.log1p(-np.exp(nla - nlb))
else:
sla, slb = _norm_logsf(a), _norm_logsf(b)
logdelta = sla + np.log1p(-np.exp(slb - sla))
return logdelta
def _truncnorm_logpdf_scalar(x, a, b):
with np.errstate(invalid='ignore'):
if np.isscalar(x):
if x < a:
return -np.inf
if x > b:
return -np.inf
shp = np.shape(x)
x = np.atleast_1d(x)
out = np.full_like(x, np.nan, dtype=np.double)
condlta, condgtb = (x < a), (x > b)
if np.any(condlta):
np.place(out, condlta, -np.inf)
if np.any(condgtb):
np.place(out, condgtb, -np.inf)
cond_inner = ~condlta & ~condgtb
if np.any(cond_inner):
_logdelta = _truncnorm_get_logdelta_scalar(a, b)
np.place(out, cond_inner, _norm_logpdf(x[cond_inner]) - _logdelta)
return (out[0] if (shp == ()) else out)
def _truncnorm_pdf_scalar(x, a, b):
with np.errstate(invalid='ignore'):
if np.isscalar(x):
if x < a:
return 0.0
if x > b:
return 0.0
shp = np.shape(x)
x = np.atleast_1d(x)
out = np.full_like(x, np.nan, dtype=np.double)
condlta, condgtb = (x < a), (x > b)
if np.any(condlta):
np.place(out, condlta, 0.0)
if np.any(condgtb):
np.place(out, condgtb, 0.0)
cond_inner = ~condlta & ~condgtb
if np.any(cond_inner):
delta = _truncnorm_get_delta_scalar(a, b)
if delta > 0:
np.place(out, cond_inner, _norm_pdf(x[cond_inner]) / delta)
else:
np.place(out, cond_inner,
np.exp(_truncnorm_logpdf_scalar(x[cond_inner], a, b)))
return (out[0] if (shp == ()) else out)
def _truncnorm_logcdf_scalar(x, a, b):
with np.errstate(invalid='ignore'):
if np.isscalar(x):
if x <= a:
return -np.inf
if x >= b:
return 0
shp = np.shape(x)
x = np.atleast_1d(x)
out = np.full_like(x, np.nan, dtype=np.double)
condlea, condgeb = (x <= a), (x >= b)
if np.any(condlea):
np.place(out, condlea, -np.inf)
if np.any(condgeb):
np.place(out, condgeb, 0.0)
cond_inner = ~condlea & ~condgeb
if np.any(cond_inner):
delta = _truncnorm_get_delta_scalar(a, b)
if delta > 0:
np.place(out, cond_inner,
np.log((_norm_cdf(x[cond_inner]) - _norm_cdf(a)) / delta))
else:
with np.errstate(divide='ignore'):
if a < 0:
nla, nlb = _norm_logcdf(a), _norm_logcdf(b)
tab = np.log1p(-np.exp(nla - nlb))
nlx = _norm_logcdf(x[cond_inner])
tax = np.log1p(-np.exp(nla - nlx))
np.place(out, cond_inner, nlx + tax - (nlb + tab))
else:
sla = _norm_logsf(a)
slb = _norm_logsf(b)
np.place(out, cond_inner,
np.log1p(-np.exp(_norm_logsf(x[cond_inner]) - sla))
- np.log1p(-np.exp(slb - sla)))
return (out[0] if (shp == ()) else out)
def _truncnorm_cdf_scalar(x, a, b):
with np.errstate(invalid='ignore'):
if np.isscalar(x):
if x <= a:
return -0
if x >= b:
return 1
shp = np.shape(x)
x = np.atleast_1d(x)
out = np.full_like(x, np.nan, dtype=np.double)
condlea, condgeb = (x <= a), (x >= b)
if np.any(condlea):
np.place(out, condlea, 0)
if np.any(condgeb):
np.place(out, condgeb, 1.0)
cond_inner = ~condlea & ~condgeb
if np.any(cond_inner):
delta = _truncnorm_get_delta_scalar(a, b)
if delta > 0:
np.place(out, cond_inner,
(_norm_cdf(x[cond_inner]) - _norm_cdf(a)) / delta)
else:
with np.errstate(divide='ignore'):
np.place(out, cond_inner,
np.exp(_truncnorm_logcdf_scalar(x[cond_inner], a, b)))
return (out[0] if (shp == ()) else out)
def _truncnorm_logsf_scalar(x, a, b):
with np.errstate(invalid='ignore'):
if np.isscalar(x):
if x <= a:
return 0.0
if x >= b:
return -np.inf
shp = np.shape(x)
x = np.atleast_1d(x)
out = np.full_like(x, np.nan, dtype=np.double)
condlea, condgeb = (x <= a), (x >= b)
if np.any(condlea):
np.place(out, condlea, 0)
if np.any(condgeb):
np.place(out, condgeb, -np.inf)
cond_inner = ~condlea & ~condgeb
if np.any(cond_inner):
delta = _truncnorm_get_delta_scalar(a, b)
if delta > 0:
np.place(out, cond_inner, np.log((_norm_sf(x[cond_inner]) - _norm_sf(b)) / delta))
else:
with np.errstate(divide='ignore'):
if b < 0:
nla, nlb = _norm_logcdf(a), _norm_logcdf(b)
np.place(out, cond_inner,
np.log1p(-np.exp(_norm_logcdf(x[cond_inner]) - nlb))
- np.log1p(-np.exp(nla - nlb)))
else:
sla, slb = _norm_logsf(a), _norm_logsf(b)
tab = np.log1p(-np.exp(slb - sla))
slx = _norm_logsf(x[cond_inner])
tax = np.log1p(-np.exp(slb - slx))
np.place(out, cond_inner, slx + tax - (sla + tab))
return (out[0] if (shp == ()) else out)
def _truncnorm_sf_scalar(x, a, b):
with np.errstate(invalid='ignore'):
if np.isscalar(x):
if x <= a:
return 1.0
if x >= b:
return 0.0
shp = np.shape(x)
x = np.atleast_1d(x)
out = np.full_like(x, np.nan, dtype=np.double)
condlea, condgeb = (x <= a), (x >= b)
if np.any(condlea):
np.place(out, condlea, 1.0)
if np.any(condgeb):
np.place(out, condgeb, 0.0)
cond_inner = ~condlea & ~condgeb
if np.any(cond_inner):
delta = _truncnorm_get_delta_scalar(a, b)
if delta > 0:
np.place(out, cond_inner, (_norm_sf(x[cond_inner]) - _norm_sf(b)) / delta)
else:
np.place(out, cond_inner, np.exp(_truncnorm_logsf_scalar(x[cond_inner], a, b)))
return (out[0] if (shp == ()) else out)
def _norm_logcdfprime(z):
# derivative of special.log_ndtr (See special/cephes/ndtr.c)
# Differentiate formula for log Phi(z)_truncnorm_ppf
# log Phi(z) = -z^2/2 - log(-z) - log(2pi)/2 + log(1 + sum (-1)^n (2n-1)!! / z^(2n))
# Convergence of series is slow for |z| < 10, but can use d(log Phi(z))/dz = dPhi(z)/dz / Phi(z)
# Just take the first 10 terms because that is sufficient for use in _norm_ilogcdf
assert np.all(z <= -10)
lhs = -z - 1/z
denom_cons = 1/z**2
numerator = 1
pwr = 1.0
denom_total, numerator_total = 0, 0
sign = -1
for i in range(1, 11):
pwr *= denom_cons
numerator *= 2 * i - 1
term = sign * numerator * pwr
denom_total += term
numerator_total += term * (2 * i) / z
sign = -sign
return lhs - numerator_total / (1 + denom_total)
def _norm_ilogcdf(y):
"""Inverse function to _norm_logcdf==sc.log_ndtr."""
# Apply approximate Newton-Raphson
# Only use for very negative values of y.
# At minimum requires y <= -(log(2pi)+2^2)/2 ~= -2.9
# Much better convergence for y <= -10
z = -np.sqrt(-2 * (y + np.log(2*np.pi)/2))
for _ in range(4):
z = z - (_norm_logcdf(z) - y) / _norm_logcdfprime(z)
return z
def _truncnorm_ppf_scalar(q, a, b):
shp = np.shape(q)
q = np.atleast_1d(q)
out = np.zeros(np.shape(q))
condle0, condge1 = (q <= 0), (q >= 1)
if np.any(condle0):
out[condle0] = a
if np.any(condge1):
out[condge1] = b
delta = _truncnorm_get_delta_scalar(a, b)
cond_inner = ~condle0 & ~condge1
if np.any(cond_inner):
qinner = q[cond_inner]
if delta > 0:
if a > 0:
sa, sb = _norm_sf(a), _norm_sf(b)
np.place(out, cond_inner,
_norm_isf(qinner * sb + sa * (1.0 - qinner)))
else:
na, nb = _norm_cdf(a), _norm_cdf(b)
np.place(out, cond_inner, _norm_ppf(qinner * nb + na * (1.0 - qinner)))
elif np.isinf(b):
np.place(out, cond_inner,
-_norm_ilogcdf(np.log1p(-qinner) + _norm_logsf(a)))
elif np.isinf(a):
np.place(out, cond_inner,
_norm_ilogcdf(np.log(q) + _norm_logcdf(b)))
else:
if b < 0:
# Solve norm_logcdf(x) = norm_logcdf(a) + log1p(q * (expm1(norm_logcdf(b) - norm_logcdf(a)))
# = nla + log1p(q * expm1(nlb - nla))
# = nlb + log(q) + log1p((1-q) * exp(nla - nlb)/q)
def _f_cdf(x, c):
return _norm_logcdf(x) - c
nla, nlb = _norm_logcdf(a), _norm_logcdf(b)
values = nlb + np.log(q[cond_inner])
C = np.exp(nla - nlb)
if C:
one_minus_q = (1 - q)[cond_inner]
values += np.log1p(one_minus_q * C / q[cond_inner])
x = [optimize.zeros.brentq(_f_cdf, a, b, args=(c,),
maxiter=TRUNCNORM_MAX_BRENT_ITERS)for c in values]
np.place(out, cond_inner, x)
else:
# Solve norm_logsf(x) = norm_logsf(b) + log1p((1-q) * (expm1(norm_logsf(a) - norm_logsf(b)))
# = slb + log1p((1-q)[cond_inner] * expm1(sla - slb))
# = sla + log(1-q) + log1p(q * np.exp(slb - sla)/(1-q))
def _f_sf(x, c):
return _norm_logsf(x) - c
sla, slb = _norm_logsf(a), _norm_logsf(b)
one_minus_q = (1-q)[cond_inner]
values = sla + np.log(one_minus_q)
C = np.exp(slb - sla)
if C:
values += np.log1p(q[cond_inner] * C / one_minus_q)
x = [optimize.zeros.brentq(_f_sf, a, b, args=(c,),
maxiter=TRUNCNORM_MAX_BRENT_ITERS) for c in values]
np.place(out, cond_inner, x)
out[out < a] = a
out[out > b] = b
return (out[0] if (shp == ()) else out)
class truncnorm_gen(rv_continuous):
r"""A truncated normal continuous random variable.
%(before_notes)s
Notes
-----
The standard form of this distribution is a standard normal truncated to
the range [a, b] --- notice that a and b are defined over the domain of the
standard normal. To convert clip values for a specific mean and standard
deviation, use::
a, b = (myclip_a - my_mean) / my_std, (myclip_b - my_mean) / my_std
`truncnorm` takes :math:`a` and :math:`b` as shape parameters.
%(after_notes)s
%(example)s
"""
def _argcheck(self, a, b):
return a < b
def _get_support(self, a, b):
return a, b
def _pdf(self, x, a, b):
if np.isscalar(a) and np.isscalar(b):
return _truncnorm_pdf_scalar(x, a, b)
a, b = np.atleast_1d(a), np.atleast_1d(b)
if a.size == 1 and b.size == 1:
return _truncnorm_pdf_scalar(x, a.item(), b.item())
it = np.nditer([x, a, b, None], [],
[['readonly'], ['readonly'], ['readonly'], ['writeonly','allocate']])
for (_x, _a, _b, _ld) in it:
_ld[...] = _truncnorm_pdf_scalar(_x, _a, _b)
return it.operands[3]
def _logpdf(self, x, a, b):
if np.isscalar(a) and np.isscalar(b):
return _truncnorm_logpdf_scalar(x, a, b)
a, b = np.atleast_1d(a), np.atleast_1d(b)
if a.size == 1 and b.size == 1:
return _truncnorm_logpdf_scalar(x, a.item(), b.item())
it = np.nditer([x, a, b, None], [],
[['readonly'], ['readonly'], ['readonly'], ['writeonly','allocate']])
for (_x, _a, _b, _ld) in it:
_ld[...] = _truncnorm_logpdf_scalar(_x, _a, _b)
return it.operands[3]
def _cdf(self, x, a, b):
if np.isscalar(a) and np.isscalar(b):
return _truncnorm_cdf_scalar(x, a, b)
a, b = np.atleast_1d(a), np.atleast_1d(b)
if a.size == 1 and b.size == 1:
return _truncnorm_cdf_scalar(x, a.item(), b.item())
out = None
it = np.nditer([x, a, b, out], [],
[['readonly'], ['readonly'], ['readonly'], ['writeonly', 'allocate']])
for (_x, _a, _b, _p) in it:
_p[...] = _truncnorm_cdf_scalar(_x, _a, _b)
return it.operands[3]
def _logcdf(self, x, a, b):
if np.isscalar(a) and np.isscalar(b):
return _truncnorm_logcdf_scalar(x, a, b)
a, b = np.atleast_1d(a), np.atleast_1d(b)
if a.size == 1 and b.size == 1:
return _truncnorm_logcdf_scalar(x, a.item(), b.item())
it = np.nditer([x, a, b, None], [],
[['readonly'], ['readonly'], ['readonly'], ['writeonly', 'allocate']])
for (_x, _a, _b, _p) in it:
_p[...] = _truncnorm_logcdf_scalar(_x, _a, _b)
return it.operands[3]
def _sf(self, x, a, b):
if np.isscalar(a) and np.isscalar(b):
return _truncnorm_sf_scalar(x, a, b)
a, b = np.atleast_1d(a), np.atleast_1d(b)
if a.size == 1 and b.size == 1:
return _truncnorm_sf_scalar(x, a.item(), b.item())
out = None
it = np.nditer([x, a, b, out], [],
[['readonly'], ['readonly'], ['readonly'], ['writeonly', 'allocate']])
for (_x, _a, _b, _p) in it:
_p[...] = _truncnorm_sf_scalar(_x, _a, _b)
return it.operands[3]
def _logsf(self, x, a, b):
if np.isscalar(a) and np.isscalar(b):
return _truncnorm_logsf_scalar(x, a, b)
a, b = np.atleast_1d(a), np.atleast_1d(b)
if a.size == 1 and b.size == 1:
return _truncnorm_logsf_scalar(x, a.item(), b.item())
out = None
it = np.nditer([x, a, b, out], [],
[['readonly'], ['readonly'], ['readonly'], ['writeonly', 'allocate']])
for (_x, _a, _b, _p) in it:
_p[...] = _truncnorm_logsf_scalar(_x, _a, _b)
return it.operands[3]
def _ppf(self, q, a, b):
if np.isscalar(a) and np.isscalar(b):
return _truncnorm_ppf_scalar(q, a, b)
a, b = np.atleast_1d(a), np.atleast_1d(b)
if a.size == 1 and b.size == 1:
return _truncnorm_ppf_scalar(q, a.item(), b.item())
out = None
it = np.nditer([q, a, b, out], [],
[['readonly'], ['readonly'], ['readonly'], ['writeonly', 'allocate']])
for (_q, _a, _b, _x) in it:
_x[...] = _truncnorm_ppf_scalar(_q, _a, _b)
return it.operands[3]
def _munp(self, n, a, b):
def n_th_moment(n, a, b):
"""
Returns n-th moment. Defined only if n >= 0.
Function cannot broadcast due to the loop over n
"""
pA, pB = self._pdf([a, b], a, b)
probs = [pA, -pB]
moments = [0, 1]
for k in range(1, n+1):
# a or b might be infinite, and the corresponding pdf value
# is 0 in that case, but nan is returned for the
# multiplication. However, as b->infinity, pdf(b)*b**k -> 0.
# So it is safe to use _lazywhere to avoid the nan.
vals = _lazywhere(probs, [probs, [a, b]],
lambda x, y: x * y**(k-1), fillvalue=0)
mk = np.sum(vals) + (k-1) * moments[-2]
moments.append(mk)
return moments[-1]
return _lazywhere((n >= 0) & (a == a) & (b == b), (n, a, b),
np.vectorize(n_th_moment, otypes=[np.float64]),
np.nan)
def _stats(self, a, b, moments='mv'):
pA, pB = self._pdf(np.array([a, b]), a, b)
m1 = pA - pB
mu = m1
# use _lazywhere to avoid nan (See detailed comment in _munp)
probs = [pA, -pB]
vals = _lazywhere(probs, [probs, [a, b]], lambda x, y: x*y,
fillvalue=0)
m2 = 1 + np.sum(vals)
vals = _lazywhere(probs, [probs, [a-mu, b-mu]], lambda x, y: x*y,
fillvalue=0)
# mu2 = m2 - mu**2, but not as numerically stable as:
# mu2 = (a-mu)*pA - (b-mu)*pB + 1
mu2 = 1 + np.sum(vals)
vals = _lazywhere(probs, [probs, [a, b]], lambda x, y: x*y**2,
fillvalue=0)
m3 = 2*m1 + np.sum(vals)
vals = _lazywhere(probs, [probs, [a, b]], lambda x, y: x*y**3,
fillvalue=0)
m4 = 3*m2 + np.sum(vals)
mu3 = m3 + m1 * (-3*m2 + 2*m1**2)
g1 = mu3 / np.power(mu2, 1.5)
mu4 = m4 + m1*(-4*m3 + 3*m1*(2*m2 - m1**2))
g2 = mu4 / mu2**2 - 3
return mu, mu2, g1, g2
def _rvs(self, a, b, size=None, random_state=None):
# if a and b are scalar, use _rvs_scalar, otherwise need to create
# output by iterating over parameters
if np.isscalar(a) and np.isscalar(b):
out = self._rvs_scalar(a, b, size, random_state=random_state)
elif a.size == 1 and b.size == 1:
out = self._rvs_scalar(a.item(), b.item(), size, random_state=random_state)
else:
# When this method is called, size will be a (possibly empty)
# tuple of integers. It will not be None; if `size=None` is passed
# to `rvs()`, size will be the empty tuple ().
a, b = np.broadcast_arrays(a, b)
# a and b now have the same shape.
# `shp` is the shape of the blocks of random variates that are
# generated for each combination of parameters associated with
# broadcasting a and b.
# bc is a tuple the same length as size. The values
# in bc are bools. If bc[j] is True, it means that
# entire axis is filled in for a given combination of the
# broadcast arguments.
shp, bc = _check_shape(a.shape, size)
# `numsamples` is the total number of variates to be generated
# for each combination of the input arguments.
numsamples = int(np.prod(shp))
# `out` is the array to be returned. It is filled in in the
# loop below.
out = np.empty(size)
it = np.nditer([a, b],
flags=['multi_index'],
op_flags=[['readonly'], ['readonly']])
while not it.finished:
# Convert the iterator's multi_index into an index into the
# `out` array where the call to _rvs_scalar() will be stored.
# Where bc is True, we use a full slice; otherwise we use the
# index value from it.multi_index. len(it.multi_index) might
# be less than len(bc), and in that case we want to align these
# two sequences to the right, so the loop variable j runs from
# -len(size) to 0. This doesn't cause an IndexError, as
# bc[j] will be True in those cases where it.multi_index[j]
# would cause an IndexError.
idx = tuple((it.multi_index[j] if not bc[j] else slice(None))
for j in range(-len(size), 0))
out[idx] = self._rvs_scalar(it[0], it[1], numsamples, random_state).reshape(shp)
it.iternext()
if size == ():
out = out.item()
return out
def _rvs_scalar(self, a, b, numsamples=None, random_state=None):
if not numsamples:
numsamples = 1
# prepare sampling of rvs
size1d = tuple(np.atleast_1d(numsamples))
N = np.prod(size1d) # number of rvs needed, reshape upon return
# Calculate some rvs
U = random_state.uniform(low=0, high=1, size=N)
x = self._ppf(U, a, b)
rvs = np.reshape(x, size1d)
return rvs
truncnorm = truncnorm_gen(name='truncnorm', momtype=1)
# FIXME: RVS does not work.
class tukeylambda_gen(rv_continuous):
r"""A Tukey-Lamdba continuous random variable.
%(before_notes)s
Notes
-----
A flexible distribution, able to represent and interpolate between the
following distributions:
- Cauchy (:math:`lambda = -1`)
- logistic (:math:`lambda = 0`)
- approx Normal (:math:`lambda = 0.14`)
- uniform from -1 to 1 (:math:`lambda = 1`)
`tukeylambda` takes a real number :math:`lambda` (denoted ``lam``
in the implementation) as a shape parameter.
%(after_notes)s
%(example)s
"""
def _argcheck(self, lam):
return np.ones(np.shape(lam), dtype=bool)
def _pdf(self, x, lam):
Fx = np.asarray(sc.tklmbda(x, lam))
Px = Fx**(lam-1.0) + (np.asarray(1-Fx))**(lam-1.0)
Px = 1.0/np.asarray(Px)
return np.where((lam <= 0) | (abs(x) < 1.0/np.asarray(lam)), Px, 0.0)
def _cdf(self, x, lam):
return sc.tklmbda(x, lam)
def _ppf(self, q, lam):
return sc.boxcox(q, lam) - sc.boxcox1p(-q, lam)
def _stats(self, lam):
return 0, _tlvar(lam), 0, _tlkurt(lam)
def _entropy(self, lam):
def integ(p):
return np.log(pow(p, lam-1)+pow(1-p, lam-1))
return integrate.quad(integ, 0, 1)[0]
tukeylambda = tukeylambda_gen(name='tukeylambda')
class FitUniformFixedScaleDataError(FitDataError):
def __init__(self, ptp, fscale):
self.args = (
"Invalid values in `data`. Maximum likelihood estimation with "
"the uniform distribution and fixed scale requires that "
"data.ptp() <= fscale, but data.ptp() = %r and fscale = %r." %
(ptp, fscale),
)
class uniform_gen(rv_continuous):
r"""A uniform continuous random variable.
In the standard form, the distribution is uniform on ``[0, 1]``. Using
the parameters ``loc`` and ``scale``, one obtains the uniform distribution
on ``[loc, loc + scale]``.
%(before_notes)s
%(example)s
"""
def _rvs(self, size=None, random_state=None):
return random_state.uniform(0.0, 1.0, size)
def _pdf(self, x):
return 1.0*(x == x)
def _cdf(self, x):
return x
def _ppf(self, q):
return q
def _stats(self):
return 0.5, 1.0/12, 0, -1.2
def _entropy(self):
return 0.0
def fit(self, data, *args, **kwds):
"""
Maximum likelihood estimate for the location and scale parameters.
`uniform.fit` uses only the following parameters. Because exact
formulas are used, the parameters related to optimization that are
available in the `fit` method of other distributions are ignored
here. The only positional argument accepted is `data`.
Parameters
----------
data : array_like
Data to use in calculating the maximum likelihood estimate.
floc : float, optional
Hold the location parameter fixed to the specified value.
fscale : float, optional
Hold the scale parameter fixed to the specified value.
Returns
-------
loc, scale : float
Maximum likelihood estimates for the location and scale.
Notes
-----
An error is raised if `floc` is given and any values in `data` are
less than `floc`, or if `fscale` is given and `fscale` is less
than ``data.max() - data.min()``. An error is also raised if both
`floc` and `fscale` are given.
Examples
--------
>>> from scipy.stats import uniform
We'll fit the uniform distribution to `x`:
>>> x = np.array([2, 2.5, 3.1, 9.5, 13.0])
For a uniform distribution MLE, the location is the minimum of the
data, and the scale is the maximum minus the minimum.
>>> loc, scale = uniform.fit(x)
>>> loc
2.0
>>> scale
11.0
If we know the data comes from a uniform distribution where the support
starts at 0, we can use `floc=0`:
>>> loc, scale = uniform.fit(x, floc=0)
>>> loc
0.0
>>> scale
13.0
Alternatively, if we know the length of the support is 12, we can use
`fscale=12`:
>>> loc, scale = uniform.fit(x, fscale=12)
>>> loc
1.5
>>> scale
12.0
In that last example, the support interval is [1.5, 13.5]. This
solution is not unique. For example, the distribution with ``loc=2``
and ``scale=12`` has the same likelihood as the one above. When
`fscale` is given and it is larger than ``data.max() - data.min()``,
the parameters returned by the `fit` method center the support over
the interval ``[data.min(), data.max()]``.
"""
if len(args) > 0:
raise TypeError("Too many arguments.")
floc = kwds.pop('floc', None)
fscale = kwds.pop('fscale', None)
_remove_optimizer_parameters(kwds)
if floc is not None and fscale is not None:
# This check is for consistency with `rv_continuous.fit`.
raise ValueError("All parameters fixed. There is nothing to "
"optimize.")
data = np.asarray(data)
if not np.isfinite(data).all():
raise RuntimeError("The data contains non-finite values.")
# MLE for the uniform distribution
# --------------------------------
# The PDF is
#
# f(x, loc, scale) = {1/scale for loc <= x <= loc + scale
# {0 otherwise}
#
# The likelihood function is
# L(x, loc, scale) = (1/scale)**n
# where n is len(x), assuming loc <= x <= loc + scale for all x.
# The log-likelihood is
# l(x, loc, scale) = -n*log(scale)
# The log-likelihood is maximized by making scale as small as possible,
# while keeping loc <= x <= loc + scale. So if neither loc nor scale
# are fixed, the log-likelihood is maximized by choosing
# loc = x.min()
# scale = x.ptp()
# If loc is fixed, it must be less than or equal to x.min(), and then
# the scale is
# scale = x.max() - loc
# If scale is fixed, it must not be less than x.ptp(). If scale is
# greater than x.ptp(), the solution is not unique. Note that the
# likelihood does not depend on loc, except for the requirement that
# loc <= x <= loc + scale. All choices of loc for which
# x.max() - scale <= loc <= x.min()
# have the same log-likelihood. In this case, we choose loc such that
# the support is centered over the interval [data.min(), data.max()]:
# loc = x.min() = 0.5*(scale - x.ptp())
if fscale is None:
# scale is not fixed.
if floc is None:
# loc is not fixed, scale is not fixed.
loc = data.min()
scale = data.ptp()
else:
# loc is fixed, scale is not fixed.
loc = floc
scale = data.max() - loc
if data.min() < loc:
raise FitDataError("uniform", lower=loc, upper=loc + scale)
else:
# loc is not fixed, scale is fixed.
ptp = data.ptp()
if ptp > fscale:
raise FitUniformFixedScaleDataError(ptp=ptp, fscale=fscale)
# If ptp < fscale, the ML estimate is not unique; see the comments
# above. We choose the distribution for which the support is
# centered over the interval [data.min(), data.max()].
loc = data.min() - 0.5*(fscale - ptp)
scale = fscale
# We expect the return values to be floating point, so ensure it
# by explicitly converting to float.
return float(loc), float(scale)
uniform = uniform_gen(a=0.0, b=1.0, name='uniform')
class vonmises_gen(rv_continuous):
r"""A Von Mises continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `vonmises` and `vonmises_line` is:
.. math::
f(x, \kappa) = \frac{ \exp(\kappa \cos(x)) }{ 2 \pi I_0(\kappa) }
for :math:`-\pi \le x \le \pi`, :math:`\kappa > 0`. :math:`I_0` is the
modified Bessel function of order zero (`scipy.special.i0`).
`vonmises` is a circular distribution which does not restrict the
distribution to a fixed interval. Currently, there is no circular
distribution framework in scipy. The ``cdf`` is implemented such that
``cdf(x + 2*np.pi) == cdf(x) + 1``.
`vonmises_line` is the same distribution, defined on :math:`[-\pi, \pi]`
on the real line. This is a regular (i.e. non-circular) distribution.
`vonmises` and `vonmises_line` take ``kappa`` as a shape parameter.
%(after_notes)s
%(example)s
"""
def _rvs(self, kappa, size=None, random_state=None):
return random_state.vonmises(0.0, kappa, size=size)
def _pdf(self, x, kappa):
# vonmises.pdf(x, \kappa) = exp(\kappa * cos(x)) / (2*pi*I[0](\kappa))
return np.exp(kappa * np.cos(x)) / (2*np.pi*sc.i0(kappa))
def _cdf(self, x, kappa):
return _stats.von_mises_cdf(kappa, x)
def _stats_skip(self, kappa):
return 0, None, 0, None
def _entropy(self, kappa):
return (-kappa * sc.i1(kappa) / sc.i0(kappa) +
np.log(2 * np.pi * sc.i0(kappa)))
vonmises = vonmises_gen(name='vonmises')
vonmises_line = vonmises_gen(a=-np.pi, b=np.pi, name='vonmises_line')
class wald_gen(invgauss_gen):
r"""A Wald continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `wald` is:
.. math::
f(x) = \frac{1}{\sqrt{2\pi x^3}} \exp(- \frac{ (x-1)^2 }{ 2x })
for :math:`x >= 0`.
`wald` is a special case of `invgauss` with ``mu=1``.
%(after_notes)s
%(example)s
"""
_support_mask = rv_continuous._open_support_mask
def _rvs(self, size=None, random_state=None):
return random_state.wald(1.0, 1.0, size=size)
def _pdf(self, x):
# wald.pdf(x) = 1/sqrt(2*pi*x**3) * exp(-(x-1)**2/(2*x))
return invgauss._pdf(x, 1.0)
def _logpdf(self, x):
return invgauss._logpdf(x, 1.0)
def _cdf(self, x):
return invgauss._cdf(x, 1.0)
def _stats(self):
return 1.0, 1.0, 3.0, 15.0
wald = wald_gen(a=0.0, name="wald")
class wrapcauchy_gen(rv_continuous):
r"""A wrapped Cauchy continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `wrapcauchy` is:
.. math::
f(x, c) = \frac{1-c^2}{2\pi (1+c^2 - 2c \cos(x))}
for :math:`0 \le x \le 2\pi`, :math:`0 < c < 1`.
`wrapcauchy` takes ``c`` as a shape parameter for :math:`c`.
%(after_notes)s
%(example)s
"""
def _argcheck(self, c):
return (c > 0) & (c < 1)
def _pdf(self, x, c):
# wrapcauchy.pdf(x, c) = (1-c**2) / (2*pi*(1+c**2-2*c*cos(x)))
return (1.0-c*c)/(2*np.pi*(1+c*c-2*c*np.cos(x)))
def _cdf(self, x, c):
output = np.zeros(x.shape, dtype=x.dtype)
val = (1.0+c)/(1.0-c)
c1 = x < np.pi
c2 = 1-c1
xp = np.extract(c1, x)
xn = np.extract(c2, x)
if np.any(xn):
valn = np.extract(c2, np.ones_like(x)*val)
xn = 2*np.pi - xn
yn = np.tan(xn/2.0)
on = 1.0-1.0/np.pi*np.arctan(valn*yn)
np.place(output, c2, on)
if np.any(xp):
valp = np.extract(c1, np.ones_like(x)*val)
yp = np.tan(xp/2.0)
op = 1.0/np.pi*np.arctan(valp*yp)
np.place(output, c1, op)
return output
def _ppf(self, q, c):
val = (1.0-c)/(1.0+c)
rcq = 2*np.arctan(val*np.tan(np.pi*q))
rcmq = 2*np.pi-2*np.arctan(val*np.tan(np.pi*(1-q)))
return np.where(q < 1.0/2, rcq, rcmq)
def _entropy(self, c):
return np.log(2*np.pi*(1-c*c))
wrapcauchy = wrapcauchy_gen(a=0.0, b=2*np.pi, name='wrapcauchy')
class gennorm_gen(rv_continuous):
r"""A generalized normal continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `gennorm` is [1]_:
.. math::
f(x, \beta) = \frac{\beta}{2 \Gamma(1/\beta)} \exp(-|x|^\beta)
:math:`\Gamma` is the gamma function (`scipy.special.gamma`).
`gennorm` takes ``beta`` as a shape parameter for :math:`\beta`.
For :math:`\beta = 1`, it is identical to a Laplace distribution.
For :math:`\beta = 2`, it is identical to a normal distribution
(with ``scale=1/sqrt(2)``).
See Also
--------
laplace : Laplace distribution
norm : normal distribution
References
----------
.. [1] "Generalized normal distribution, Version 1",
https://en.wikipedia.org/wiki/Generalized_normal_distribution#Version_1
%(example)s
"""
def _pdf(self, x, beta):
return np.exp(self._logpdf(x, beta))
def _logpdf(self, x, beta):
return np.log(0.5*beta) - sc.gammaln(1.0/beta) - abs(x)**beta
def _cdf(self, x, beta):
c = 0.5 * np.sign(x)
# evaluating (.5 + c) first prevents numerical cancellation
return (0.5 + c) - c * sc.gammaincc(1.0/beta, abs(x)**beta)
def _ppf(self, x, beta):
c = np.sign(x - 0.5)
# evaluating (1. + c) first prevents numerical cancellation
return c * sc.gammainccinv(1.0/beta, (1.0 + c) - 2.0*c*x)**(1.0/beta)
def _sf(self, x, beta):
return self._cdf(-x, beta)
def _isf(self, x, beta):
return -self._ppf(x, beta)
def _stats(self, beta):
c1, c3, c5 = sc.gammaln([1.0/beta, 3.0/beta, 5.0/beta])
return 0., np.exp(c3 - c1), 0., np.exp(c5 + c1 - 2.0*c3) - 3.
def _entropy(self, beta):
return 1. / beta - np.log(.5 * beta) + sc.gammaln(1. / beta)
gennorm = gennorm_gen(name='gennorm')
class halfgennorm_gen(rv_continuous):
r"""The upper half of a generalized normal continuous random variable.
%(before_notes)s
Notes
-----
The probability density function for `halfgennorm` is:
.. math::
f(x, \beta) = \frac{\beta}{\Gamma(1/\beta)} \exp(-|x|^\beta)
for :math:`x > 0`. :math:`\Gamma` is the gamma function
(`scipy.special.gamma`).
`gennorm` takes ``beta`` as a shape parameter for :math:`\beta`.
For :math:`\beta = 1`, it is identical to an exponential distribution.
For :math:`\beta = 2`, it is identical to a half normal distribution
(with ``scale=1/sqrt(2)``).
See Also
--------
gennorm : generalized normal distribution
expon : exponential distribution
halfnorm : half normal distribution
References
----------
.. [1] "Generalized normal distribution, Version 1",
https://en.wikipedia.org/wiki/Generalized_normal_distribution#Version_1
%(example)s
"""
def _pdf(self, x, beta):
# beta
# halfgennorm.pdf(x, beta) = ------------- exp(-|x|**beta)
# gamma(1/beta)
return np.exp(self._logpdf(x, beta))
def _logpdf(self, x, beta):
return np.log(beta) - sc.gammaln(1.0/beta) - x**beta
def _cdf(self, x, beta):
return sc.gammainc(1.0/beta, x**beta)
def _ppf(self, x, beta):
return sc.gammaincinv(1.0/beta, x)**(1.0/beta)
def _sf(self, x, beta):
return sc.gammaincc(1.0/beta, x**beta)
def _isf(self, x, beta):
return sc.gammainccinv(1.0/beta, x)**(1.0/beta)
def _entropy(self, beta):
return 1.0/beta - np.log(beta) + sc.gammaln(1.0/beta)
halfgennorm = halfgennorm_gen(a=0, name='halfgennorm')
class crystalball_gen(rv_continuous):
r"""
Crystalball distribution
%(before_notes)s
Notes
-----
The probability density function for `crystalball` is:
.. math::
f(x, \beta, m) = \begin{cases}
N \exp(-x^2 / 2), &\text{for } x > -\beta\\
N A (B - x)^{-m} &\text{for } x \le -\beta
\end{cases}
where :math:`A = (m / |\beta|)^n \exp(-\beta^2 / 2)`,
:math:`B = m/|\beta| - |\beta|` and :math:`N` is a normalisation constant.
`crystalball` takes :math:`\beta > 0` and :math:`m > 1` as shape
parameters. :math:`\beta` defines the point where the pdf changes
from a power-law to a Gaussian distribution. :math:`m` is the power
of the power-law tail.
References
----------
.. [1] "Crystal Ball Function",
https://en.wikipedia.org/wiki/Crystal_Ball_function
%(after_notes)s
.. versionadded:: 0.19.0
%(example)s
"""
def _pdf(self, x, beta, m):
"""
Return PDF of the crystalball function.
--
| exp(-x**2 / 2), for x > -beta
crystalball.pdf(x, beta, m) = N * |
| A * (B - x)**(-m), for x <= -beta
--
"""
N = 1.0 / (m/beta / (m-1) * np.exp(-beta**2 / 2.0) +
_norm_pdf_C * _norm_cdf(beta))
def rhs(x, beta, m):
return np.exp(-x**2 / 2)
def lhs(x, beta, m):
return ((m/beta)**m * np.exp(-beta**2 / 2.0) *
(m/beta - beta - x)**(-m))
return N * _lazywhere(x > -beta, (x, beta, m), f=rhs, f2=lhs)
def _logpdf(self, x, beta, m):
"""
Return the log of the PDF of the crystalball function.
"""
N = 1.0 / (m/beta / (m-1) * np.exp(-beta**2 / 2.0) +
_norm_pdf_C * _norm_cdf(beta))
def rhs(x, beta, m):
return -x**2/2
def lhs(x, beta, m):
return m*np.log(m/beta) - beta**2/2 - m*np.log(m/beta - beta - x)
return np.log(N) + _lazywhere(x > -beta, (x, beta, m), f=rhs, f2=lhs)
def _cdf(self, x, beta, m):
"""
Return CDF of the crystalball function
"""
N = 1.0 / (m/beta / (m-1) * np.exp(-beta**2 / 2.0) +
_norm_pdf_C * _norm_cdf(beta))
def rhs(x, beta, m):
return ((m/beta) * np.exp(-beta**2 / 2.0) / (m-1) +
_norm_pdf_C * (_norm_cdf(x) - _norm_cdf(-beta)))
def lhs(x, beta, m):
return ((m/beta)**m * np.exp(-beta**2 / 2.0) *
(m/beta - beta - x)**(-m+1) / (m-1))
return N * _lazywhere(x > -beta, (x, beta, m), f=rhs, f2=lhs)
def _ppf(self, p, beta, m):
N = 1.0 / (m/beta / (m-1) * np.exp(-beta**2 / 2.0) +
_norm_pdf_C * _norm_cdf(beta))
pbeta = N * (m/beta) * np.exp(-beta**2/2) / (m - 1)
def ppf_less(p, beta, m):
eb2 = np.exp(-beta**2/2)
C = (m/beta) * eb2 / (m-1)
N = 1/(C + _norm_pdf_C * _norm_cdf(beta))
return (m/beta - beta -
((m - 1)*(m/beta)**(-m)/eb2*p/N)**(1/(1-m)))
def ppf_greater(p, beta, m):
eb2 = np.exp(-beta**2/2)
C = (m/beta) * eb2 / (m-1)
N = 1/(C + _norm_pdf_C * _norm_cdf(beta))
return _norm_ppf(_norm_cdf(-beta) + (1/_norm_pdf_C)*(p/N - C))
return _lazywhere(p < pbeta, (p, beta, m), f=ppf_less, f2=ppf_greater)
def _munp(self, n, beta, m):
"""
Returns the n-th non-central moment of the crystalball function.
"""
N = 1.0 / (m/beta / (m-1) * np.exp(-beta**2 / 2.0) +
_norm_pdf_C * _norm_cdf(beta))
def n_th_moment(n, beta, m):
"""
Returns n-th moment. Defined only if n+1 < m
Function cannot broadcast due to the loop over n
"""
A = (m/beta)**m * np.exp(-beta**2 / 2.0)
B = m/beta - beta
rhs = (2**((n-1)/2.0) * sc.gamma((n+1)/2) *
(1.0 + (-1)**n * sc.gammainc((n+1)/2, beta**2 / 2)))
lhs = np.zeros(rhs.shape)
for k in range(n + 1):
lhs += (sc.binom(n, k) * B**(n-k) * (-1)**k / (m - k - 1) *
(m/beta)**(-m + k + 1))
return A * lhs + rhs
return N * _lazywhere(n + 1 < m, (n, beta, m),
np.vectorize(n_th_moment, otypes=[np.float64]),
np.inf)
def _argcheck(self, beta, m):
"""
Shape parameter bounds are m > 1 and beta > 0.
"""
return (m > 1) & (beta > 0)
crystalball = crystalball_gen(name='crystalball', longname="A Crystalball Function")
def _argus_phi(chi):
"""
Utility function for the argus distribution
used in the CDF and norm of the Argus Funktion
"""
return _norm_cdf(chi) - chi * _norm_pdf(chi) - 0.5
class argus_gen(rv_continuous):
r"""
Argus distribution
%(before_notes)s
Notes
-----
The probability density function for `argus` is:
.. math::
f(x, \chi) = \frac{\chi^3}{\sqrt{2\pi} \Psi(\chi)} x \sqrt{1-x^2}
\exp(-\chi^2 (1 - x^2)/2)
for :math:`0 < x < 1` and :math:`\chi > 0`, where
.. math::
\Psi(\chi) = \Phi(\chi) - \chi \phi(\chi) - 1/2
with :math:`\Phi` and :math:`\phi` being the CDF and PDF of a standard
normal distribution, respectively.
`argus` takes :math:`\chi` as shape a parameter.
References
----------
.. [1] "ARGUS distribution",
https://en.wikipedia.org/wiki/ARGUS_distribution
%(after_notes)s
.. versionadded:: 0.19.0
%(example)s
"""
def _pdf(self, x, chi):
y = 1.0 - x**2
A = chi**3 / (_norm_pdf_C * _argus_phi(chi))
return A * x * np.sqrt(y) * np.exp(-chi**2 * y / 2)
def _cdf(self, x, chi):
return 1.0 - self._sf(x, chi)
def _sf(self, x, chi):
return _argus_phi(chi * np.sqrt(1 - x**2)) / _argus_phi(chi)
def _rvs(self, chi, size=None, random_state=None):
chi = np.asarray(chi)
if chi.size == 1:
out = self._rvs_scalar(chi, numsamples=size,
random_state=random_state)
else:
shp, bc = _check_shape(chi.shape, size)
numsamples = int(np.prod(shp))
out = np.empty(size)
it = np.nditer([chi],
flags=['multi_index'],
op_flags=[['readonly']])
while not it.finished:
idx = tuple((it.multi_index[j] if not bc[j] else slice(None))
for j in range(-len(size), 0))
r = self._rvs_scalar(it[0], numsamples=numsamples,
random_state=random_state)
out[idx] = r.reshape(shp)
it.iternext()
if size == ():
out = out[()]
return out
def _rvs_scalar(self, chi, numsamples=None, random_state=None):
# if chi <= 2.611:
# use rejection method, see Devroye:
# Non-Uniform Random Variate Generation, 1986, section II.3.2.
# write: self.pdf = c * g(x) * h(x), where
# h is [0,1]-valued and g is a density
# g(x) = d1 * chi**2 * x * exp(-chi**2 * (1 - x**2) / 2), 0 <= x <= 1
# h(x) = sqrt(1 - x**2), 0 <= x <= 1
# Integrating g, we get:
# G(x) = d1 * exp(-chi**2 * (1 - x**2) / 2) - d2
# d1 and d2 are determined by G(0) = 0 and G(1) = 1
# d1 = 1 / (1 - exp(-0.5 * chi**2))
# d2 = 1 / (exp(0.5 * chi**2) - 1)
# => G(x) = (exp(chi**2 * x**2 /2) - 1) / (exp(chi**2 / 2) - 1)
# expected number of iterations is c with
# c = -np.expm1(-0.5 * chi**2) * chi / (_norm_pdf_C * _argus_phi(chi))
# note that G can be inverted easily, so we can sample
# rvs from this distribution
# G_inv(y) = sqrt(2 * log(1 + (exp(chi**2 / 2) - 1) * y) / chi**2)
# to avoid an overflow of exp(chi**2 / 2), it is convenient to write
# G_inv(y) = sqrt(1 + 2 * log(exp(-chi**2 / 2) * (1-y) + y) / chi**2)
#
# if chi > 2.611:
# use ratio of uniforms method applied to a transformed variable of X
# (X is ARGUS with parameter chi):
# Y = chi * sqrt(1 - X**2) has density proportional to
# u**2 * exp(-u**2 / 2) on [0, chi] (Maxwell distribution conditioned
# on [0, chi]). Apply ratio of uniforms to this density to generate
# samples of Y and convert back to X
#
# The expected number of iterations using the rejection method
# increases with increasing chi, whereas the expected number of
# iterations using the ratio of uniforms method decreases with
# increasing chi. The crossover occurs where
# chi*(1 - exp(-0.5*chi**2)) = 8*sqrt(2)*exp(-1.5) => chi ~ 2.611
# Switching algorithms at chi=2.611 means that the expected number of
# iterations is always below 2.2.
if chi <= 2.611:
# use rejection method
size1d = tuple(np.atleast_1d(numsamples))
N = int(np.prod(size1d))
x = np.zeros(N)
echi = np.exp(-chi**2 / 2)
simulated = 0
while simulated < N:
k = N - simulated
u = random_state.uniform(size=k)
v = random_state.uniform(size=k)
# acceptance condition: u <= h(G_inv(v)). This simplifies to
z = 2 * np.log(echi * (1 - v) + v) / chi**2
accept = (u**2 + z <= 0)
num_accept = np.sum(accept)
if num_accept > 0:
# rvs follow a distribution with density g: rvs = G_inv(v)
rvs = np.sqrt(1 + z[accept])
x[simulated:(simulated + num_accept)] = rvs
simulated += num_accept
return np.reshape(x, size1d)
else:
# use ratio of uniforms method
def f(x):
return np.where((x >= 0) & (x <= chi),
np.exp(2*np.log(x) - x**2/2), 0)
umax = np.sqrt(2) / np.exp(0.5)
vmax = 4 / np.exp(1)
z = rvs_ratio_uniforms(f, umax, 0, vmax, size=numsamples,
random_state=random_state)
return np.sqrt(1 - z*z / chi**2)
def _stats(self, chi):
chi2 = chi**2
phi = _argus_phi(chi)
m = np.sqrt(np.pi/8) * chi * sc.ive(1, chi2/4) / phi
v = (1 - 3 / chi2 + chi * _norm_pdf(chi) / phi) - m**2
return m, v, None, None
argus = argus_gen(name='argus', longname="An Argus Function", a=0.0, b=1.0)
class rv_histogram(rv_continuous):
"""
Generates a distribution given by a histogram.
This is useful to generate a template distribution from a binned
datasample.
As a subclass of the `rv_continuous` class, `rv_histogram` inherits from it
a collection of generic methods (see `rv_continuous` for the full list),
and implements them based on the properties of the provided binned
datasample.
Parameters
----------
histogram : tuple of array_like
Tuple containing two array_like objects
The first containing the content of n bins
The second containing the (n+1) bin boundaries
In particular the return value np.histogram is accepted
Notes
-----
There are no additional shape parameters except for the loc and scale.
The pdf is defined as a stepwise function from the provided histogram
The cdf is a linear interpolation of the pdf.
.. versionadded:: 0.19.0
Examples
--------
Create a scipy.stats distribution from a numpy histogram
>>> import scipy.stats
>>> import numpy as np
>>> data = scipy.stats.norm.rvs(size=100000, loc=0, scale=1.5, random_state=123)
>>> hist = np.histogram(data, bins=100)
>>> hist_dist = scipy.stats.rv_histogram(hist)
Behaves like an ordinary scipy rv_continuous distribution
>>> hist_dist.pdf(1.0)
0.20538577847618705
>>> hist_dist.cdf(2.0)
0.90818568543056499
PDF is zero above (below) the highest (lowest) bin of the histogram,
defined by the max (min) of the original dataset
>>> hist_dist.pdf(np.max(data))
0.0
>>> hist_dist.cdf(np.max(data))
1.0
>>> hist_dist.pdf(np.min(data))
7.7591907244498314e-05
>>> hist_dist.cdf(np.min(data))
0.0
PDF and CDF follow the histogram
>>> import matplotlib.pyplot as plt
>>> X = np.linspace(-5.0, 5.0, 100)
>>> plt.title("PDF from Template")
>>> plt.hist(data, density=True, bins=100)
>>> plt.plot(X, hist_dist.pdf(X), label='PDF')
>>> plt.plot(X, hist_dist.cdf(X), label='CDF')
>>> plt.show()
"""
_support_mask = rv_continuous._support_mask
def __init__(self, histogram, *args, **kwargs):
"""
Create a new distribution using the given histogram
Parameters
----------
histogram : tuple of array_like
Tuple containing two array_like objects
The first containing the content of n bins
The second containing the (n+1) bin boundaries
In particular the return value np.histogram is accepted
"""
self._histogram = histogram
if len(histogram) != 2:
raise ValueError("Expected length 2 for parameter histogram")
self._hpdf = np.asarray(histogram[0])
self._hbins = np.asarray(histogram[1])
if len(self._hpdf) + 1 != len(self._hbins):
raise ValueError("Number of elements in histogram content "
"and histogram boundaries do not match, "
"expected n and n+1.")
self._hbin_widths = self._hbins[1:] - self._hbins[:-1]
self._hpdf = self._hpdf / float(np.sum(self._hpdf * self._hbin_widths))
self._hcdf = np.cumsum(self._hpdf * self._hbin_widths)
self._hpdf = np.hstack([0.0, self._hpdf, 0.0])
self._hcdf = np.hstack([0.0, self._hcdf])
# Set support
kwargs['a'] = self.a = self._hbins[0]
kwargs['b'] = self.b = self._hbins[-1]
super(rv_histogram, self).__init__(*args, **kwargs)
def _pdf(self, x):
"""
PDF of the histogram
"""
return self._hpdf[np.searchsorted(self._hbins, x, side='right')]
def _cdf(self, x):
"""
CDF calculated from the histogram
"""
return np.interp(x, self._hbins, self._hcdf)
def _ppf(self, x):
"""
Percentile function calculated from the histogram
"""
return np.interp(x, self._hcdf, self._hbins)
def _munp(self, n):
"""Compute the n-th non-central moment."""
integrals = (self._hbins[1:]**(n+1) - self._hbins[:-1]**(n+1)) / (n+1)
return np.sum(self._hpdf[1:-1] * integrals)
def _entropy(self):
"""Compute entropy of distribution"""
res = _lazywhere(self._hpdf[1:-1] > 0.0,
(self._hpdf[1:-1],),
np.log,
0.0)
return -np.sum(self._hpdf[1:-1] * res * self._hbin_widths)
def _updated_ctor_param(self):
"""
Set the histogram as additional constructor argument
"""
dct = super(rv_histogram, self)._updated_ctor_param()
dct['histogram'] = self._histogram
return dct
# Collect names of classes and objects in this module.
pairs = list(globals().copy().items())
_distn_names, _distn_gen_names = get_distribution_names(pairs, rv_continuous)
__all__ = _distn_names + _distn_gen_names + ['rv_histogram']