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136 lines
4.9 KiB
TeX
136 lines
4.9 KiB
TeX
\section{\module{random} ---
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Generate pseudo-random numbers}
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\declaremodule{standard}{random}
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\modulesynopsis{Generate pseudo-random numbers with various common
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distributions.}
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This module implements pseudo-random number generators for various
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distributions: on the real line, there are functions to compute normal
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or Gaussian, lognormal, negative exponential, gamma, and beta
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distributions. For generating distribution of angles, the circular
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uniform and von Mises distributions are available.
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The \module{random} module supports the \emph{Random Number
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Generator} interface, described in section \ref{rng-objects}. This
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interface of the module, as well as the distribution-specific
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functions described below, all use the pseudo-random generator
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provided by the \refmodule{whrandom} module.
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The following functions are defined to support specific distributions,
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and all return real values. Function parameters are named after the
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corresponding variables in the distribution's equation, as used in
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common mathematical practice; most of these equations can be found in
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any statistics text. These are expected to become part of the Random
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Number Generator interface in a future release.
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\begin{funcdesc}{betavariate}{alpha, beta}
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Beta distribution. Conditions on the parameters are
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\code{\var{alpha} > -1} and \code{\var{beta} > -1}.
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Returned values range between 0 and 1.
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\end{funcdesc}
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\begin{funcdesc}{cunifvariate}{mean, arc}
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Circular uniform distribution. \var{mean} is the mean angle, and
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\var{arc} is the range of the distribution, centered around the mean
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angle. Both values must be expressed in radians, and can range
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between 0 and \emph{pi}. Returned values will range between
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\code{\var{mean} - \var{arc}/2} and \code{\var{mean} + \var{arc}/2}.
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\end{funcdesc}
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\begin{funcdesc}{expovariate}{lambd}
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Exponential distribution. \var{lambd} is 1.0 divided by the desired
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mean. (The parameter would be called ``lambda'', but that is a
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reserved word in Python.) Returned values will range from 0 to
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positive infinity.
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\end{funcdesc}
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\begin{funcdesc}{gamma}{alpha, beta}
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Gamma distribution. (\emph{Not} the gamma function!) Conditions on
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the parameters are \code{\var{alpha} > -1} and \code{\var{beta} > 0}.
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\end{funcdesc}
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\begin{funcdesc}{gauss}{mu, sigma}
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Gaussian distribution. \var{mu} is the mean, and \var{sigma} is the
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standard deviation. This is slightly faster than the
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\function{normalvariate()} function defined below.
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\end{funcdesc}
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\begin{funcdesc}{lognormvariate}{mu, sigma}
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Log normal distribution. If you take the natural logarithm of this
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distribution, you'll get a normal distribution with mean \var{mu} and
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standard deviation \var{sigma}. \var{mu} can have any value, and
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\var{sigma} must be greater than zero.
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\end{funcdesc}
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\begin{funcdesc}{normalvariate}{mu, sigma}
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Normal distribution. \var{mu} is the mean, and \var{sigma} is the
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standard deviation.
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\end{funcdesc}
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\begin{funcdesc}{vonmisesvariate}{mu, kappa}
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\var{mu} is the mean angle, expressed in radians between 0 and 2*\emph{pi},
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and \var{kappa} is the concentration parameter, which must be greater
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than or equal to zero. If \var{kappa} is equal to zero, this
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distribution reduces to a uniform random angle over the range 0 to
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2*\emph{pi}.
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\end{funcdesc}
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\begin{funcdesc}{paretovariate}{alpha}
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Pareto distribution. \var{alpha} is the shape parameter.
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\end{funcdesc}
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\begin{funcdesc}{weibullvariate}{alpha, beta}
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Weibull distribution. \var{alpha} is the scale parameter and
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\var{beta} is the shape parameter.
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\end{funcdesc}
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\begin{seealso}
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\seemodule{whrandom}{The standard Python random number generator.}
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\end{seealso}
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\subsection{The Random Number Generator Interface
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\label{rng-objects}}
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% XXX This *must* be updated before a future release!
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The \dfn{Random Number Generator} interface describes the methods
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which are available for all random number generators. This will be
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enhanced in future releases of Python.
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In this release of Python, the modules \refmodule{random},
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\refmodule{whrandom}, and instances of the
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\class{whrandom.whrandom} class all conform to this interface.
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\begin{funcdesc}{choice}{seq}
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Chooses a random element from the non-empty sequence \var{seq} and
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returns it.
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\end{funcdesc}
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\begin{funcdesc}{randint}{a, b}
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\deprecated{2.0}{Use \function{randrange()} instead.}
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Returns a random integer \var{N} such that
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\code{\var{a} <= \var{N} <= \var{b}}.
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\end{funcdesc}
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\begin{funcdesc}{random}{}
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Returns the next random floating point number in the range [0.0
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... 1.0).
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\end{funcdesc}
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\begin{funcdesc}{randrange}{\optional{start,} stop\optional{, step}}
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Return a randomly selected element from \code{range(\var{start},
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\var{stop}, \var{step})}. This is equivalent to
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\code{choice(range(\var{start}, \var{stop}, \var{step}))}.
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\versionadded{1.5.2}
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\end{funcdesc}
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\begin{funcdesc}{uniform}{a, b}
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Returns a random real number \var{N} such that
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\code{\var{a} <= \var{N} < \var{b}}.
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\end{funcdesc}
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