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gmx::GammaDistribution< RealType > Class Template Reference

#include <gromacs/random/gammadistribution.h>

Description

template<class RealType = real>
class gmx::GammaDistribution< RealType >

Gamma distribution.

The C++ standard library does provide a gamma distribution, but when using libstdc++-4.4.7 with at least gcc-4.6 or icc-14.0 the headers produce errors. Even for newer compilers, libstdc++ and libc++ appear to use different algorithms to generate it, which means their values differ in contrast to the uniform and normal distributions where they are identical. To avoid both the gcc-4.6 bug and make it easier to use GROMACS unit tests that depend on random numbers we have our own implementation.

Be warned that the gamma distribution works like the standard normal distribution and keeps drawing values from the random engine in a loop, so you want to make sure you use a random stream with a very large margin to make sure you do not run out of random numbers in an unlucky case (which will lead to an exception with the GROMACS default random engine).

The gamma distribution is defined as

\[ p(x|\alpha,\beta) = \frac{1}{\Gamma(\alpha)\beta^{alpha}} x^{\alpha - 1} e^{-\frac{x}{\beta}}, x\geq 0 \]

Template Parameters
RealTypeFloating-point type, real by default in GROMACS.

Classes

class  param_type
 Gamma distribution parameters. More...
 

Public Types

typedef RealType result_type
 Type of values returned.
 

Public Member Functions

 GammaDistribution (result_type alpha=1.0, result_type beta=1.0)
 Construct new distribution with given floating-point parameters. More...
 
 GammaDistribution (const param_type &param)
 Construct new distribution from parameter class. More...
 
void reset ()
 Flush all internal saved values.
 
template<class Rng >
result_type operator() (Rng &g)
 Return values from gamma distribution with internal parameters. More...
 
template<class Rng >
result_type operator() (Rng &g, const param_type &param)
 Return value from gamma distribution with given parameters. More...
 
result_type alpha () const
 Return the first parameter of gamma distribution.
 
result_type beta () const
 Return the second parameter of gamma distribution.
 
param_type param () const
 Return the full parameter class of gamma distribution.
 
result_type min () const
 Smallest value that can be returned from gamma distribution.
 
result_type max () const
 Largest value that can be returned from gamma distribution.
 
bool operator== (const GammaDistribution &x) const
 True if two gamma distributions will produce the same values. More...
 
bool operator!= (const GammaDistribution &x) const
 True if two gamma distributions will produce different values. More...
 

Constructor & Destructor Documentation

template<class RealType = real>
gmx::GammaDistribution< RealType >::GammaDistribution ( result_type  alpha = 1.0,
result_type  beta = 1.0 
)
inlineexplicit

Construct new distribution with given floating-point parameters.

Parameters
alphaFirst parameter of gamma distribution
betaSecond parameter of gamma distribution
template<class RealType = real>
gmx::GammaDistribution< RealType >::GammaDistribution ( const param_type param)
inlineexplicit

Construct new distribution from parameter class.

Parameters
paramParameter class as defined inside gmx::GammaDistribution.

Member Function Documentation

template<class RealType = real>
bool gmx::GammaDistribution< RealType >::operator!= ( const GammaDistribution< RealType > &  x) const
inline

True if two gamma distributions will produce different values.

Parameters
xInstance to compare with.
template<class RealType = real>
template<class Rng >
result_type gmx::GammaDistribution< RealType >::operator() ( Rng &  g)
inline

Return values from gamma distribution with internal parameters.

Template Parameters
RngRandom engine class
Parameters
gRandom engine
template<class RealType = real>
template<class Rng >
result_type gmx::GammaDistribution< RealType >::operator() ( Rng &  g,
const param_type param 
)
inline

Return value from gamma distribution with given parameters.

Template Parameters
RngRandom engine class
Parameters
gRandom engine
paramParameters to use
template<class RealType = real>
bool gmx::GammaDistribution< RealType >::operator== ( const GammaDistribution< RealType > &  x) const
inline

True if two gamma distributions will produce the same values.

Parameters
xInstance to compare with.

The documentation for this class was generated from the following file: