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std::gamma_distribution

From cppreference.com
< cpp‎ | numeric‎ | random
 
 
 
Pseudo-random number generation
Uniform random bit generators
Engines and engine adaptors
Non-deterministic generator
Distributions
Uniform distributions
Bernoulli distributions
Poisson distributions
gamma_distribution
(C++11)
Normal distributions
Sampling distributions
Seed Sequences
(C++11)
C library
 
std::gamma_distribution
 
Defined in header <random>
template< class RealType = double >
class gamma_distribution;
(since C++11)

Produces random positive floating-point values x, distributed according to probability density function:

P(x|α,β) =
e-x/β
βα
· Γ(α)
· xα-1

where α is known as the shape parameter and β is known as the scale parameter. The shape parameter is sometimes denoted by the letter k and the scale parameter is sometimes denoted by the letter θ.

For floating-point α, the value obtained is the sum of α independent exponentially distributed random variables, each of which has a mean of β.

std::gamma_distribution satisfies RandomNumberDistribution.

Contents

[edit] Template parameters

RealType - The result type generated by the generator. The effect is undefined if this is not one of float, double, or long double.

[edit] Member types

Member type Definition
result_type RealType
param_type(C++11) the type of the parameter set, see RandomNumberDistribution.

[edit] Member functions

constructs new distribution
(public member function) [edit]
(C++11)
resets the internal state of the distribution
(public member function) [edit]
Generation
generates the next random number in the distribution
(public member function) [edit]
Characteristics
returns the distribution parameters
(public member function) [edit]
(C++11)
gets or sets the distribution parameter object
(public member function) [edit]
(C++11)
returns the minimum potentially generated value
(public member function) [edit]
(C++11)
returns the maximum potentially generated value
(public member function) [edit]

[edit] Non-member functions

(C++11)(C++11)(removed in C++20)
compares two distribution objects
(function) [edit]
performs stream input and output on pseudo-random number distribution
(function template) [edit]

[edit] Example

#include <iostream>
#include <iomanip>
#include <string>
#include <map>
#include <random>
int main()
{
    std::random_device rd;
    std::mt19937 gen(rd());
 
    // A gamma distribution with alpha=1, and beta=2
    // approximates an exponential distribution.
    std::gamma_distribution<> d(1,2);
 
    std::map<int, int> hist;
    for(int n=0; n<10000; ++n) {
        ++hist[2*d(gen)];
    }
    for(auto p : hist) {
        if (p.second/100. > 0.5)
            std::cout
                << std::fixed << std::setprecision(1) 
                << p.first/2.0 << '-' << (p.first+1)/2.0 << ' '
                << std::string(p.second/100, '*') << '\n';
    }
}

Possible output:

0.0-0.5 **********************
0.5-1.0 ****************
1.0-1.5 *************
1.5-2.0 **********
2.0-2.5 ********
2.5-3.0 ******
3.0-3.5 *****
3.5-4.0 ****
4.0-4.5 ***
4.5-5.0 **
5.0-5.5 **
5.5-6.0 *
6.0-6.5 *
6.5-7.0
7.0-7.5
7.5-8.0

[edit] External links

Weisstein, Eric W. "Gamma Distribution." From MathWorld--A Wolfram Web Resource.