std::gamma_distribution
From cppreference.com
| 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) =
· xα-1e-x/β βα
· Γ(α)
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.
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.
|
Member types
| Member type | Definition |
result_type (C++11)
|
RealType
|
param_type (C++11)
|
the type of the parameter set, see RandomNumberDistribution. |
Member functions
(C++11) |
constructs new distribution (public member function) |
(C++11) |
resets the internal state of the distribution (public member function) |
Generation | |
(C++11) |
generates the next random number in the distribution (public member function) |
Characteristics | |
(C++11) |
returns the distribution parameters (public member function) |
(C++11) |
gets or sets the distribution parameter object (public member function) |
(C++11) |
returns the minimum potentially generated value (public member function) |
(C++11) |
returns the maximum potentially generated value (public member function) |
Non-member functions
(C++11)(C++11)(removed in C++20) |
compares two distribution objects (function) |
(C++11) |
performs stream input and output on pseudo-random number distribution (function template) |
Example
Run this code
#include <iomanip>
#include <iostream>
#include <map>
#include <random>
#include <string>
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 const& [x, y] : hist)
if (y / 100.0 > 0.5)
std::cout << std::fixed << std::setprecision(1)
<< x / 2.0 << '-' << (x + 1) / 2.0 << ' '
<< std::string(y / 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
External links
| Weisstein, Eric W. "Gamma Distribution." From MathWorld — A Wolfram Web Resource. |
