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| Member functions | ||||
| Generation | ||||
| Characteristics | ||||
| Non-member functions | ||||
(C++11)(C++11)(until C++20) | ||||
(C++11)(C++11) |
Defined in header <random> | ||
template<class RealType=double> class gamma_distribution; | (since C++11) | |
Produces random positive floating-point valuesx, distributed according to probability density function:
| e-x/β |
| βα · Γ(α) |
whereα is known as theshape parameter andβ is known as thescale parameter. The shape parameter is sometimes denoted by the letterk and the scale parameter is sometimes denoted by the letterθ.
For floating-pointα, the value obtained is the sum ofα independent exponentially distributed random variables, each of which has a mean ofβ.
std::gamma_distribution satisfiesRandomNumberDistribution.
Contents |
| RealType | - | The result type generated by the generator. The effect is undefined if this is not one offloat,double, orlongdouble. |
| Member type | Definition |
result_type(C++11) | RealType |
param_type(C++11) | the type of the parameter set, seeRandomNumberDistribution. |
(C++11) | constructs new distribution (public member function)[edit] |
(C++11) | resets the internal state of the distribution (public member function)[edit] |
Generation | |
(C++11) | generates the next random number in the distribution (public member function)[edit] |
Characteristics | |
(C++11) | 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] |
(C++11)(C++11)(removed in C++20) | compares two distribution objects (function)[edit] |
(C++11) | performs stream input and output on pseudo-random number distribution (function template)[edit] |
#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(autoconst&[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 *6.5-7.07.0-7.57.5-8.0
| Weisstein, Eric W. "Gamma Distribution." From MathWorld — A Wolfram Web Resource. |
