26 Numerics library [numerics]

26.6 Random number generation [rand]

26.6.8 Random number distribution class templates [rand.dist]

26.6.8.4 Poisson distributions [rand.dist.pois]

26.6.8.4.3 Class template gamma_distribution [rand.dist.pois.gamma]

1

A gamma_distribution random number distribution produces random numbers x > 0 distributed according to the probability density function\[% p(x\,|\,\alpha,\beta) = \frac{e^{-x/\beta}}{\beta^{\alpha} \cdot \Gamma(\alpha)} \, \cdot \, x^{\, \alpha-1} \; \mbox{.} \]

template<class RealType = double>
 class gamma_distribution{
public:
 // types
 using result_type = RealType;
 using param_type  = unspecified;

 // constructors and reset functions
 explicit gamma_distribution(RealType alpha = 1.0, RealType beta = 1.0);
 explicit gamma_distribution(const param_type& parm);
 void reset();

 // generating functions
 template<class URBG>
   result_type operator()(URBG& g);
 template<class URBG>
   result_type operator()(URBG& g, const param_type& parm);

 // property functions
 RealType alpha() const;
 RealType beta() const;
 param_type param() const;
 void param(const param_type& parm);
 result_type min() const;
 result_type max() const;
};

explicit gamma_distribution(RealType alpha = 1.0, RealType beta = 1.0);

2

Requires: 0 < alpha and 0 < beta .

3

Effects: Constructs a gamma_distribution object; alpha and beta correspond to the parameters of the distribution.

RealType alpha() const;

4

Returns: The value of the alpha parameter with which the object was constructed.

RealType beta() const;

5

Returns: The value of the beta parameter with which the object was constructed.