26 Numerics library [numerics]

26.5 Random number generation [rand]

26.5.8 Random number distribution class templates [rand.dist] Poisson distributions [rand.dist.pois] Class template poisson_distribution [rand.dist.pois.poisson]

A poisson_distribution random number distribution produces integer values i ≥ 0 distributed according to the discrete probability function \[%
      = \frac{ e^{-\mu} \mu^{i} }
             { i\,! }
\; \mbox{.}
\] The distribution parameter μ is also known as this distribution's mean.

template<class IntType = int>
 class poisson_distribution{
 // types
 typedef IntType result_type;
 typedef unspecified param_type;

 // constructors and reset functions
 explicit poisson_distribution(double mean = 1.0);
 explicit poisson_distribution(const param_type& parm);
 void reset();

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

 // property functions
 double mean() const;
 param_type param() const;
 void param(const param_type& parm);
 result_type min() const;
 result_type max() const;

explicit poisson_distribution(double mean = 1.0);

Requires: 0 < mean .

Effects: Constructs a poisson_distribution object; mean corresponds to the parameter of the distribution.

double mean() const;

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