class template
<random>

std::poisson_distribution

template <class IntType = int> class poisson_distribution;
Poisson distribution
Random number distribution that produces integers according to a Poisson distribution, which is described by the following probability mass function:



This distribution produces random integers where each value represents a specific count of independent events occurring within a fixed interval, based on the observed mean rate at which they appear to happen (μ).

The distribution parameter, mean (μ), is set on construction.

To produce a random value following this distribution, call its member function operator().

Template parameters

IntType
An integer type. Aliased as member type result_type.
By default, this is int.

Member types

The following aliases are member types of poisson_distribution:

member typedefinitionnotes
result_typeThe first template parameter (IntType)The type of the numbers generated (defaults to int)
param_typenot specifiedThe type returned by member param.

Member functions


Distribution parameters


Non-member functions


Example

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
// poisson_distribution
#include <iostream>
#include <random>

int main()
{
  const int nrolls = 10000; // number of experiments
  const int nstars = 100;   // maximum number of stars to distribute

  std::default_random_engine generator;
  std::poisson_distribution<int> distribution(4.1);

  int p[10]={};

  for (int i=0; i<nrolls; ++i) {
    int number = distribution(generator);
    if (number<10) ++p[number];
  }

  std::cout << "poisson_distribution (mean=4.1):" << std::endl;
  for (int i=0; i<10; ++i)
    std::cout << i << ": " << std::string(p[i]*nstars/nrolls,'*') << std::endl;

  return 0;
}

Possible output:

poisson_distribution (mean=4.1):
0: *
1: ******
2: *************
3: *******************
4: *******************
5: ***************
6: ***********
7: ******
8: ***
9: *


See also