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poisson_distribution Class

Generates a Poisson distribution.

template<class IntType = int>
class poisson_distribution
{
public:
    // types
    typedef IntType result_type;
    struct 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& gen);
    template<class URNG>
    result_type operator()(URNG& gen, 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;
};

IntType

The integer result type, defaults to int. For possible types, see <random>.

The template class describes a distribution that produces values of a user-specified integral type with a Poisson distribution. The following table links to articles about individual members.

poisson_distribution::poisson_distribution

poisson_distribution::mean

poisson_distribution::param

poisson_distribution::operator()

poisson_distribution::param_type

The property function mean() returns the value for stored distribution parameter mean.

For more information about distribution classes and their members, see <random>.

For detailed information about the Poisson distribution, see the Wolfram MathWorld article Poisson Distribution.

 

// compile with: /EHsc /W4
#include <random> 
#include <iostream>
#include <iomanip>
#include <string>
#include <map>

void test(const double p, const int s) {

    // uncomment to use a non-deterministic generator
    //    std::random_device gen;
    std::mt19937 gen(1701);

    std::poisson_distribution<> distr(p);

    std::cout << std::endl;
    std::cout << "min() == " << distr.min() << std::endl;
    std::cout << "max() == " << distr.max() << std::endl;
    std::cout << "p() == " << std::fixed << std::setw(11) << std::setprecision(10) << distr.mean() << std::endl;

    // generate the distribution as a histogram
    std::map<int, int> histogram;
    for (int i = 0; i < s; ++i) {
        ++histogram[distr(gen)];
    }

    // print results
    std::cout << "Distribution for " << s << " samples:" << std::endl;
    for (const auto& elem : histogram) {
        std::cout << std::setw(5) << elem.first << ' ' << std::string(elem.second, ':') << std::endl;
    }
    std::cout << std::endl;
}

int main()
{
    double p_dist = 1.0;

    int samples = 100;

    std::cout << "Use CTRL-Z to bypass data entry and run using default values." << std::endl;
    std::cout << "Enter a floating point value for the 'mean' distribution parameter (must be greater than zero): ";
    std::cin >> p_dist;
    std::cout << "Enter an integer value for the sample count: ";
    std::cin >> samples;

    test(p_dist, samples);
}

First test:

Use CTRL-Z to bypass data entry and run using default values.
Enter a floating point value for the 'mean' distribution parameter (must be greater than zero): 1
Enter an integer value for the sample count: 100

min() == 0
max() == 2147483647
p() == 1.0000000000
Distribution for 100 samples:
    0 ::::::::::::::::::::::::::::::
    1 ::::::::::::::::::::::::::::::::::::::
    2 :::::::::::::::::::::::
    3 ::::::::
    5 :

Second test:

Use CTRL-Z to bypass data entry and run using default values.
Enter a floating point value for the 'mean' distribution parameter (must be greater than zero): 10
Enter an integer value for the sample count: 100

min() == 0
max() == 2147483647
p() == 10.0000000000
Distribution for 100 samples:
    3 :
    4 ::
    5 ::
    6 ::::::::
    7 ::::
    8 ::::::::
    9 ::::::::::::::
   10 ::::::::::::
   11 ::::::::::::::::
   12 :::::::::::::::
   13 ::::::::
   14 ::::::
   15 :
   16 ::
   17 :

Header: <random>

Namespace: std

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