NAME¶

std::geometric_distribution - std::geometric_distribution

Synopsis¶

Defined in header <random>
template< class IntType = int > (since C++11)
class geometric_distribution;

Produces random non-negative integer values i, distributed according to discrete
probability function:

\(P(i|p) = p \cdot (1-p)^i\)P(i|p) = p · (1 − p)i

The value represents the number of failures in a series of independent yes/no trials
(each succeeds with probability p), before exactly 1 success occurs.

std::geometric_distribution<>(p) is exactly equivalent to
std::negative_binomial_distribution<>(1, p). It is also the discrete counterpart of
std::exponential_distribution.

std::geometric_distribution satisfies RandomNumberDistribution.

Template parameters¶

The result type generated by the generator. The effect is undefined if
IntType - this is not one of short, int, long, long long, unsigned short, unsigned
int, unsigned long, or unsigned long long.

Member types¶

Member type Definition
result_type (C++11) IntType
param_type (C++11) the type of the parameter set, see RandomNumberDistribution.

Member functions¶

constructor constructs new distribution
(C++11) (public member function)
reset resets the internal state of the distribution
(C++11) (public member function)

Generation¶

operator() generates the next random number in the distribution
(C++11) (public member function)

Characteristics¶

p returns the p distribution parameter (probability of a trial
(C++11) generating true)
(public member function)
param gets or sets the distribution parameter object
(C++11) (public member function)
min returns the minimum potentially generated value
(C++11) (public member function)
max returns the maximum potentially generated value
(C++11) (public member function)

Non-member functions¶

operator==
operator!= compares two distribution objects
(C++11) (function)
(C++11)(removed in C++20)
operator<< performs stream input and output on pseudo-random number
operator>> distribution
(C++11) (function template)

Example¶

std::geometric_distribution<>(0.5) is the default and represents the number of coin
tosses that are required to get heads.

// Run this code

#include <iomanip>
#include <iostream>
#include <map>
#include <random>
#include <string>

int main()
{
std::random_device rd;
std::mt19937 gen(rd());

std::geometric_distribution<> d;
// same as
// std::negative_binomial_distribution<> d(1, 0.5):

std::map<int, int> hist;
for (int n = 0; n != 10000; ++n)
++hist[d(gen)];

for (auto [x, y] : hist)
{
const char c = x < 10 ? x + '0' : x - 10 + 'a';
std::cout << c << ' ' << std::string(y / 100, '*') << '\n';
}
}

Possible output:¶

0 *************************************************
1 *************************
2 ************
3 ******
4 **
5 *
6
7
8
9

External links¶

Weisstein, Eric W. "Geometric Distribution." From MathWorld — A Wolfram Web
Resource.