NAME¶

std::exponential_distribution - std::exponential_distribution

Synopsis¶

Defined in header <random>
template< class RealType = double > (since C++11)
class exponential_distribution;

Produces random non-negative floating-point values \(\small x\)x, distributed
according to probability density function:

\(\small P(x|\lambda) = \lambda e^{-\lambda x}\)P(x|λ) = λe-λx

The value obtained is the time/distance until the next random event if random events
occur at constant rate \(\small\lambda\)λ per unit of time/distance. For example,
this distribution describes the time between the clicks of a Geiger counter or the
distance between point mutations in a DNA strand.

This is the continuous counterpart of std::geometric_distribution.

std::exponential_distribution satisfies RandomNumberDistribution.

Template parameters¶

RealType - The result type generated by the generator. The effect is undefined if
this is not one of float, double, or long double.

Member types¶

Member type Definition
result_type (C++11) RealType
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¶

lambda returns the lambda distribution parameter (rate of events)
(C++11) (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)

Notes¶

Some implementations may occasionally return infinity if RealType is float. This is
LWG issue 2524.

Example¶

// Run this code

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

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

// if particles decay once per second on average,
// how much time, in seconds, until the next one?
std::exponential_distribution<> d(1);

std::map<int, int> hist;
for (int n = 0; n != 10000; ++n)
++hist[2 * d(gen)];

for (auto const& [x, y] : hist)
std::cout << std::fixed << std::setprecision(1)
<< x / 2.0 << '-' << (x + 1) / 2.0 << ' '
<< std::string(y / 200, '*') << '\n';
}

Possible output:¶

0.0-0.5 *******************
0.5-1.0 ***********
1.0-1.5 *******
1.5-2.0 ****
2.0-2.5 **
2.5-3.0 *
3.0-3.5
3.5-4.0

External links¶

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