std::exponential_distribution(3)

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)
                 (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 <iostream>
 #include <iomanip>
 #include <random>
 #include <string>
 #include <map>
 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 p : hist) {
         std::cout << std::fixed << std::setprecision(1)
                   << p.first/2.0 << '-' << (p.first+1)/2.0 << ' '
                   << std::string(p.second/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.