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std::expone...distribution(3)  C++ Standard Libary std::expone...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, dis-
     tributed
	according to probability density function:

	\(\small P(x|\lambda) = \lambda e^{-\lambda x}\)P(x|I>>) = I>>e-I>>x

	The value obtained is the time/distance until the next random  event  if
     random events
	occur  at  constant rate \(\small\lambda\)I>> 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 un-
     defined 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 RandomNumberDis-
     tribution.

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  A  Wol-
     fram Web
	Resource.

http://cppreference.com 	   2024.06.10	   std::expone...distribution(3)
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