[rand.dist.pois.exp]

# 29 Numerics library [[numerics]](./#numerics)

## 29.5 Random number generation [[rand]](rand#dist.pois.exp)

### 29.5.9 Random number distribution class templates [[rand.dist]](rand.dist#pois.exp)

#### 29.5.9.4 Poisson distributions [[rand.dist.pois]](rand.dist.pois#exp)

#### 29.5.9.4.2 Class template exponential_distribution [rand.dist.pois.exp]

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/numerics.tex#L5245)

An exponential_distribution random number distribution
produces random numbers x>0 distributed according to
the probability density function in Formula [29.9](#eq:rand.dist.pois.exp)[.](#1.sentence-1)

p(x|Î»)=Î»eâÎ»x(29.9)

[ð](#lib:exponential_distribution_)

namespace std {template<class RealType = double>class exponential_distribution {public:// typesusing result_type = RealType; using param_type = *unspecified*; // constructors and reset functions exponential_distribution() : exponential_distribution(1.0) {}explicit exponential_distribution(RealType lambda); explicit exponential_distribution(const param_type& parm); void reset(); // equality operatorsfriend bool operator==(const exponential_distribution& x, const exponential_distribution& y); // generating functionstemplate<class URBG> result_type operator()(URBG& g); template<class URBG> result_type operator()(URBG& g, const param_type& parm); // property functions RealType lambda() const;
 param_type param() const; void param(const param_type& parm);
 result_type min() const;
 result_type max() const; // inserters and extractorstemplate<class charT, class traits>friend basic_ostream<charT, traits>&operator<<(basic_ostream<charT, traits>& os, const exponential_distribution& x); template<class charT, class traits>friend basic_istream<charT, traits>&operator>>(basic_istream<charT, traits>& is, exponential_distribution& x); };}

[ð](#lib:exponential_distribution,constructor)

`explicit exponential_distribution(RealType lambda);
`

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/numerics.tex#L5305)

*Preconditions*: 0<lambda[.](#2.sentence-1)

[3](#3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/numerics.tex#L5309)

*Remarks*: lambda corresponds to the parameter of the distribution[.](#3.sentence-1)

[ð](#lib:lambda,exponential_distribution)

`RealType lambda() const;
`

[4](#4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/numerics.tex#L5320)

*Returns*: The value of the lambda parameter
 with which the object was constructed[.](#4.sentence-1)
