[#absl-poisson_distribution]
= xref:absl.adoc[absl]::poisson_distribution
:relfileprefix: ../
:mrdocs:

A distribution that generates discrete Poisson‐distributed variates.

== Synopsis

Declared in `<absl/random/poisson_distribution.h>`

[source,cpp,subs="verbatim,replacements,macros,-callouts"]
----
template<typename IntType = int>
class poisson_distribution;
----

== Description

Generates discrete variates conforming to a Poisson distribution. p(n) = (meanˆn / n!) exp(‐mean)

Depending on the parameter, the distribution selects one of the following algorithms: * The standard algorithm, attributed to Knuth, extended using a split method for larger values * The "Ratio of Uniforms as a convenient method for sampling from classical discrete distributions", Stadlober, 1989. http://www.sciencedirect.com/science/article/pii/0377042790903495

NOTE: param_type.mean() is a double, which permits values larger than poisson_distribution<IntType>::max(), however this should be avoided and the distribution results are limited to the max() value.

The goals of this implementation are to provide good performance while still being thread‐safe: This limits the implementation to not using lgamma provided by <math.h>.

== Types

[cols="1,4"]
|===
| Name| Description
| xref:absl/poisson_distribution/param_type.adoc[`param_type`] 
| The parameter set of the distribution.
|===


== Type Aliases

[cols="1,4"]
|===
| Name| Description
| xref:absl/poisson_distribution/result_type.adoc[`result_type`] 
| The type of the values produced by the distribution.
|===


== Member Functions

[cols="1,4"]
|===
| Name| Description
| xref:absl/poisson_distribution/2constructor-03.adoc[`poisson_distribution`]         [.small]#[constructor]#
| Constructors
| xref:absl/poisson_distribution/max.adoc[`max`] 
| Returns the largest value the distribution can produce.
| xref:absl/poisson_distribution/mean.adoc[`mean`] 
| Returns the mean of the distribution.
| xref:absl/poisson_distribution/min.adoc[`min`] 
| Returns the smallest value the distribution can produce.
| xref:absl/poisson_distribution/operator_call-09.adoc[`operator()`] 
| Function call operators
| xref:absl/poisson_distribution/param-00.adoc[`param`] 
| `param` overloads
| xref:absl/poisson_distribution/reset.adoc[`reset`] 
| Resets the internal state of the distribution.
|===


== Friends

[cols="1,4"]
|===
| Name| Description
| `xref:absl/operator_not_eq-09d.adoc[absl::operator!=]`
| Compares two distributions for inequality.
| `xref:absl/operator_eq-021.adoc[absl::operator==]`
| Compares two distributions for equality.
|===


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