npm package: @stdlib/stats-base-dists-poisson-quantile
Description: Poisson distribution quantile function.
Author: The Stdlib Authors
Latest version: 0.2.3

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# Quantile Function

[![NPM version][npm-image]][npm-url] [![Build Status][test-image]][test-url] [![Coverage Status][coverage-image]][coverage-url] <!-- [![dependencies][dependencies-image]][dependencies-url] -->

> [Poisson][poisson-distribution] distribution [quantile function][quantile-function].

<section class="intro">

The [quantile function][quantile-function] for a [Poisson][poisson-distribution] random variable returns for `0 <= p <= 1` the smallest nonnegative integer for which

<!-- <equation class="equation" label="eq:poisson_condition" align="center" raw="F(x;\lambda) \ge p" alt="Quantile condition."> -->

<div class="equation" align="center" data-raw-text="F(x;\lambda) \ge p" data-equation="eq:poisson_condition">
    <img src="https://cdn.jsdelivr.net/gh/stdlib-js/stdlib@51534079fef45e990850102147e8945fb023d1d0/lib/node_modules/@stdlib/stats/base/dists/poisson/quantile/docs/img/equation_poisson_condition.svg" alt="Quantile condition.">
    <br>
</div>

<!-- </equation> -->

where `F` is the cumulative distribution function (CDF) of a Poisson distribution with mean parameter `lambda > 0`.

</section>

<!-- /.intro -->

<section class="installation">

## Installation

```bash
npm install @stdlib/stats-base-dists-poisson-quantile
```

</section>

<section class="usage">

## Usage

```javascript
var quantile = require( '@stdlib/stats-base-dists-poisson-quantile' );
```

#### quantile( p, lambda )

Evaluates the [quantile function][quantile-function] for a [Poisson][poisson-distribution] distribution with mean parameter `lambda` at a probability `p`.

```javascript
var y = quantile( 0.5, 2.0 );
// returns 2

y = quantile( 0.9, 4.0 );
// returns 7

y = quantile( 0.1, 200.0 );
// returns 182
```

If provided an input probability `p` outside the interval `[0,1]`, the function returns `NaN`.

```javascript
var y = quantile( 1.9, 0.5 );
// returns NaN

y = quantile( -0.1, 0.5 );
// returns NaN
```

If provided `NaN` as any argument, the function returns `NaN`.

```javascript
var y = quantile( NaN, 1.0 );
// returns NaN

y = quantile( 0.0, NaN );
// returns NaN
```

If provided a negative `lambda`, the function returns `NaN`.

```javascript
var y = quantile( 0.4, -1.0 );
// returns NaN
```

If provided `lambda = 0`, the function evaluates the [quantile function][quantile-function] of a [degenerate distribution][degenerate-distribution] centered at `0.0`.

```javascript
var y = quantile( 0.1, 0.0 );
// returns 0.0

y = quantile( 0.9, 0.0 );
// returns 0.0
```

#### quantile.factory( lambda )

Returns a function for evaluating the [quantile function][quantile-function] of a [Poisson][poisson-distribution] distribution with mean parameter `lambda`.

```javascript
var myquantile = quantile.factory( 5.0 );
var y = myquantile( 0.4 );
// returns 4

y = myquantile( 0.8 );
// returns 7

y = myquantile( 1.0 );
// returns Infinity
```

</section>

<!-- /.usage -->

<section class="examples">

## Examples

<!-- eslint no-undef: "error" -->

```javascript
var uniform = require( '@stdlib/random-array-uniform' );
var logEachMap = require( '@stdlib/console-log-each-map' );
var quantile = require( '@stdlib/stats-base-dists-poisson-quantile' );

var opts = {
    'dtype': 'float64'
};
var p = uniform( 10, 0.0, 1.0, opts );
var lambda = uniform( 10, 0.0, 10.0, opts );

logEachMap( 'p: %0.4f, λ: %0.4f, Q(p;λ): %0.4f', p, lambda, quantile );
```

</section>

<!-- /.examples -->

<!-- Section for related `stdlib` packages. Do not manually edit this section, as it is automatically populated. -->

<section class="related">

</section>

<!-- /.related -->

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<section class="main-repo" >

* * *

## Notice

This package is part of [stdlib][stdlib], a standard library for JavaScript and Node.js, with an emphasis on numerical and scientific computing. The library provides a collection of robust, high performance libraries for mathematics, statistics, streams, utilities, and more.

For more information on the project, filing bug reports and feature requests, and guidance on how to develop [stdlib][stdlib], see the main project [repository][stdlib].

#### Community

[![Chat][chat-image]][chat-url]

---

## License

See [LICENSE][stdlib-license].


## Copyright

Copyright &copy; 2016-2026. The Stdlib [Authors][stdlib-authors].

</section>

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[stdlib-license]: https://raw.githubusercontent.com/stdlib-js/stats-base-dists-poisson-quantile/main/LICENSE

[poisson-distribution]: https://en.wikipedia.org/wiki/Poisson_distribution

[quantile-function]: https://en.wikipedia.org/wiki/Quantile_function

[degenerate-distribution]: https://en.wikipedia.org/wiki/Degenerate_distribution

</section>

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