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# Quantile Function

> [Binomial][binomial-distribution] distribution [quantile function][quantile-function].

<section class="intro">

The [quantile function][quantile-function] for a [binomial][binomial-distribution] random variable returns, for any `r` satisfying `0 <= r <= 1`, the value `x` for which the relation

<!-- <equation class="equation" label="eq:binomial_quantile_function" align="center" raw="F(x-1;n,p) < r \le F(x;n,p)" alt="Quantile value for a binomial distribution."> -->

<div class="equation" align="center" data-raw-text="F(x-1;n,p) &lt; r \le F(x;n,p)" data-equation="eq:binomial_quantile_function">
    <img src="https://cdn.rawgit.com/stdlib-js/stdlib/7e0a95722efd9c771b129597380c63dc6715508b/lib/node_modules/@stdlib/stats/base/dists/binomial/quantile/docs/img/equation_binomial_quantile_function.svg" alt="Quantile value for a binomial distribution.">
    <br>
</div>

<!-- </equation> -->

holds, where `F` is the cumulative distribution function (CDF) of a binomial random variable, `n` is the number of trials, and `0 <= p <= 1` is the success probability.

</section>

<!-- /.intro -->

<section class="usage">

## Usage

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

#### quantile( r, n, p )

Evaluates the [quantile function][quantile-function] for a [binomial][binomial-distribution] distribution with number of trials `n` and success probability `p` at value `r`.

```javascript
var y = quantile( 0.4, 20, 0.2 );
// returns 3

y = quantile( 0.8, 20, 0.2 );
// returns 5

y = quantile( 0.5, 10, 0.4 );
// returns 4

y = quantile( 0.0, 10, 0.4 );
// returns 0

y = quantile( 1.0, 10, 0.4 );
// returns 10
```

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

```javascript
var y = quantile( NaN, 20, 0.5 );
// returns NaN

y = quantile( 0.2, NaN, 0.5 );
// returns NaN

y = quantile( 0.2, 20, NaN );
// returns NaN
```

If provided a number of trials `n` which is not a nonnegative integer, the function returns `NaN`.

```javascript
var y = quantile( 0.5, 1.5, 0.5 );
// returns NaN

y = quantile( 0.5, -2.0, 0.5 );
// returns NaN
```

If provided a success probability `p` outside of `[0,1]`, the function returns `NaN`.

```javascript
var y = quantile( 0.5, 20, -1.0 );
// returns NaN

y = quantile( 0.5, 20, 1.5 );
// returns NaN
```

#### quantile.factory( n, p )

Returns a function for evaluating the [quantile function][quantile-function] of a [binomial][binomial-distribution] distribution with number of trials `n` and success probability `p`.

```javascript
var myquantile = quantile.factory( 10, 0.5 );

var y = myquantile( 0.1 );
// returns 3

y = myquantile( 0.9 );
// returns 7
```

</section>

<!-- /.usage -->

<section class="examples">

## Examples

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

```javascript
var randu = require( '@stdlib/random/base/randu' );
var round = require( '@stdlib/math/base/special/round' );
var quantile = require( '@stdlib/stats/base/dists/binomial/quantile' );

var r;
var i;
var n;
var p;
var y;

for ( i = 0; i < 10; i++ ) {
    r = randu();
    n = round( randu() * 100.0 );
    p = randu();
    y = quantile( r, n, p );
    console.log( 'r: %d, n: %d, p: %d, Q(r;n,p): %d', r.toFixed( 4 ), n, p.toFixed( 4 ) );
}
```

</section>

<!-- /.examples -->

<section class="links">

[binomial-distribution]: https://en.wikipedia.org/wiki/Binomial_distribution

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

</section>

<!-- /.links -->
