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# Probability Density Function

> [Exponential][exponential-distribution] distribution probability density function (PDF).

<section class="intro">

The [probability density function][pdf] (PDF) for an [exponential][exponential-distribution] random variable is

<!-- <equation class="equation" label="eq:exponential_pdf" align="center" raw="f(x;\lambda) = \begin{cases} \lambda e^{-\lambda x} & x \ge 0 \\ 0 & x < 0 \end{cases}" alt="Probability density function (PDF) for a Exponential distribution."> -->

<div class="equation" align="center" data-raw-text="f(x;\lambda) = \begin{cases} \lambda e^{-\lambda x} &amp; x \ge 0 \\ 0 &amp; x &lt; 0 \end{cases}" data-equation="eq:exponential_pdf">
    <img src="https://cdn.rawgit.com/stdlib-js/stdlib/7e0a95722efd9c771b129597380c63dc6715508b/lib/node_modules/@stdlib/stats/base/dists/exponential/pdf/docs/img/equation_exponential_pdf.svg" alt="Probability density function (PDF) for a Exponential distribution.">
    <br>
</div>

<!-- </equation> -->

where `λ` is the rate parameter.

</section>

<!-- /.intro -->

<section class="usage">

## Usage

```javascript
var pdf = require( '@stdlib/stats/base/dists/exponential/pdf' );
```

#### pdf( x, lambda )

Evaluates the [probability density function][pdf] (PDF) for an [exponential][exponential-distribution] distribution with rate parameter `lambda`.

```javascript
var y = pdf( 2.0, 0.3 );
// returns ~0.165

y = pdf( 2.0, 1.0 );
// returns ~0.135
```

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

```javascript
var y = pdf( NaN, 0.0 );
// returns NaN

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

If provided `lambda < 0`, the function returns `NaN`.

```javascript
var y = pdf( 2.0, -1.0 );
// returns NaN
```

#### pdf.factory( lambda )

Partially apply `lambda` to create a reusable `function` for evaluating the [PDF][pdf].

```javascript
var mypdf = pdf.factory( 0.1 );

var y = mypdf( 8.0 );
// returns ~0.045

y = mypdf( 5.0 );
// returns ~0.06
```

</section>

<!-- /.usage -->

<section class="examples">

## Examples

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

```javascript
var randu = require( '@stdlib/random/base/randu' );
var pdf = require( '@stdlib/stats/base/dists/exponential/pdf' );

var lambda;
var x;
var y;
var i;

for ( i = 0; i < 10; i++ ) {
    x = randu() * 10.0;
    lambda = randu() * 10.0;
    y = pdf( x, lambda );
    console.log( 'x: %d, λ: %d, f(x;λ): %d', x, lambda, y );
}
```

</section>

<!-- /.examples -->

<section class="links">

[pdf]: https://en.wikipedia.org/wiki/Probability_density_function

[exponential-distribution]: https://en.wikipedia.org/wiki/Exponential_distribution

</section>

<!-- /.links -->
