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# Logarithm of Probability Mass Function

> Evaluate the natural logarithm of the [probability mass function][pmf] (PMF) for a [discrete uniform][discrete-uniform-distribution] distribution.

<section class="intro">

The [probability mass function][pmf] (PMF) for a [discrete uniform][discrete-uniform-distribution] random variable is

<!-- <equation class="equation" label="eq:discrete_uniform_pmf" align="center" raw="P(X=x;a,b)=\begin{cases} \frac{1}{b - a + 1} & \text{for } x \in \{ a, \ldots, b \} \\ 0 & \text{otherwise} \end{cases}" alt="Probability mass function (PMF) for a discrete uniform distribution."> -->

<div class="equation" align="center" data-raw-text="P(X=x;a,b)=\begin{cases} \frac{1}{b - a + 1} &amp; \text{for } x \in \{ a, \ldots, b \} \\ 0 &amp; \text{otherwise} \end{cases}" data-equation="eq:discrete_uniform_pmf">
    <img src="https://cdn.rawgit.com/stdlib-js/stdlib/7e0a95722efd9c771b129597380c63dc6715508b/lib/node_modules/@stdlib/stats/base/dists/discrete-uniform/logpmf/docs/img/equation_discrete_uniform_pmf.svg" alt="Probability mass function (PMF) for a discrete uniform distribution.">
    <br>
</div>

<!-- </equation> -->

where `a` is the minimum support and `b` is the maximum support of the distribution. The parameters must satisfy `a <= b`.

</section>

<!-- /.intro -->

<section class="usage">

## Usage

```javascript
var logpmf = require( '@stdlib/stats/base/dists/discrete-uniform/logpmf' );
```

#### logpmf( x, a, b )

Evaluates the natural logarithm of the [probability mass function][pmf] (PMF) for a [discrete uniform][discrete-uniform-distribution] distribution with parameters `a` (minimum support) and `b` (maximum support).

```javascript
var y = logpmf( 2.0, 0, 4 );
// returns ~-1.609

y = logpmf( 5.0, 0, 4 );
// returns -Infinity

y = logpmf( 3, -4, 4 );
// returns ~-2.197
```

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

```javascript
var y = logpmf( NaN, -2, 2 );
// returns NaN

y = logpmf( 1.0, NaN, 4 );
// returns NaN

y = logpmf( 2.0, 0, NaN );
// returns NaN
```

If `a` or `b` is not an integer value, the function returns `NaN`.

```javascript
var y = logpmf( 2.0, 1, 5.5 );
// returns NaN
```

If provided `a > b`, the function returns `NaN`.

```javascript
var y = logpmf( 2.0, 3, 2 );
// returns NaN
```

#### logpmf.factory( a, b )

Returns a `function` for evaluating the [PMF][pmf] for a [discrete uniform][discrete-uniform-distribution] distribution with parameters `a` (minimum support) and `b` (maximum support).

```javascript
var myLogPMF = logpmf.factory( 6, 7 );
var y = myLogPMF( 7.0 );
// returns ~-0.693

y = myLogPMF( 5.0 );
// returns -Infinity
```

</section>

<!-- /.usage -->

<section class="examples">

## Examples

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

```javascript
var randint = require( '@stdlib/random/base/discrete-uniform' );
var logpmf = require( '@stdlib/stats/base/dists/discrete-uniform/logpmf' );

var randa = randint.factory( 0, 10 );
var randb = randint.factory();
var a;
var b;
var x;
var y;
var i;

for ( i = 0; i < 25; i++ ) {
    a = randa();
    x = randb( a, a+randa() );
    b = randb( a, a+randa() );
    y = logpmf( x, a, b );
    console.log( 'x: %d, a: %d, b: %d, ln(P(X=x;a,b)): %d', x.toFixed( 4 ), a.toFixed( 4 ), b.toFixed( 4 ), y.toFixed( 4 ) );
}
```

</section>

<!-- /.examples -->

<section class="links">

[pmf]: https://en.wikipedia.org/wiki/Probability_mass_function

[discrete-uniform-distribution]: https://en.wikipedia.org/wiki/Discrete_uniform_distribution

</section>

<!-- /.links -->
