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# factorial2

> [Double factorial][double-factorial] function.

<section class="intro">

The [double factorial][double-factorial] of a number `n`, denoted `n!!`, is defined as the product of all the positive integers up to `n` that have the same parity (odd or even) as `n`.

Thus, for example, `5!!` is `5 * 3 * 1 = 15` and `8!!` is `8 * 6 * 4 * 2 = 384`.

</section>

<!-- /.intro -->

<section class="usage">

## Usage

```javascript
var factorial2 = require( '@stdlib/math/base/special/factorial2' );
```

#### factorial2( n )

Evaluates the [double factorial][double-factorial] of `n`.

```javascript
var v = factorial2( 2 );
// returns 2

v = factorial2( 3 );
// returns 3

v = factorial2( 0 );
// returns 1

v = factorial2( 4 );
// returns 8

v = factorial2( 5 );
// returns 15

v = factorial2( NaN );
// returns NaN

v = factorial2( 301 );
// returns Infinity
```

</section>

<!-- /.usage -->

<section class="examples">

## Examples

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

```javascript
var oneTo = require( '@stdlib/array/base/one-to' );
var factorial2 = require( '@stdlib/math/base/special/factorial2' );

var values = oneTo( 300 );

var i;
for ( i = 0; i < values.length; i++ ) {
    console.log( 'f(%d): %d', values[ i ], factorial2( values[ i ] ) );
}
```

</section>

<!-- /.examples -->

<!-- C interface documentation. -->

* * *

<section class="c">

## C APIs

<!-- Section to include introductory text. Make sure to keep an empty line after the intro `section` element and another before the `/section` close. -->

<section class="intro">

</section>

<!-- /.intro -->

<!-- C usage documentation. -->

<section class="usage">

### Usage

```c
#include "stdlib/math/base/special/factorial2.h"
```

#### stdlib_base_factorial2( n )

Evaluates the [double factorial][double-factorial] of `n`.

```c
double out = stdlib_base_factorial2( 3.0 );
// returns 3.0
```

The function accepts the following arguments:

-   **n**: `[in] double` input value.

```c
double stdlib_base_factorial2( const double n );
```

</section>

<!-- /.usage -->

<!-- C API usage notes. Make sure to keep an empty line after the `section` element and another before the `/section` close. -->

<section class="notes">

</section>

<!-- /.notes -->

<!-- C API usage examples. -->

<section class="examples">

### Examples

```c
#include "stdlib/math/base/special/factorial2.h"
#include <stdio.h>

int main( void ) {
    const double x[] = { 1.0, 10.0, 100.0, 301.0, 302.0 };

    double b;
    int i;
    for ( i = 0; i < 5; i++ ){
        b = stdlib_base_factorial2( x[ i ] );
        printf ( "factorial2(%lf) = %lf\n", x[ i ], b );
    }
}
```

</section>

<!-- /.examples -->

</section>

<!-- /.c -->

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

<section class="related">

* * *

## See Also

-   <span class="package-name">[`@stdlib/math/base/special/factorial`][@stdlib/math/base/special/factorial]</span><span class="delimiter">: </span><span class="description">evaluate a factorial.</span>

</section>

<!-- /.related -->

<!-- Section for all links. Make sure to keep an empty line after the `section` element and another before the `/section` close. -->

<section class="links">

[double-factorial]: https://en.wikipedia.org/wiki/Double_factorial

<!-- <related-links> -->

[@stdlib/math/base/special/factorial]: https://github.com/stdlib-js/math/tree/main/base/special/factorial

<!-- </related-links> -->

</section>

<!-- /.links -->
