npm package: @stdlib/math-base-special-factorial2f
Description: Evaluate the double factorial function as a single-precision floating-point number.
Author: The Stdlib Authors
Latest version: 0.1.1

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# factorial2f

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

> Evaluate the [double factorial][double-factorial] function as a single-precision floating-point number.

<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="installation">

## Installation

```bash
npm install @stdlib/math-base-special-factorial2f
```

</section>

<section class="usage">

## Usage

```javascript
var factorial2f = require( '@stdlib/math-base-special-factorial2f' );
```

#### factorial2f( n )

Evaluates the [double factorial][double-factorial] of `n` as a single-precision floating-point number.

```javascript
var v = factorial2f( 3 );
// returns 3

v = factorial2f( 4 );
// returns 8

v = factorial2f( 10 );
// returns 3840
```

If `n > 56`, the function returns `NaN`, as larger [double factorial][double-factorial] values cannot be safely represented in [single-precision floating-point format][ieee754].

```javascript
var v = factorial2f( 57 );
// returns Infinity
```

If not provided a nonnegative integer value, the function returns `NaN`.

```javascript
var v = factorial2f( 3.14 );
// returns NaN

v = factorial2f( -1 );
// returns NaN
```

If provided `NaN`, the function returns `NaN`.

```javascript
var v = factorial2f( NaN );
// returns NaN
```

</section>

<!-- /.usage -->

<section class="examples">

## Examples

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

```javascript
var discreteUniform = require( '@stdlib/random-array-discrete-uniform' );
var logEachMap = require( '@stdlib/console-log-each-map' );
var factorial2f = require( '@stdlib/math-base-special-factorial2f' );

var x = discreteUniform( 10, 0, 56, {
    'dtype': 'int32'
});

logEachMap( 'factorial2f(%d) = %0.1f', x, factorial2f );
```

</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/factorial2f.h"
```

#### stdlib_base_factorial2f( n )

Evaluates the [double factorial][double-factorial] of `n` as a single-precision floating-point number.

```c
float out = stdlib_base_factorial2f( 3.0f );
// returns 3.0f
```

The function accepts the following arguments:

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

```c
float stdlib_base_factorial2f( const float 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/factorial2f.h"
#include <stdio.h>

int main( void ) {
    const float x[] = { 1.0f, 10.0f, 50.0f, 56.0f, 57.0f };

    float b;
    int i;
    for ( i = 0; i < 5; i++ ) {
        b = stdlib_base_factorial2f( x[ i ] );
        printf ( "factorial2f(%f) = %f\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">

</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>

<!-- /.stdlib -->

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[stdlib-license]: https://raw.githubusercontent.com/stdlib-js/math-base-special-factorial2f/main/LICENSE

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

[ieee754]: https://en.wikipedia.org/wiki/IEEE_754-1985

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</section>

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