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# Identity Function

> Evaluate the [identity function][identity-function] of a double-precision floating-point number.

<section class="intro">

The [identity-function][identity-function] is defined as

<!-- <equation class="equation" label="eq:identity_function" align="center" raw="f(x) = x" alt="Identity function"> -->

<div class="equation" align="center" data-raw-text="f(x) = x" data-equation="eq:identity_function">
    <img src="https://cdn.rawgit.com/stdlib-js/stdlib/ad7afa5d7ec1b1596f8a4828153d8c2e87a90161/lib/node_modules/@stdlib/math/base/special/identity/docs/img/equation_identity_function.svg" alt="Identity function">
    <br>
</div>

<!-- </equation> -->

for all `x`.

</section>

<!-- /.intro -->

<section class="usage">

## Usage

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

#### identity( x )

Evaluates the [identity function][identity-function] for a double-precision floating-point number.

```javascript
var v = identity( -1.0 );
// returns -1.0

v = identity( 2.0 );
// returns 2.0

v = identity( 0.0 );
// returns 0.0

v = identity( -0.0 );
// returns -0.0

v = identity( NaN );
// returns NaN
```

</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 identity = require( '@stdlib/math/base/special/identity' );

var rand;
var i;

for ( i = 0; i < 100; i++ ) {
    rand = round( randu() * 100.0 ) - 50.0;
    console.log( 'identity(%d) = %d', rand, identity( rand ) );
}
```

</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/identity.h"
```

#### stdlib_base_identity( x )

Evaluates the identity function for a double-precision floating-point number.

```c
double y = stdlib_base_identity( 2.0 );
// returns 2.0
```

The function accepts the following arguments:

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

```c
double stdlib_base_identity( const double x );
```

</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/identity.h"
#include <stdio.h>

int main() {
    double x[] = { 3.14, -3.14, 0.0, 0.0/0.0 };

    double y;
    int i;
    for ( i = 0; i < 4; i++ ) {
        y = stdlib_base_identity( x[ i ] );
        printf( "f(%lf) = %lf\n", x[ i ], y );
    }
}
```

</section>

<!-- /.examples -->

</section>

<!-- /.c -->

<section class="links">

[identity-function]: https://en.wikipedia.org/wiki/Identity_function

</section>

<!-- /.links -->
