# Identity Function
> Evaluate the [identity function][identity-function] of a double-precision floating-point number.
The [identity-function][identity-function] is defined as
for all `x`.
## 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
```
## Examples
```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 ) );
}
```
* * *
## C APIs
### 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 );
```
### Examples
```c
#include "stdlib/math/base/special/identity.h"
#include
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 );
}
}
```
[identity-function]: https://en.wikipedia.org/wiki/Identity_function
