# Reciprocal Square Root
> Compute the reciprocal of the principal [square root][square-root] of a single-precision floating-point number.
The reciprocal of the principal [square root][square-root] is defined as
## Usage
```javascript
var rsqrtf = require( '@stdlib/math/base/special/rsqrtf' );
```
#### rsqrtf( x )
Computes the reciprocal (inverse) square root of a single-precision floating-point number.
```javascript
var v = rsqrtf( 1.0 );
// returns 1.0
v = rsqrtf( 4.0 );
// returns 0.5
v = rsqrtf( 0.0 );
// returns Infinity
v = rsqrtf( NaN );
// returns NaN
v = rsqrtf( Infinity );
// returns 0.0
```
For negative numbers, the reciprocal square root is **not** defined.
```javascript
var v = rsqrtf( -4.0 );
// returns NaN
```
## Examples
```javascript
var randu = require( '@stdlib/random/base/randu' );
var round = require( '@stdlib/math/base/special/round' );
var rsqrtf = require( '@stdlib/math/base/special/rsqrtf' );
var x;
var i;
for ( i = 0; i < 100; i++ ) {
x = round( randu() * 100.0 );
console.log( 'rsqrt(%d) = %d', x, rsqrtf( x ) );
}
```
* * *
## C APIs
### Usage
```c
#include "stdlib/math/base/special/rsqrtf.h"
```
#### stdlib_base_rsqrtf( x )
Computes the reciprocal (inverse) [square root][square-root] of a single-precision floating-point number.
```c
float y = stdlib_base_rsqrtf( 4.0 );
// returns 0.5
```
The function accepts the following arguments:
- **x**: `[in] float` input value.
```c
float stdlib_base_rsqrtf( const float x );
```
### Examples
```c
#include "stdlib/math/base/special/rsqrtf.h"
#include
int main() {
float x[] = { 3.14f, 9.0f, 0.0f, 0.0f/0.0f };
float y;
int i;
for ( i = 0; i < 4; i++ ) {
y = stdlib_base_rsqrtf( x[ i ] );
printf( "rsqrt(%f) = %f\n", x[ i ], y );
}
}
```
[square-root]: https://en.wikipedia.org/wiki/Square_root
