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# smskrsqrt

> Compute the [reciprocal square root][@stdlib/math/base/special/rsqrtf] for each element in a single-precision floating-point strided array according to a strided mask array.

<section class="intro">

</section>

<!-- /.intro -->

<section class="usage">

## Usage

```javascript
var smskrsqrt = require( '@stdlib/math/strided/special/smskrsqrt' );
```

#### smskrsqrt( N, x, sx, m, sm, y, sy )

Computes the [reciprocal square root][@stdlib/math/base/special/rsqrtf] for each element in a single-precision floating-point strided array `x` according to a strided mask array and assigns the results to elements in a single-precision floating-point strided array `y`.

```javascript
var Float32Array = require( '@stdlib/array/float32' );
var Uint8Array = require( '@stdlib/array/uint8' );

var x = new Float32Array( [ 0.0, 4.0, 9.0, 12.0, 24.0 ] );
var m = new Uint8Array( [ 0, 0, 1, 0, 1 ] );
var y = new Float32Array( x.length );

smskrsqrt( x.length, x, 1, m, 1, y, 1 );
// y => <Float32Array>[ Infinity, 0.5, 0.0, ~0.289, 0.0 ]
```

The function accepts the following arguments:

-   **N**: number of indexed elements.
-   **x**: input [`Float32Array`][@stdlib/array/float32].
-   **sx**: index increment for `x`.
-   **m**: mask [`Uint8Array`][@stdlib/array/uint8].
-   **sm**: index increment for `m`.
-   **y**: output [`Float32Array`][@stdlib/array/float32].
-   **sy**: index increment for `y`.

The `N` and stride parameters determine which strided array elements are accessed at runtime. For example, to index every other value in `x` and to index the first `N` elements of `y` in reverse order,

```javascript
var Float32Array = require( '@stdlib/array/float32' );
var Uint8Array = require( '@stdlib/array/uint8' );

var x = new Float32Array( [ 0.0, 4.0, 9.0, 12.0, 24.0, 64.0 ] );
var m = new Uint8Array( [ 0, 0, 1, 0, 1, 1 ] );
var y = new Float32Array( [ 0.0, 0.0, 0.0, 0.0, 0.0, 0.0 ] );

smskrsqrt( 3, x, 2, m, 2, y, -1 );
// y => <Float32Array>[ 0.0, 0.0, Infinity, 0.0, 0.0, 0.0 ]
```

Note that indexing is relative to the first index. To introduce an offset, use [`typed array`][@stdlib/array/float32] views.

```javascript
var Float32Array = require( '@stdlib/array/float32' );
var Uint8Array = require( '@stdlib/array/uint8' );

// Initial arrays...
var x0 = new Float32Array( [ 0.0, 4.0, 9.0, 12.0, 24.0, 64.0 ] );
var m0 = new Uint8Array( [ 0, 0, 1, 0, 1, 1 ] );
var y0 = new Float32Array( [ 0.0, 0.0, 0.0, 0.0, 0.0, 0.0 ] );

// Create offset views...
var x1 = new Float32Array( x0.buffer, x0.BYTES_PER_ELEMENT*1 ); // start at 2nd element
var m1 = new Uint8Array( m0.buffer, m0.BYTES_PER_ELEMENT*1 ); // start at 2nd element
var y1 = new Float32Array( y0.buffer, y0.BYTES_PER_ELEMENT*3 ); // start at 4th element

smskrsqrt( 3, x1, -2, m1, -2, y1, 1 );
// y0 => <Float32Array>[ 0.0, 0.0, 0.0, 0.0, ~0.289, 0.5 ]
```

#### smskrsqrt.ndarray( N, x, sx, ox, m, sm, om, y, sy, oy )

Computes the [reciprocal square root][@stdlib/math/base/special/rsqrtf] for each element in a single-precision floating-point strided array `x` according to a strided mask array and assigns the results to elements in a single-precision floating-point strided array `y` using alternative indexing semantics.

```javascript
var Float32Array = require( '@stdlib/array/float32' );
var Uint8Array = require( '@stdlib/array/uint8' );

var x = new Float32Array( [ 0.0, 4.0, 9.0, 12.0, 24.0 ] );
var m = new Uint8Array( [ 0, 0, 1, 0, 1 ] );
var y = new Float32Array( [ 0.0, 0.0, 0.0, 0.0, 0.0 ] );

smskrsqrt.ndarray( x.length, x, 1, 0, m, 1, 0, y, 1, 0 );
// y => <Float32Array>[ Infinity, 0.5, 0.0, ~0.289, 0.0 ]
```

The function accepts the following additional arguments:

-   **ox**: starting index for `x`.
-   **om**: starting index for `m`.
-   **oy**: starting index for `y`.

While [`typed array`][@stdlib/array/float32] views mandate a view offset based on the underlying `buffer`, the offset parameters support indexing semantics based on starting indices. For example, to index every other value in `x` starting from the second value and to index the last `N` elements in `y`,

```javascript
var Float32Array = require( '@stdlib/array/float32' );
var Uint8Array = require( '@stdlib/array/uint8' );

var x = new Float32Array( [ 0.0, 4.0, 9.0, 12.0, 24.0, 64.0 ] );
var m = new Uint8Array( [ 0, 0, 1, 0, 1, 1 ] );
var y = new Float32Array( [ 0.0, 0.0, 0.0, 0.0, 0.0, 0.0 ] );

smskrsqrt.ndarray( 3, x, 2, 1, m, 2, 1, y, -1, y.length-1 );
// y => <Float32Array>[ 0.0, 0.0, 0.0, 0.0, ~0.289, 0.5 ]
```

</section>

<!-- /.usage -->

<section class="notes">

</section>

<!-- /.notes -->

<section class="examples">

## Examples

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

```javascript
var uniform = require( '@stdlib/random/base/uniform' );
var Float32Array = require( '@stdlib/array/float32' );
var Uint8Array = require( '@stdlib/array/uint8' );
var smskrsqrt = require( '@stdlib/math/strided/special/smskrsqrt' );

var x = new Float32Array( 10 );
var m = new Uint8Array( 10 );
var y = new Float32Array( 10 );

var i;
for ( i = 0; i < x.length; i++ ) {
    x[ i ] = uniform( 0.0, 200.0 );
    if ( uniform( 0.0, 1.0 ) < 0.5 ) {
        m[ i ] = 1;
    }
}
console.log( x );
console.log( m );
console.log( y );

smskrsqrt.ndarray( x.length, x, 1, 0, m, 1, 0, y, -1, y.length-1 );
console.log( y );
```

</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/strided/special/smskrsqrt.h"
```

#### stdlib_strided_smskrsqrt( N, \*X, strideX, \*Mask, strideMask, \*Y, strideY )

Computes the [reciprocal square root][@stdlib/math/base/special/rsqrtf] for each element in a single-precision floating-point strided array `X` according to a strided mask array and assigns the results to elements in a single-precision floating-point strided array `Y`.

```c
#include <stdint.h>

float X[] = { 0.0, 4.0, 9.0, 12.0, 24.0, 64.0, 81.0, 101.0 };
uint8_t Mask[] = { 0, 0, 1, 0, 1, 1, 0, 0 };
float Y[] = { 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0 };

int64_t N = 4;

stdlib_strided_smskrsqrt( N, X, 2, Mask, 2, Y, 2 );
```

The function accepts the following arguments:

-   **N**: `[in] int64_t` number of indexed elements.
-   **X**: `[in] float*` input array.
-   **strideX**: `[in] int64_t` index increment for `X`.
-   **Mask**: `[in] uint8_t*` mask array.
-   **strideMask**: `[in] int64_t` index increment for `Mask`.
-   **Y**: `[out] float*` output array.
-   **strideY**: `[in] int64_t` index increment for `Y`.

```c
void stdlib_strided_smskrsqrt( const int64_t N, const float *X, const int64_t strideX, const uint8_t *Mask, const int64_t strideMask, float *Y, const int64_t strideY );
```

</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/strided/special/smskrsqrt.h"
#include <stdint.h>
#include <stdio.h>

int main() {
    // Create an input strided array:
    float X[] = { 0.0, 4.0, 9.0, 12.0, 24.0, 64.0, 81.0, 101.0 };

    // Create a mask strided array:
    uint8_t M[] = { 0, 0, 1, 0, 1, 1, 0, 0 };

    // Create an output strided array:
    float Y[] = { 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0 };

    // Specify the number of elements:
    int64_t N = 4;

    // Specify the stride lengths:
    int64_t strideX = 2;
    int64_t strideM = 2;
    int64_t strideY = 2;

    // Compute the results:
    stdlib_strided_smskrsqrt( N, X, strideX, M, strideM, Y, strideY );

    // Print the results:
    for ( int i = 0; i < 8; i++ ) {
        printf( "Y[ %i ] = %f\n", i, Y[ i ] );
    }
}
```

</section>

<!-- /.examples -->

</section>

<!-- /.c -->

<section class="links">

[@stdlib/array/float32]: https://github.com/stdlib-js/array-float32

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

[@stdlib/array/uint8]: https://github.com/stdlib-js/array-uint8

</section>

<!-- /.links -->
