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# gwhere

> Take elements from one of two strided arrays depending on a condition.

<section class="intro">

</section>

<!-- /.intro -->

<section class="usage">

## Usage

```javascript
var gwhere = require( '@stdlib/blas/ext/base/gwhere' );
```

#### gwhere( N, condition, strideC, x, strideX, y, strideY, out, strideOut )

Takes elements from one of two strided arrays depending on a condition.

```javascript
var condition = [ 1, 0, 1 ];
var x = [ 1.0, 2.0, 3.0 ];
var y = [ 4.0, 5.0, 6.0 ];
var out = [ 0.0, 0.0, 0.0 ];

gwhere( 3, condition, 1, x, 1, y, 1, out, 1 );
// out => [ 1.0, 5.0, 3.0 ]
```

The function has the following parameters:

-   **N**: number of indexed elements.
-   **condition**: condition [`Array`][mdn-array] or [`typed array`][mdn-typed-array].
-   **strideC**: stride length for `condition`.
-   **x**: first input [`Array`][mdn-array] or [`typed array`][mdn-typed-array].
-   **strideX**: stride length for `x`.
-   **y**: second input [`Array`][mdn-array] or [`typed array`][mdn-typed-array].
-   **strideY**: stride length for `y`.
-   **out**: output [`Array`][mdn-array] or [`typed array`][mdn-typed-array].
-   **strideOut**: stride length for `out`.

The `N` and stride parameters determine which elements in the strided arrays are accessed at runtime. For example, to select from every other element:

```javascript
var condition = [ 1, 0, 0, 1, 1, 0 ];
var x = [ 1.0, 2.0, 3.0, 4.0, 5.0, 6.0 ];
var y = [ 7.0, 8.0, 9.0, 10.0, 11.0, 12.0 ];
var out = [ 0.0, 0.0, 0.0 ];

gwhere( 3, condition, 2, x, 2, y, 2, out, 1 );
// out => [ 1.0, 9.0, 5.0 ]
```

Note that indexing is relative to the first index. To introduce an offset, use [`typed array`][mdn-typed-array] views.

<!-- eslint-disable stdlib/capitalized-comments, max-len -->

```javascript
var Float64Array = require( '@stdlib/array/float64' );

// Initial arrays...
var condition0 = new Float64Array( [ 0.0, 1.0, 0.0, 1.0, 0.0, 1.0 ] );
var x0 = new Float64Array( [ 1.0, 2.0, 3.0, 4.0, 5.0, 6.0 ] );
var y0 = new Float64Array( [ 7.0, 8.0, 9.0, 10.0, 11.0, 12.0 ] );
var out0 = new Float64Array( [ 0.0, 0.0, 0.0, 0.0, 0.0, 0.0 ] );

// Create offset views...
var condition1 = new Float64Array( condition0.buffer, condition0.BYTES_PER_ELEMENT*1 ); // start at 2nd element
var x1 = new Float64Array( x0.buffer, x0.BYTES_PER_ELEMENT*1 ); // start at 2nd element
var y1 = new Float64Array( y0.buffer, y0.BYTES_PER_ELEMENT*1 ); // start at 2nd element
var out1 = new Float64Array( out0.buffer, out0.BYTES_PER_ELEMENT*1 ); // start at 2nd element

gwhere( 3, condition1, 2, x1, 2, y1, 2, out1, 1 );
// out0 => <Float64Array>[ 0.0, 2.0, 4.0, 6.0, 0.0, 0.0 ]
```

<!-- lint disable maximum-heading-length -->

#### gwhere.ndarray( N, condition, strideC, offsetC, x, strideX, offsetX, y, strideY, offsetY, out, strideOut, offsetOut )

<!-- lint enable maximum-heading-length -->

Takes elements from one of two strided arrays depending on a condition using alternative indexing semantics.

```javascript
var condition = [ 1, 0, 1 ];
var x = [ 1.0, 2.0, 3.0 ];
var y = [ 4.0, 5.0, 6.0 ];
var out = [ 0.0, 0.0, 0.0 ];

gwhere.ndarray( 3, condition, 1, 0, x, 1, 0, y, 1, 0, out, 1, 0 );
// out => [ 1.0, 5.0, 3.0 ]
```

The function has the following additional parameters:

-   **offsetC**: starting index for `condition`.
-   **offsetX**: starting index for `x`.
-   **offsetY**: starting index for `y`.
-   **offsetOut**: starting index for `out`.

While [`typed array`][mdn-typed-array] views mandate a view offset based on the underlying buffer, offset parameters support indexing semantics based on starting indices. For example, to select from every other element starting from the second element:

```javascript
var condition = [ 0, 1, 0, 0, 0, 1 ];
var x = [ 1.0, 2.0, 3.0, 4.0, 5.0, 6.0 ];
var y = [ 7.0, 8.0, 9.0, 10.0, 11.0, 12.0 ];
var out = [ 0.0, 0.0, 0.0 ];

gwhere.ndarray( 3, condition, 2, 1, x, 2, 1, y, 2, 1, out, 1, 0 );
// out => [ 2.0, 10.0, 6.0 ]
```

</section>

<!-- /.usage -->

<section class="notes">

## Notes

-   If `N <= 0`, both functions return `out` unchanged.
-   Both functions support array-like objects having getter and setter accessors for array element access (e.g., [`@stdlib/array/base/accessor`][@stdlib/array/base/accessor]).
-   Depending on the environment, the typed versions ([`dwhere`][@stdlib/blas/ext/base/dwhere], [`swhere`][@stdlib/blas/ext/base/swhere], etc.) are likely to be significantly more performant.

</section>

<!-- /.notes -->

<section class="examples">

## Examples

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

```javascript
var bernoulli = require( '@stdlib/random/array/bernoulli' );
var discreteUniform = require( '@stdlib/random/array/discrete-uniform' );
var zeros = require( '@stdlib/array/base/zeros' );
var gwhere = require( '@stdlib/blas/ext/base/gwhere' );

var condition = bernoulli( 20, 0.5, {
    'dtype': 'generic'
});
console.log( condition );
var x = discreteUniform( 20, 0, 100, {
    'dtype': 'generic'
});
console.log( x );
var y = discreteUniform( 20, -100, 0, {
    'dtype': 'generic'
});
console.log( y );

var out = zeros( condition.length );
console.log( out );

gwhere( condition.length, condition, 1, x, 1, y, 1, out, 1 );
console.log( out );
```

</section>

<!-- /.examples -->

<!-- Section for related `stdlib` packages. Do not manually edit this section, as it is automatically populated. -->

<section class="related">

</section>

<!-- /.related -->

<!-- Section for all links. Make sure to keep an empty line after the `section` element and another before the `/section` close. -->

<section class="links">

[mdn-array]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array

[mdn-typed-array]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray

[@stdlib/array/base/accessor]: https://www.npmjs.com/package/@stdlib/array-base-accessor

[@stdlib/blas/ext/base/dwhere]: https://github.com/stdlib-js/blas/tree/main/ext/base/dwhere

[@stdlib/blas/ext/base/swhere]: https://github.com/stdlib-js/blas/tree/main/ext/base/swhere

</section>

<!-- /.links -->
