npm package: @stdlib/array-base-where
Description: Take elements from either one of two arrays depending on a condition.
Author: The Stdlib Authors
Latest version: 0.0.3

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# where

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

<section class="installation">

## Installation

```bash
npm install @stdlib/array-base-where
```

</section>

<section class="usage">

## Usage

```javascript
var where = require( '@stdlib/array-base-where' );
```

#### where( condition, x, y )

Takes elements from either `x` or `y` depending on a condition.

```javascript
var x = [ 1, 2, 3, 4 ];
var y = [ 5, 6, 7, 8 ];

var condition = [ true, false, true, false ];

var z = where( condition, x, y );
// returns [ 1, 6, 3, 8 ]
```

The function supports the following parameters:

-   **condition**: array of values indicating whether to take an element from either `x` or `y`. If a condition element is truthy, the function takes a respective element from `x`; otherwise, the function takes a respective element from `y`. If non-empty, must be broadcast [compatible][@stdlib/ndarray/base/broadcast-shapes] with the resolved output array length.
-   **x**: first input array. If `condition` is non-empty, must be broadcast [compatible][@stdlib/ndarray/base/broadcast-shapes] with the resolved output array length.
-   **y**: second input array. If `condition` is non-empty, must be broadcast [compatible][@stdlib/ndarray/base/broadcast-shapes] with the resolved output array length.

When all input arrays are non-empty, the function supports broadcasting single-element arrays to the resolved output array length, which is equal to the maximum length of all provided input arrays.

```javascript
var x = [ 1, 2, 3, 4 ];
var y = [ 5 ];

var condition = [ true, false, true, false ];

var z = where( condition, x, y );
// returns [ 1, 5, 3, 5 ]

z = where( condition, y, x );
// returns [ 5, 2, 5, 4 ]

z = where( [ true ], x, y );
// returns [ 1, 2, 3, 4 ]

z = where( [ false ], x, y );
// returns [ 5, 5, 5, 5 ]

z = where( condition, [ 1 ], y );
// returns [ 1, 5, 1, 5 ]
```

If `condition` is an empty array, the function returns an empty array.

```javascript
var x = [ 1, 2, 3, 4 ];
var y = [ 5, 6, 7, 8 ];

var condition = [];

var z = where( condition, x, y );
// returns []
```

#### where.assign( condition, x, y, out, stride, offset )

Takes elements from either `x` or `y` depending on a condition and assigns the values to elements in a provided output array.

```javascript
var x = [ 1, 2, 3, 4 ];
var y = [ 5, 6, 7, 8 ];

var out = [ 0, 0, 0, 0 ];
var condition = [ true, false, true, false ];

var arr = where.assign( condition, x, y, out, 1, 0 );
// returns [ 1, 6, 3, 8 ]

var bool = ( arr === out );
// returns true
```

The function supports the following parameters:

-   **condition**: array of values indicating whether to take an element from either `x` or `y`. If a condition element is truthy, the function takes a respective element from `x`; otherwise, the function takes a respective element from `y`. If non-empty, must be broadcast [compatible][@stdlib/ndarray/base/broadcast-shapes] with the output array.
-   **x**: first input array. If `condition` is non-empty, must be broadcast [compatible][@stdlib/ndarray/base/broadcast-shapes] with the output array.
-   **y**: second input array. If `condition` is non-empty, must be broadcast [compatible][@stdlib/ndarray/base/broadcast-shapes] with the output array.
-   **out**: output array.
-   **stride**: output array stride.
-   **offset**: output array offset.

The function supports broadcasting single-element arrays to the output array length.

```javascript
var x = [ 1, 2, 3, 4 ];
var y = [ 5 ];

var condition = [ true, false, true, false ];

var out = [ 0, 0, 0, 0 ];
var arr = where.assign( condition, x, y, out, 1, 0 );
// returns [ 1, 5, 3, 5 ]

out = [ 0, 0, 0, 0 ];
arr = where.assign( condition, y, x, out, 1, 0 );
// returns [ 5, 2, 5, 4 ]

out = [ 0, 0, 0, 0 ];
arr = where.assign( [ true ], x, y, out, 1, 0 );
// returns [ 1, 2, 3, 4 ]

out = [ 0, 0, 0, 0 ];
arr = where.assign( [ false ], x, y, out, 1, 0 );
// returns [ 5, 5, 5, 5 ]

out = [ 0, 0, 0, 0 ];
arr = where.assign( condition, [ 1 ], y, out, 1, 0 );
// returns [ 1, 5, 1, 5 ]
```

When `condition` is an empty array, the function returns the output array unchanged.

```javascript
var x = [ 1, 2, 3, 4 ];
var y = [ 5, 6, 7, 8 ];

var out = [ 0, 0, 0, 0 ];
var condition = [];

var arr = where.assign( condition, x, y, out, 1, 0 );
// returns [ 0, 0, 0, 0 ]

var bool = ( arr === out );
// returns true
```

</section>

<!-- /.usage -->

<section class="notes">

</section>

<!-- /.notes -->

<section class="examples">

## Examples

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

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

var opts = {
    'dtype': 'generic'
};

// Generate an array of indicator values:
var condition = bernoulli( 20, 0.9, opts );
console.log( condition );

// Generate an array of random values:
var x = discreteUniform( condition.length, 0, 10, opts );
console.log( x );

// Define an array containing a broadcasted "missing" value:
var y = [ NaN ];

// Return an array with randomly placed missing values:
var z = where( condition, x, y );
console.log( z );
```

</section>

<!-- /.examples -->

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

<section class="related">

</section>

<!-- /.related -->

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<section class="main-repo" >

* * *

## Notice

This package is part of [stdlib][stdlib], a standard library for JavaScript and Node.js, with an emphasis on numerical and scientific computing. The library provides a collection of robust, high performance libraries for mathematics, statistics, streams, utilities, and more.

For more information on the project, filing bug reports and feature requests, and guidance on how to develop [stdlib][stdlib], see the main project [repository][stdlib].

#### Community

[![Chat][chat-image]][chat-url]

---

## License

See [LICENSE][stdlib-license].


## Copyright

Copyright &copy; 2016-2026. The Stdlib [Authors][stdlib-authors].

</section>

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[stdlib]: https://github.com/stdlib-js/stdlib

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[umd]: https://github.com/umdjs/umd
[es-module]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Modules

[deno-url]: https://github.com/stdlib-js/array-base-where/tree/deno
[deno-readme]: https://github.com/stdlib-js/array-base-where/blob/deno/README.md
[umd-url]: https://github.com/stdlib-js/array-base-where/tree/umd
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[stdlib-license]: https://raw.githubusercontent.com/stdlib-js/array-base-where/main/LICENSE

[@stdlib/ndarray/base/broadcast-shapes]: https://www.npmjs.com/package/@stdlib/ndarray-base-broadcast-shapes

</section>

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