@stdlib/array
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Arrays.
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> Take elements from either one of two arrays depending on a condition.
<section class="usage">
```javascript
var where = require( '@stdlib/array/base/where' );
```
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 []
```
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">
<!-- 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 -->
<!-- Section for all links. Make sure to keep an empty line after the `section` element and another before the `/section` close. -->
<section class="links">
[@stdlib/ndarray/base/broadcast-shapes]: https://www.npmjs.com/package/@stdlib/ndarray-base-broadcast-shapes
</section>
<!-- /.links -->