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where

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

Usage

To use in Observable,

where = require( 'https://cdn.jsdelivr.net/gh/stdlib-js/array-base-where@umd/browser.js' )

To vendor stdlib functionality and avoid installing dependency trees for Node.js, you can use the UMD server build:

var where = require( 'path/to/vendor/umd/array-base-where/index.js' )

To include the bundle in a webpage,

<script type="text/javascript" src="https://cdn.jsdelivr.net/gh/stdlib-js/array-base-where@umd/browser.js"></script>

If no recognized module system is present, access bundle contents via the global scope:

<script type="text/javascript">
(function () {
    window.where;
})();
</script>

where( condition, x, y )

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

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 with the resolved output array length.
• x: first input array. If condition is non-empty, must be broadcast compatible with the resolved output array length.
• y: second input array. If condition is non-empty, must be broadcast compatible 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.

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.

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.

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 with the output array.
• x: first input array. If condition is non-empty, must be broadcast compatible with the output array.
• y: second input array. If condition is non-empty, must be broadcast compatible 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.

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.

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

Examples

<!DOCTYPE html>
<html lang="en">
<body>
<script type="text/javascript" src="https://cdn.jsdelivr.net/gh/stdlib-js/random-array-discrete-uniform@umd/browser.js"></script>
<script type="text/javascript" src="https://cdn.jsdelivr.net/gh/stdlib-js/random-array-bernoulli@umd/browser.js"></script>
<script type="text/javascript" src="https://cdn.jsdelivr.net/gh/stdlib-js/array-base-where@umd/browser.js"></script>
<script type="text/javascript">
(function () {

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 );

})();
</script>
</body>
</html>

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Notice

This package is part of 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, see the main project repository.

Community

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License

See LICENSE.

Copyright

Copyright © 2016-2025. The Stdlib Authors.