# bifurcate

> Split values into two groups.


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## Usage

``` javascript
var bifurcate = require( '@stdlib/utils/bifurcate' );
```

#### bifurcate( collection, \[options,\] filter )

Splits values into two groups. If an element in `filter` is truthy, the corresponding element in `collection` belongs to the first group; otherwise, the `collection` element belongs to the second group.

``` javascript
var arr = [ 'beep', 'boop', 'foo', 'bar' ];
var filter = [ true, true, false, true ];

var out = bifurcate( arr, filter );
// returns [ [ 'beep', 'boop', 'bar' ], [ 'foo' ] ]
```

The function accepts the following `options`:

* `returns`: specifies the output format. If the option equals `'values'`, the function outputs element values. If option equals `'indices'`, the function outputs element indices. If the option equals `'*'`, the function outputs both element indices and values. Default: `'values'`.

By default, the function returns element values. To return element indices, set the `returns` option to `'indices'`.

``` javascript
var arr = [ 'beep', 'boop', 'foo', 'bar' ];
var filter = [ true, true, false, true ];

var opts = {
    'returns': 'indices'  
};
var out = bifurcate( arr, opts, filter );
// returns [ [ 0, 1, 3 ], [ 2 ] ]
```

To return index-element pairs, set the `returns` option to `'*'`.

``` javascript
var arr = [ 'beep', 'boop', 'foo', 'bar' ];
var filter = [ true, true, false, true ];

var opts = {
    'returns': '*'  
};
var out = bifurcate( arr, opts, filter );
// returns [ [ [ 0, 'beep' ], [ 1, 'boop ], [ 3, 'bar' ] ], [ [ 2, 'foo' ] ] ]
```

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## Notes

* Both `collection` and `filter` __must__ be collections. A collection may be either an [`Array`][mdn-array], [`Typed Array`][mdn-typed-array], or an array-like [`Object`][mdn-object] (excluding `strings` and `functions`).

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## Examples

``` javascript
var randu = require( '@stdlib/math/base/random/randu' );
var floor = require( '@stdlib/math/base/special/floor' );
var bifurcate = require( '@stdlib/utils/bifurcate' );

var vals;
var arr;
var out;
var f;
var i;
var j;

vals = [ 'beep', 'boop', 'foo', 'bar', 'woot', 'woot' ];

// Generate a random collection...
arr = new Array( 100 );
for ( i = 0; i < arr.length; i++ ) {
    j = floor( randu()*vals.length );
    arr[ i ] = vals[ j ];
}

// Randomly assign collection values to a group...
f = new Array( arr.length );
for ( i = 0; i < arr.length; i++ ) {
    f[ i ] = ( randu() < 0.5 );
}

// Compute the groups:
out = bifurcate( arr, f );
console.log( out );
```

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

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