# bifurcateBy
> Split values into two groups according to a predicate function.
## Usage
```javascript
var bifurcateBy = require( '@stdlib/utils/bifurcate-by' );
```
#### bifurcateBy( collection, \[options,] predicate )
Splits values into two groups according to a `predicate` function, which specifies which group an element in the input `collection` belongs to. If a `predicate` function returns a truthy value, a collection element belongs to the first group; otherwise, a collection element belongs to the second group.
```javascript
function predicate( v ) {
return v[ 0 ] === 'b';
}
var arr = [ 'beep', 'boop', 'foo', 'bar' ];
var out = bifurcateBy( arr, predicate );
// returns [ [ 'beep', 'boop', 'bar' ], [ 'foo' ] ]
```
A `predicate` function is provided two arguments:
- `value`: collection element
- `index`: collection index
```javascript
function predicate( v, i ) {
console.log( '%d: %s', i, v );
return v[ 0 ] === 'b';
}
var arr = [ 'beep', 'boop', 'foo', 'bar' ];
var out = bifurcateBy( arr, predicate );
// 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 the option equals `'indices'`, the function outputs element indices. If the option equals `'*'`, the function outputs both element indices and values. Default: `'values'`.
- `thisArg`: execution context.
By default, the function returns element values. To return element indices, set the `returns` option to `'indices'`.
```javascript
function predicate( v ) {
return v[ 0 ] === 'b';
}
var arr = [ 'beep', 'boop', 'foo', 'bar' ];
var opts = {
'returns': 'indices'
};
var out = bifurcateBy( arr, opts, predicate );
// returns [ [ 0, 1, 3 ], [ 2 ] ]
```
To return index-element pairs, set the `returns` option to `'*'`.
```javascript
function predicate( v ) {
return v[ 0 ] === 'b';
}
var arr = [ 'beep', 'boop', 'foo', 'bar' ];
var opts = {
'returns': '*'
};
var out = bifurcateBy( arr, opts, predicate );
// returns [ [ [ 0, 'beep' ], [ 1, 'boop' ], [ 3, 'bar' ] ], [ [ 2, 'foo' ] ] ]
```
To set the `predicate` execution context, provide a `thisArg`.
```javascript
function predicate( v ) {
this.count += 1;
return v[ 0 ] === 'b';
}
var context = {
'count': 0
};
var opts = {
'thisArg': context
};
var arr = [ 'beep', 'boop', 'foo', 'bar' ];
var out = bifurcateBy( arr, opts, predicate );
// returns [ [ 'beep', 'boop', 'bar' ], [ 'foo' ] ]
console.log( context.count );
// => 4
```
## Notes
- 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`).
## Examples
```javascript
var randu = require( '@stdlib/random/base/randu' );
var floor = require( '@stdlib/math/base/special/floor' );
var bifurcateBy = require( '@stdlib/utils/bifurcate-by' );
var vals;
var arr;
var out;
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 ];
}
function predicate( v ) {
return v[ 0 ] === 'b';
}
// Compute the groups:
out = bifurcateBy( arr, predicate );
console.log( out );
```
[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