# group
> Group values as arrays associated with distinct keys.
## Usage
```javascript
var group = require( '@stdlib/utils/group' );
```
#### group( collection, \[options,] groups )
Groups values as arrays associated with distinct keys.
```javascript
var arr = [ 'beep', 'boop', 'foo', 'bar' ];
var groups = [ 'b', 'b', 'f', 'b' ];
var out = group( arr, groups );
// returns { 'b': [ 'beep', 'boop', 'bar' ], 'f': [ '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'`.
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 groups = [ 'b', 'b', 'f', 'b' ];
var opts = {
'returns': 'indices'
};
var out = group( arr, opts, groups );
// returns { 'b': [ 0, 1, 3 ], 'f': [ 2 ] }
```
To return index-element pairs, set the `returns` option to `'*'`.
```javascript
var arr = [ 'beep', 'boop', 'foo', 'bar' ];
var groups = [ 'b', 'b', 'f', 'b' ];
var opts = {
'returns': '*'
};
var out = group( arr, opts, groups );
// returns { 'b': [ [ 0, 'beep' ], [ 1, 'boop' ], [ 3, 'bar' ] ], 'f': [ [ 2, 'foo' ] ] }
```
## Notes
- Both `collection` and `groups` **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`).
- Each value in `groups` should resolve to a value which can be serialized as an `object` key. As a counterexample,
```javascript
var arr = [ 'beep', 'boop', 'foo', 'bar' ];
var groups = [ {}, {}, {}, {} ];
var out = group( arr, groups );
// returns { '[object Object]': [ 'beep', 'boop', 'foo', 'bar' ] }
```
while each "group" is unique, all collection elements resolve to the same group because each group identifier serializes to the same `string`.
## Examples
```javascript
var randu = require( '@stdlib/random/base/randu' );
var floor = require( '@stdlib/math/base/special/floor' );
var group = require( '@stdlib/utils/group' );
var vals;
var arr;
var out;
var g;
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 groups...
g = new Array( arr.length );
for ( i = 0; i < arr.length; i++ ) {
g[ i ] = floor( randu()*vals.length );
}
// Compute the groups:
out = group( arr, g );
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