Usage

var group = require( '@stdlib/utils/group' );

group( collection, [options,] groups )

Groups values as arrays associated with distinct keys.

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'.

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 '*'.

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, Typed Array, or an array-like 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,

  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

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 );

See Also

• @stdlib/utils/bifurcate: split values into two groups.
• @stdlib/utils/count-by: group values according to an indicator function and return group counts.
• @stdlib/utils/group-by: group values according to an indicator function.