groupBy

Group values according to an indicator function.

Usage

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

groupBy( collection, [options,] indicator )

Groups values according to an indicator function, which specifies which group an element in the input collection belongs to.

function indicator( v ) {
    return v[ 0 ];
}
var arr = [ 'beep', 'boop', 'foo', 'bar' ];

var out = groupBy( arr, indicator );
// returns { 'b': [ 'beep', 'boop', 'bar' ], 'f': [ 'foo' ] }

An indicator function is provided two arguments:

  • value: collection element
  • index: collection index
function indicator( v, i ) {
    console.log( '%d: %s', i, v );
    return v[ 0 ];
}
var arr = [ 'beep', 'boop', 'foo', 'bar' ];

var out = groupBy( arr, indicator );
// 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'.
  • thisArg: execution context.

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

function indicator( v ) {
    return v[ 0 ];
}
var arr = [ 'beep', 'boop', 'foo', 'bar' ];

var opts = {
    'returns': 'indices'
};
var out = groupBy( arr, opts, indicator );
// returns { 'b': [ 0, 1, 3 ], 'f': [ 2 ] }

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

function indicator( v ) {
    return v[ 0 ];
}
var arr = [ 'beep', 'boop', 'foo', 'bar' ];

var opts = {
    'returns': '*'
};
var out = groupBy( arr, opts, indicator );
// returns { 'b': [ [ 0, 'beep' ], [ 1, 'boop ], [ 3, 'bar' ] ], 'f': [ [ 2, 'foo' ] ] }

To set the indicator execution context, provide a thisArg.

function indicator( v ) {
    this.count += 1;
    return v[ 0 ];
}
var context = {
    'count': 0
};
var opts = {
    'thisArg': context
};
var arr = [ 'beep', 'boop', 'foo', 'bar' ];

var out = groupBy( arr, opts, indicator );
// returns { 'b': [ 'beep', 'boop', 'bar' ], 'f': [ 'foo' ] }

console.log( context.count );
// => 4

Notes

  • A collection may be either an Array, Typed Array, or an array-like Object (excluding strings and functions).

  • The value returned by an indicator function should be a value which can be serialized as an object key. As a counterexample,

    function indicator( v ) {
        return {};
    }
    var arr = [ 'beep', 'boop', 'foo', 'bar' ];
    
    var out = groupBy( arr, indicator );
    // returns { '[object Object]': [ 'beep', 'boop', 'foo', 'bar' ] }
    

    while each group identifier 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 groupBy = require( '@stdlib/utils/group-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 indicator( v ) {
    // Use the first letter of each element to define groups:
    return v[ 0 ];
}

// Compute the groups:
out = groupBy( arr, indicator );
console.log( out );