@stdlib/utils-group-by NPM

groupBy

Group values according to an indicator function.

Installation

npm install @stdlib/utils-group-by

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 );

Notice

This package is part of stdlib, a standard library for JavaScript and Node.js, with an emphasis on numerical and scientific computing. The library provides a collection of robust, high performance libraries for mathematics, statistics, streams, utilities, and more.

For more information on the project, filing bug reports and feature requests, and guidance on how to develop stdlib, see the main project repository.