group

    Group values as arrays associated with distinct keys.

    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 );