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 );