tabulateBy

Generate a frequency table according to an indicator function.

Usage

var tabulateBy = require( '@stdlib/utils/tabulate-by' );

tabulateBy( collection[, options,] indicator )

Generates a frequency table according to an indicator function, i.e., a function which specifies how to categorize an element in the input collection.

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

var out = tabulateBy( arr, indicator );
// returns [ [ 'b', 3, 0.75 ], [ 'f', 1, 0.25 ] ]

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 = tabulateBy( arr, indicator );
// returns [ [ 'b', 3, 0.75 ], [ 'f', 1, 0.25 ] ]

The function accepts the following options:

thisArg: execution context.

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 = tabulateBy( arr, opts, indicator );
// returns [ [ 'b', 3, 0.75 ], [ 'f', 1, 0.25 ] ]

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

The returned frequency table is an array of arrays. Each sub-array corresponds to a unique value in the input collection and is structured as follows:

0: unique value
1: value count
2: frequency percentage

Notes

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

Examples

var randu = require( '@stdlib/random/base/randu' );
var floor = require( '@stdlib/math/base/special/floor' );
var tabulateBy = require( '@stdlib/utils/tabulate-by' );

var vals;
var arr;
var out;
var i;
var j;

function indicator( value ) {
    return value[ 0 ];
}

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 ];
}

// Generate a frequency table:
out = tabulateBy( arr, indicator );
console.log( out );