README

tabulateBy

Generate a frequency table according to an indicator function.

Installation

npm install @stdlib/utils-tabulate-by

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 );

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.

Community

License

See LICENSE.

@stdlib/utils-tabulate-by

Usage no npm install needed!