README

countBy

Group values according to an indicator function and return group counts.

Installation

npm install @stdlib/utils-count-by

Usage

var countBy = require( '@stdlib/utils-count-by' );

countBy( collection, [options,] indicator )

Groups values according to an indicator function, i.e., a function which specifies which group an element in the input collection belongs to, and returns group counts.

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

var out = countBy( arr, indicator );
// returns { 'b': 3, 'f': 1 }

An indicator function is provided two arguments:

value: collection element
index: collection index

function indicator( v, i ) {
    console.log( '%d: %d', i, v );
    return v[ 0 ];
}
var arr = [ 'beep', 'boop', 'foo', 'bar' ];

var out = countBy( arr, indicator );
// returns { 'b': 3, 'f': 1 }

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 = countBy( arr, opts, indicator );
// returns { 'b': 3, 'f': 1 }

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 = countBy( arr, indicator );
// returns { '[object Object]': 4 }
```
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 countBy = require( '@stdlib/utils-count-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 group counts:
out = countBy( 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-count-by

Usage no npm install needed!