README.md

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groupIndicesBy

Group element indices according to an indicator function.

Usage

import groupIndicesBy from 'https://cdn.jsdelivr.net/gh/stdlib-js/array-base-group-indices-by@deno/mod.js';

groupIndicesBy( x, indicator[, thisArg] )

Groups element indices according to an indicator function.

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

var x = [ 'beep', 'boop', 'foo', 'bar' ];

var out = groupIndicesBy( x, indicator );
// returns { 'b': [ 0, 1, 3 ], 'f': [ 2 ] }

An indicator function is provided the following arguments:

• value: current array element.
• index: current array element index.
• arr: input array.

To set the indicator function execution context, provide a thisArg.

function indicator( v ) {
    this.count += 1;
    return v[ 0 ];
}

var x = [ 'beep', 'boop', 'foo', 'bar' ];

var context = {
    'count': 0
};
var out = groupIndicesBy( x, indicator, context );
// returns { 'b': [ 0, 1, 3 ], 'f': [ 2 ] }

var cnt = context.count;
// returns 4

Notes

• 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 x = [ 'beep', 'boop', 'foo', 'bar' ];
  
  var out = groupIndicesBy( x, indicator );
  // returns { '[object Object]': [ 0, 1, 2, 3 ] }

  while each group identifier is unique, all input array elements resolve to the same group because each group identifier serializes to the same string.

Examples

import discreteUniform from 'https://cdn.jsdelivr.net/gh/stdlib-js/random-array-discrete-uniform@deno/mod.js';
import take from 'https://cdn.jsdelivr.net/gh/stdlib-js/array-base-take-indexed@deno/mod.js';
import groupIndicesBy from 'https://cdn.jsdelivr.net/gh/stdlib-js/array-base-group-indices-by@deno/mod.js';

function indicator( v ) {
    // Use the first letter of each element to define groups:
    return v[ 0 ];
}

// Define an initial array of values:
var values = [ 'beep', 'boop', 'foo', 'bar', 'woot', 'woot' ];

// Sample from the initial array to generate a random collection:
var indices = discreteUniform( 100, 0, values.length-1, {
    'dtype': 'generic'
});
var x = take( values, indices );
// returns [...]

// Group the values:
var out = groupIndicesBy( x, indicator );
// returns {...}

console.log( out );

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Notice

This package is part of stdlib, a standard library 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

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License

See LICENSE.

Copyright

Copyright © 2016-2024. The Stdlib Authors.