https://github.com/stdlib-js/utils-map-reduce-right
Perform a single-pass map-reduce operation against each element in an array while iterating from right to left and return the accumulated result.
https://github.com/stdlib-js/utils-map-reduce-right
accumulate accumulation accumulator aggregate iterate javascript map map-reduce node node-js nodejs reduce reducer reduction stdlib transform util utilities utility utils
Last synced: 27 days ago
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Perform a single-pass map-reduce operation against each element in an array while iterating from right to left and return the accumulated result.
- Host: GitHub
- URL: https://github.com/stdlib-js/utils-map-reduce-right
- Owner: stdlib-js
- License: apache-2.0
- Created: 2022-01-23T07:08:04.000Z (over 3 years ago)
- Default Branch: main
- Last Pushed: 2025-04-07T00:43:01.000Z (about 2 months ago)
- Last Synced: 2025-04-12T06:57:41.212Z (about 2 months ago)
- Topics: accumulate, accumulation, accumulator, aggregate, iterate, javascript, map, map-reduce, node, node-js, nodejs, reduce, reducer, reduction, stdlib, transform, util, utilities, utility, utils
- Language: JavaScript
- Homepage: https://github.com/stdlib-js/stdlib
- Size: 2.74 MB
- Stars: 2
- Watchers: 3
- Forks: 0
- Open Issues: 0
-
Metadata Files:
- Readme: README.md
- Changelog: CHANGELOG.md
- Contributing: CONTRIBUTING.md
- License: LICENSE
- Code of conduct: CODE_OF_CONDUCT.md
- Citation: CITATION.cff
- Security: SECURITY.md
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README
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# mapReduceRight
[![NPM version][npm-image]][npm-url] [![Build Status][test-image]][test-url] [![Coverage Status][coverage-image]][coverage-url]
> Perform a single-pass map-reduce operation against each element in an array while iterating from right to left and return the accumulated result.
## Installation
```bash
npm install @stdlib/utils-map-reduce-right
```
Alternatively,
- To load the package in a website via a `script` tag without installation and bundlers, use the [ES Module][es-module] available on the [`esm`][esm-url] branch (see [README][esm-readme]).
- If you are using Deno, visit the [`deno`][deno-url] branch (see [README][deno-readme] for usage intructions).
- For use in Observable, or in browser/node environments, use the [Universal Module Definition (UMD)][umd] build available on the [`umd`][umd-url] branch (see [README][umd-readme]).
The [branches.md][branches-url] file summarizes the available branches and displays a diagram illustrating their relationships.
To view installation and usage instructions specific to each branch build, be sure to explicitly navigate to the respective README files on each branch, as linked to above.
## Usage
```javascript
var mapReduceRight = require( '@stdlib/utils-map-reduce-right' );
```
#### mapReduceRight( arr, initial, mapper, reducer\[, thisArg ] )
Performs a map-reduce operation against each element in an array while iterating from right to left and returns the accumulated result.
```javascript
function square( value ) {
return value * value;
}
function sum( accumulator, value ) {
return accumulator + value;
}
var arr = [ 1, 2, 3, 4 ];
var out = mapReduceRight( arr, 0, square, sum );
// returns 30
```
The function accepts both array-like objects and [`ndarray`][@stdlib/ndarray/ctor]-like objects.
```javascript
var array = require( '@stdlib/ndarray-array' );
function square( value ) {
return value * value;
}
function sum( accumulator, value ) {
return accumulator + value;
}
var opts = {
'dtype': 'generic'
};
var arr = array( [ [ 1, 2, 3 ], [ 4, 5, 6 ] ], opts );
var out = mapReduceRight( arr, 0, square, sum );
// returns 91
```
The mapping function is provided the following arguments:
- **value**: array element.
- **index**: element index.
- **arr**: input array.
The reducing function is provided the following arguments:
- **accumulator**: accumulated value.
- **value**: result of applying the mapping function to the current array element.
- **index**: element index.
- **arr**: input array.
To set the `this` context when invoking the reducing function, provide a `thisArg`.
```javascript
function square( value ) {
return value * value;
}
function sum( accumulator, value ) {
this.count += 1;
return accumulator + value;
}
var arr = [ 1, 2, 3, 4 ];
var ctx = {
'count': 0
};
var out = mapReduceRight( arr, 0, square, sum, ctx );
// returns 30
var mean = out / ctx.count;
// returns 7.5
```
## Notes
- The function supports array-like objects exposing getters and setters for array element access (e.g., [`Complex64Array`][@stdlib/array/complex64], [`Complex128Array`][@stdlib/array/complex128], etc).
```javascript
var Complex64Array = require( '@stdlib/array-complex64' );
var Complex64 = require( '@stdlib/complex-float32-ctor' );
var cceil = require( '@stdlib/math-base-special-cceil' );
var realf = require( '@stdlib/complex-float32-real' );
var imagf = require( '@stdlib/complex-float32-imag' );
function sum( acc, z ) {
var re1 = realf( acc );
var im1 = imagf( acc );
var re2 = realf( z );
var im2 = imagf( z );
return new Complex64( re1+re2, im1+im2 );
}
var x = new Complex64Array( [ 1.5, 2.5, 3.5, 4.5, 5.5, 6.5, 7.5, 8.5 ] );
var v = mapReduceRight( x, new Complex64( 0.0, 0.0 ), cceil, sum );
// returns
var re = realf( v );
// returns 20.0
var im = imagf( v );
// returns 24.0
```
- For [`ndarray`][@stdlib/ndarray/ctor]-like objects, the function performs a single-pass map-reduce operation over the entire input [`ndarray`][@stdlib/ndarray/ctor] (i.e., higher-order [`ndarray`][@stdlib/ndarray/ctor] dimensions are flattened to a single-dimension).
- When applying a function to [`ndarray`][@stdlib/ndarray/ctor]-like objects, performance will be best for [`ndarray`][@stdlib/ndarray/ctor]-like objects which are single-segment contiguous.
## Examples
```javascript
var filledarrayBy = require( '@stdlib/array-filled-by' );
var discreteUniform = require( '@stdlib/random-base-discrete-uniform' ).factory;
var naryFunction = require( '@stdlib/utils-nary-function' );
var add = require( '@stdlib/number-float64-base-add' );
var abs = require( '@stdlib/math-base-special-abs' );
var array = require( '@stdlib/ndarray-array' );
var mapReduceRight = require( '@stdlib/utils-map-reduce-right' );
function fill( i ) {
var rand = discreteUniform( -10*(i+1), 10*(i+1) );
return filledarrayBy( 10, 'generic', rand );
}
// Create a two-dimensional ndarray (i.e., a matrix):
var x = array( filledarrayBy( 10, 'generic', fill ), {
'dtype': 'generic',
'flatten': true
});
// Create an explicit unary function:
var f1 = naryFunction( abs, 1 );
// Create an explicit binary function:
var f2 = naryFunction( add, 2 );
// Compute the sum of absolute values:
var out = mapReduceRight( x, 0, f1, f2 );
console.log( 'x:' );
console.log( x.data );
console.log( 'sum: %d', out );
```
* * *
## See Also
- [`@stdlib/utils-map-right`][@stdlib/utils/map-right]: apply a function to each element in an array and assign the result to an element in an output array, iterating from right to left.
- [`@stdlib/utils-map-reduce`][@stdlib/utils/map-reduce]: perform a single-pass map-reduce operation against each element in an array and return the accumulated result.
- [`@stdlib/utils-reduce-right`][@stdlib/utils/reduce-right]: apply a function against an accumulator and each element in an array while iterating from right to left and return the accumulated result.
* * *
## Notice
This package is part of [stdlib][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][stdlib], see the main project [repository][stdlib].
#### Community
[![Chat][chat-image]][chat-url]
---
## License
See [LICENSE][stdlib-license].
## Copyright
Copyright © 2016-2025. The Stdlib [Authors][stdlib-authors].
[npm-image]: http://img.shields.io/npm/v/@stdlib/utils-map-reduce-right.svg
[npm-url]: https://npmjs.org/package/@stdlib/utils-map-reduce-right
[test-image]: https://github.com/stdlib-js/utils-map-reduce-right/actions/workflows/test.yml/badge.svg?branch=main
[test-url]: https://github.com/stdlib-js/utils-map-reduce-right/actions/workflows/test.yml?query=branch:main
[coverage-image]: https://img.shields.io/codecov/c/github/stdlib-js/utils-map-reduce-right/main.svg
[coverage-url]: https://codecov.io/github/stdlib-js/utils-map-reduce-right?branch=main
[chat-image]: https://img.shields.io/gitter/room/stdlib-js/stdlib.svg
[chat-url]: https://app.gitter.im/#/room/#stdlib-js_stdlib:gitter.im
[stdlib]: https://github.com/stdlib-js/stdlib
[stdlib-authors]: https://github.com/stdlib-js/stdlib/graphs/contributors
[umd]: https://github.com/umdjs/umd
[es-module]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Modules
[deno-url]: https://github.com/stdlib-js/utils-map-reduce-right/tree/deno
[deno-readme]: https://github.com/stdlib-js/utils-map-reduce-right/blob/deno/README.md
[umd-url]: https://github.com/stdlib-js/utils-map-reduce-right/tree/umd
[umd-readme]: https://github.com/stdlib-js/utils-map-reduce-right/blob/umd/README.md
[esm-url]: https://github.com/stdlib-js/utils-map-reduce-right/tree/esm
[esm-readme]: https://github.com/stdlib-js/utils-map-reduce-right/blob/esm/README.md
[branches-url]: https://github.com/stdlib-js/utils-map-reduce-right/blob/main/branches.md
[stdlib-license]: https://raw.githubusercontent.com/stdlib-js/utils-map-reduce-right/main/LICENSE
[@stdlib/ndarray/ctor]: https://github.com/stdlib-js/ndarray-ctor
[@stdlib/array/complex64]: https://github.com/stdlib-js/array-complex64
[@stdlib/array/complex128]: https://github.com/stdlib-js/array-complex128
[@stdlib/utils/map-right]: https://github.com/stdlib-js/utils-map-right
[@stdlib/utils/map-reduce]: https://github.com/stdlib-js/utils-map-reduce
[@stdlib/utils/reduce-right]: https://github.com/stdlib-js/utils-reduce-right