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https://github.com/stdlib-js/utils-reduce-right

Apply a function against an accumulator and each element in a collection and return the accumulated result, iterating from right to left.
https://github.com/stdlib-js/utils-reduce-right

accumulate accumulation accumulator aggregate collection iterate javascript node node-js nodejs reduce reducer reduceright reduction right stdlib util utilities utility utils

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Apply a function against an accumulator and each element in a collection and return the accumulated result, iterating from right to left.

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README 

        



  

    About stdlib...

  

  
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# reduceRight



[![NPM version][npm-image]][npm-url] [![Build Status][test-image]][test-url] [![Coverage Status][coverage-image]][coverage-url] 



> Apply a function against an accumulator and each element in an array while iterating from right to left and return the accumulated result.



## Installation



```bash

npm install @stdlib/utils-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 reduceRight = require( '@stdlib/utils-reduce-right' );

```



#### reduceRight( arr, initial, reducer\[, thisArg ] )



Applies a function against an accumulator and each element in an array while iterating from right to left and returns the accumulated result.



```javascript

function sum( accumulator, value ) {

    return accumulator + value;

}



var arr = [ 1, 2, 3, 4 ];



var out = reduceRight( arr, 0, sum );

// returns 10

```



The function accepts both array-like objects and [`ndarray`][@stdlib/ndarray/ctor]-like objects.



```javascript

var array = require( '@stdlib/ndarray-array' );



function sum( accumulator, value ) {

    return accumulator + value;

}



var opts = {

    'dtype': 'generic'

};

var arr = array( [ [ 1, 2, 3 ], [ 4, 5, 6 ] ], opts );



var out = reduceRight( arr, 0, sum );

// returns 21

```



The applied function is provided the following arguments:



-   **accumulator**: accumulated value.

-   **value**: array element.

-   **index**: element index.

-   **arr**: input array.



To set the `this` context when invoking the input function, provide a `thisArg`.



```javascript

function sum( accumulator, value ) {

    this.count += 1;

    return accumulator + value;

}



var arr = [ 1, 2, 3, 4 ];



var ctx = {

    'count': 0

};



var out = reduceRight( arr, 0, sum, ctx );

// returns 10



var mean = out / ctx.count;

// returns 2.5

```



## Notes



-   For input arrays, the function differs from [`Array.prototype.reduceRight`][mdn-array-reduceright] in the following ways:



    -   The function **requires** an `initial` value for the `accumulator`. The `initial` value is used during the first invocation of the `reducer` function.

    -   The function does **not** skip the last array element.

    -   The function does **not** skip `undefined` elements.

    -   The function does **not** support dynamic array-like objects (i.e., array-like objects whose `length` changes during execution).



-   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 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( [ 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0 ] );



    var v = reduceRight( x, new Complex64( 0.0, 0.0 ), sum );

    // returns 



    var re = realf( v );

    // returns 16.0



    var im = imagf( v );

    // returns 20.0

    ```



-   For [`ndarray`][@stdlib/ndarray/ctor]-like objects, the function performs a reduction 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/math-base-ops-add' );

var array = require( '@stdlib/ndarray-array' );

var reduceRight = require( '@stdlib/utils-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 binary function:

var f = naryFunction( add, 2 );



// Compute the sum:

var out = reduceRight( x, 0, f );



console.log( 'x:' );

console.log( x.data );



console.log( 'sum: %d', out );

```



* * *



## See Also



-   [`@stdlib/utils-for-each-right`][@stdlib/utils/for-each-right]: invoke a function for each element in a collection, iterating from right to left.

-   [`@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-reduce`][@stdlib/utils/reduce]: apply a function against an accumulator and each element in an array and return the accumulated result.

-   [`@stdlib/utils-async/reduce-right`][@stdlib/utils/async/reduce-right]: apply a function against an accumulator and each element in a collection and return the accumulated result, iterating from right to left.



* * *



## 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 © 2016-2024. The Stdlib [Authors][stdlib-authors].



[npm-image]: http://img.shields.io/npm/v/@stdlib/utils-reduce-right.svg

[npm-url]: https://npmjs.org/package/@stdlib/utils-reduce-right



[test-image]: https://github.com/stdlib-js/utils-reduce-right/actions/workflows/test.yml/badge.svg?branch=main

[test-url]: https://github.com/stdlib-js/utils-reduce-right/actions/workflows/test.yml?query=branch:main



[coverage-image]: https://img.shields.io/codecov/c/github/stdlib-js/utils-reduce-right/main.svg

[coverage-url]: https://codecov.io/github/stdlib-js/utils-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-reduce-right/tree/deno

[deno-readme]: https://github.com/stdlib-js/utils-reduce-right/blob/deno/README.md

[umd-url]: https://github.com/stdlib-js/utils-reduce-right/tree/umd

[umd-readme]: https://github.com/stdlib-js/utils-reduce-right/blob/umd/README.md

[esm-url]: https://github.com/stdlib-js/utils-reduce-right/tree/esm

[esm-readme]: https://github.com/stdlib-js/utils-reduce-right/blob/esm/README.md

[branches-url]: https://github.com/stdlib-js/utils-reduce-right/blob/main/branches.md



[stdlib-license]: https://raw.githubusercontent.com/stdlib-js/utils-reduce-right/main/LICENSE



[mdn-array-reduceright]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/ReduceRight



[@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/for-each-right]: https://github.com/stdlib-js/utils-for-each-right



[@stdlib/utils/map-right]: https://github.com/stdlib-js/utils-map-right



[@stdlib/utils/reduce]: https://github.com/stdlib-js/utils-reduce



[@stdlib/utils/async/reduce-right]: https://github.com/stdlib-js/utils-async-reduce-right