@stdlib/utils/reduce-right/README.md

Standard utilities.

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# reduceRight

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

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## 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`.

<!-- eslint-disable no-invalid-this -->

```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
```

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## 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 <Complex64>

    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.

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## Examples

<!-- eslint no-undef: "error" -->

```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 );
```

</section>

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* * *

## See Also

-   <span class="package-name">[`@stdlib/utils/for-each-right`][@stdlib/utils/for-each-right]</span><span class="delimiter">: </span><span class="description">invoke a function for each element in a collection, iterating from right to left.</span>
-   <span class="package-name">[`@stdlib/utils/map-right`][@stdlib/utils/map-right]</span><span class="delimiter">: </span><span class="description">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.</span>
-   <span class="package-name">[`@stdlib/utils/reduce`][@stdlib/utils/reduce]</span><span class="delimiter">: </span><span class="description">apply a function against an accumulator and each element in an array and return the accumulated result.</span>
-   <span class="package-name">[`@stdlib/utils/async/reduce-right`][@stdlib/utils/async/reduce-right]</span><span class="delimiter">: </span><span class="description">apply a function against an accumulator and each element in a collection and return the accumulated result, iterating from right to left.</span>

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

[@stdlib/ndarray/ctor]: https://www.npmjs.com/package/@stdlib/ndarray-ctor

[@stdlib/array/complex64]: https://www.npmjs.com/package/@stdlib/array-complex64

[@stdlib/array/complex128]: https://www.npmjs.com/package/@stdlib/array-complex128

<!-- <related-links> -->

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

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

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

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

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</section>

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