npm package: @stdlib/utils-reduce2d
Description: Reduce the number of dimensions by one of a two-dimensional nested array by applying a function against an accumulator and each element along the innermost dimension and returning the accumulation results as a one-dimensional array.
Author: The Stdlib Authors
Latest version: 0.2.3

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# reduce2d

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> Reduce the number of dimensions by one of a two-dimensional nested array by applying a function against an accumulator and each element along the innermost dimension and returning the accumulation results as a one-dimensional array.

<!-- Section to include introductory text. Make sure to keep an empty line after the intro `section` element and another before the `/section` close. -->

<section class="intro">

</section>

<!-- /.intro -->

<!-- Package usage documentation. -->

<section class="installation">

## Installation

```bash
npm install @stdlib/utils-reduce2d
```

</section>

<section class="usage">

## Usage

```javascript
var reduce2d = require( '@stdlib/utils-reduce2d' );
```

#### reduce2d( arr, initial, fcn\[, thisArg] )

Reduces the number of dimensions by one of a two-dimensional nested array by applying a function against an accumulator and each element along the innermost dimension and returning the accumulation results as a one-dimensional array.

```javascript
var naryFunction = require( '@stdlib/utils-nary-function' );
var add = require( '@stdlib/number-float64-base-add' );

var arr = [
    [ 1, 2, 3 ],
    [ 4, 5, 6 ]
];

var out = reduce2d( arr, [ 0, 0 ], naryFunction( add, 2 ) );
// returns [ 6, 15 ]
```

The applied function is provided the following arguments:

-   **accumulator**: accumulated value.
-   **value**: array element.
-   **indices**: current array element indices.
-   **arr**: input array.

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

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

```javascript
var add = require( '@stdlib/number-float64-base-add' );

function fcn( acc, v ) {
    this.count += 1;
    return add( acc, v );
}

var arr = [
    [ 1, 2, 3 ],
    [ 4, 5, 6 ]
];

var ctx = {
    'count': 0
};

var out = reduce2d( arr, [ 0, 0 ], fcn, ctx );
// returns [ 6, 15 ]

var cnt = ctx.count;
// returns 6
```

</section>

<!-- /.usage -->

<!-- Package usage notes. Make sure to keep an empty line after the `section` element and another before the `/section` close. -->

<section class="notes">

## Notes

-   The function **requires** an array-like object containing an `initial` value for the accumulator for each reduction. The number of `initial` values must equal the size of the outermost input array dimension.

</section>

<!-- /.notes -->

<!-- Package usage examples. -->

<section class="examples">

## 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/number-float64-base-add' );
var zeros = require( '@stdlib/array-base-zeros' );
var reduce2d = require( '@stdlib/utils-reduce2d' );

function fill( i ) {
    var rand = discreteUniform( -10*(i+1), 10*(i+1) );
    return filledarrayBy( 10, 'float64', rand );
}

// Create a nested array of arrays:
var x = filledarrayBy( 10, 'generic', fill );

// Create an explicit binary function:
var f = naryFunction( add, 2 );

// Compute the sums along the innermost dimension...
var y = reduce2d( x, zeros( x.length ), f );

console.log( 'x:' );
console.log( x );

console.log( 'y:' );
console.log( y );
```

</section>

<!-- /.examples -->

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<section class="references">

</section>

<!-- /.references -->

<!-- Section for related `stdlib` packages. Do not manually edit this section, as it is automatically populated. -->

<section class="related">

* * *

## See Also

-   <span class="package-name">[`@stdlib/utils-map2d`][@stdlib/utils/map2d]</span><span class="delimiter">: </span><span class="description">apply a function to each nested element in an array of arrays and assign the result to a nested element in a new array of arrays.</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>

</section>

<!-- /.related -->

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<section class="main-repo" >

* * *

## 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 &copy; 2016-2026. The Stdlib [Authors][stdlib-authors].

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

[@stdlib/utils/map2d]: https://www.npmjs.com/package/@stdlib/utils-map2d

[@stdlib/utils/reduce]: https://www.npmjs.com/package/@stdlib/utils-reduce

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</section>

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