@stdlib/utils/reduce2d/README.md

Standard utilities.

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# reduce2d

> 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.

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## 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/math/base/ops/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/math/base/ops/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
```

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

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## 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.

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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 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 );
```

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* * *

## 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>

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[@stdlib/utils/map2d]: https://github.com/stdlib-js/utils/tree/main/map2d

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

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