npm package: @stdlib/array-base-flatten2d-by
Description: Flatten a two-dimensional nested array according to a callback function.
Author: The Stdlib Authors
Latest version: 0.2.3

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# flatten2dBy

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

> Flatten a two-dimensional nested array according to a callback function.

<section class="installation">

## Installation

```bash
npm install @stdlib/array-base-flatten2d-by
```

</section>

<section class="usage">

## Usage

```javascript
var flatten2dBy = require( '@stdlib/array-base-flatten2d-by' );
```

#### flatten2dBy( x, shape, colexicographic, clbk\[, thisArg] )

Flattens a two-dimensional nested array according to a callback function.

```javascript
function scale( v ) {
    return v * 2;
}

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

var out = flatten2dBy( x, [ 2, 2 ], false, scale );
// returns [ 2, 4, 6, 8 ]
```

To flatten in colexicographic order, provide a third argument equal to `true`.

```javascript
function scale( v ) {
    return v * 2;
}

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

var out = flatten2dBy( x, [ 2, 2 ], true, scale );
// returns [ 2, 6, 4, 8 ]
```

To set the callback execution context, provide a `thisArg` argument.

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

```javascript
function scale( v ) {
    this.count += 1;
    return v * 2;
}

var x = [ [ 1, 2 ], [ 3, 4 ] ];
var ctx = {
    'count': 0
};

var out = flatten2dBy( x, [ 2, 2 ], false, scale, ctx );
// returns [ 2, 4, 6, 8 ]

var count = ctx.count;
// returns 4
```

#### flatten2dBy.assign( x, shape, colexicographic, out, stride, offset, clbk\[, thisArg] )

Flattens a two-dimensional nested array according to a callback function and assigns elements to a provided output array.

```javascript
var Float64Array = require( '@stdlib/array-float64' );

function scale( v ) {
    return v * 2;
}

var x = [ [ 1, 2 ], [ 3, 4 ] ];
var out = new Float64Array( 4 );

var y = flatten2dBy.assign( x, [ 2, 2 ], false, out, 1, 0, scale );
// returns <Float64Array>[ 2, 4, 6, 8 ]

var bool = ( y === out );
// returns true

y = flatten2dBy.assign( x, [ 2, 2 ], true, out, 1, 0, scale );
// returns <Float64Array>[ 2, 6, 4, 8 ]
```

</section>

<!-- /.usage -->

<section class="notes">

## Notes

-   A callback function is provided the following arguments:

    -   **value**: nested array element.
    -   **indices**: element indices (in lexicographic order).
    -   **arr**: the input array.

-   Both functions assume that all nested arrays have the same length (i.e., the input array is **not** a ragged array).

</section>

<!-- /.notes -->

<section class="examples">

## Examples

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

```javascript
var naryFunction = require( '@stdlib/utils-nary-function' );
var abs = require( '@stdlib/math-base-special-abs' );
var flatten2dBy = require( '@stdlib/array-base-flatten2d-by' );

var fcn = naryFunction( abs, 1 );

var x = [
    [ -1, -2, -3, -4 ],
    [ -5, -6, -7, -8 ],
    [ -9, -10, -11, -12 ],
    [ -13, -14, -15, -16 ]
];

var out = flatten2dBy( x, [ 0, 0 ], false, fcn );
// returns []

out = flatten2dBy( x, [ 0, 0 ], true, fcn );
// returns []

out = flatten2dBy( x, [ 1, 1 ], false, fcn );
// returns [ 1 ]

out = flatten2dBy( x, [ 1, 1 ], true, fcn );
// returns [ 1 ]

out = flatten2dBy( x, [ 2, 2 ], false, fcn );
// returns [ 1, 2, 5, 6 ]

out = flatten2dBy( x, [ 2, 2 ], true, fcn );
// returns [ 1, 5, 2, 6 ]

out = flatten2dBy( x, [ 3, 3 ], false, fcn );
// returns [ 1, 2, 3, 5, 6, 7, 9, 10, 11 ]

out = flatten2dBy( x, [ 3, 3 ], true, fcn );
// returns [ 1, 5, 9, 2, 6, 10, 3, 7, 11 ]

out = flatten2dBy( x, [ 4, 4 ], false, fcn );
// returns [ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16 ]

out = flatten2dBy( x, [ 4, 4 ], true, fcn );
// returns [ 1, 5, 9, 13, 2, 6, 10, 14, 3, 7, 11, 15, 4, 8, 12, 16 ]
```

</section>

<!-- /.examples -->

<!-- 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/array-base/flatten2d`][@stdlib/array/base/flatten2d]</span><span class="delimiter">: </span><span class="description">flatten a two-dimensional nested array.</span>

</section>

<!-- /.related -->

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* * *

## 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].

</section>

<!-- /.stdlib -->

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