npm package: @stdlib/ndarray-flatten-by
Description: Flatten an ndarray according to a callback function.
Author: The Stdlib Authors
Latest version: 0.1.1

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# flattenBy

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

> Flatten an [ndarray][@stdlib/ndarray/ctor] according to a callback function.

<section class="intro">

</section>

<!-- /.intro -->

<section class="installation">

## Installation

```bash
npm install @stdlib/ndarray-flatten-by
```

</section>

<section class="usage">

## Usage

```javascript
var flattenBy = require( '@stdlib/ndarray-flatten-by' );
```

#### flattenBy( x\[, options], fcn\[, thisArg] )

Flattens an [ndarray][@stdlib/ndarray/ctor] according to a callback function.

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

function scale( value ) {
    return value * 2.0;
}

var x = array( [ [ [ 1.0, 2.0 ] ], [ [ 3.0, 4.0 ] ], [ [ 5.0, 6.0 ] ] ] );
// returns <ndarray>

var y = flattenBy( x, scale );
// returns <ndarray>[ 2.0, 4.0, 6.0, 8.0, 10.0, 12.0 ]
```

The function accepts the following arguments:

-   **x**: input [ndarray][@stdlib/ndarray/ctor].
-   **options**: function options (_optional_).
-   **fcn**: callback function.
-   **thisArg**: callback execution context (_optional_).

The function accepts the following options:

-   **order**: order in which input [ndarray][@stdlib/ndarray/ctor] elements should be flattened. Must be one of the following:

    -   `'row-major'`: flatten elements in lexicographic order. For example, given a two-dimensional input [ndarray][@stdlib/ndarray/ctor] (i.e., a matrix), flattening in lexicographic order means flattening the input [ndarray][@stdlib/ndarray/ctor] row-by-row.
    -   `'column-major'`: flatten elements in colexicographic order. For example, given a two-dimensional input [ndarray][@stdlib/ndarray/ctor] (i.e., a matrix), flattening in colexicographic order means flattening the input [ndarray][@stdlib/ndarray/ctor] column-by-column.
    -   `'any'`: flatten according to the physical layout of the input [ndarray][@stdlib/ndarray/ctor] data in memory, regardless of the stated [order][@stdlib/ndarray/orders] of the input [ndarray][@stdlib/ndarray/ctor].
    -   `'same'`: flatten according to the stated [order][@stdlib/ndarray/orders] of the input [ndarray][@stdlib/ndarray/ctor].

    Default: `'row-major'`.

-   **depth**: maximum number of input [ndarray][@stdlib/ndarray/ctor] dimensions to flatten.

-   **dtype**: output ndarray [data type][@stdlib/ndarray/dtypes]. By default, the function returns an [ndarray][@stdlib/ndarray/ctor] having the same [data type][@stdlib/ndarray/dtypes] as a provided input [ndarray][@stdlib/ndarray/ctor].

By default, the function flattens all dimensions of the input [ndarray][@stdlib/ndarray/ctor]. To flatten to a desired depth, specify the `depth` option.

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

function scale( value ) {
    return value * 2.0;
}

var x = array( [ [ [ 1.0, 2.0 ] ], [ [ 3.0, 4.0 ] ], [ [ 5.0, 6.0 ] ] ] );
// returns <ndarray>

var opts = {
    'depth': 1
};

var y = flattenBy( x, opts, scale );
// returns <ndarray>[ [ 2.0, 4.0 ], [ 6.0, 8.0 ], [ 10.0, 12.0 ] ]
```

By default, the input [ndarray][@stdlib/ndarray/ctor] is flattened in lexicographic order. To flatten elements in a different order, specify the `order` option.

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

function scale( value ) {
    return value * 2.0;
}

var x = array( [ [ [ 1.0, 2.0 ] ], [ [ 3.0, 4.0 ] ], [ [ 5.0, 6.0 ] ] ] );
// returns <ndarray>

var opts = {
    'order': 'column-major'
};

var y = flattenBy( x, opts, scale );
// returns <ndarray>[ 2.0, 6.0, 10.0, 4.0, 8.0, 12.0 ]
```

By default, the output ndarray [data type][@stdlib/ndarray/dtypes] is inferred from the input [ndarray][@stdlib/ndarray/ctor]. To return an ndarray with a different [data type][@stdlib/ndarray/dtypes], specify the `dtype` option.

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

function scale( value ) {
    return value * 2.0;
}

var x = array( [ [ [ 1.0, 2.0 ] ], [ [ 3.0, 4.0 ] ], [ [ 5.0, 6.0 ] ] ] );
// returns <ndarray>

var opts = {
    'dtype': 'float32'
};
var y = flattenBy( x, opts, scale );
// returns <ndarray>[ 2.0, 4.0, 6.0, 8.0, 10.0, 12.0 ]

var dt = String( dtype( y ) );
// returns 'float32'
```

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

<!-- eslint-disable no-invalid-this, max-len -->

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

function scale( value ) {
    this.count += 1;
    return value * 2.0;
}

var x = array( [ [ [ 1.0, 2.0 ] ], [ [ 3.0, 4.0 ] ], [ [ 5.0, 6.0 ] ] ] );
// returns <ndarray>

var ctx = {
    'count': 0
};

var y = flattenBy( x, scale, ctx );
// returns <ndarray>[ 2.0, 4.0, 6.0, 8.0, 10.0, 12.0 ]

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

</section>

<!-- /.usage -->

<section class="notes">

## Notes

-   The function **always** returns a copy of input [ndarray][@stdlib/ndarray/ctor] data, even when an input [ndarray][@stdlib/ndarray/ctor] already has the desired number of dimensions.

-   The callback function is provided the following arguments:

    -   **value**: current array element.
    -   **indices**: current array element indices.
    -   **arr**: the input [ndarray][@stdlib/ndarray/ctor].

-   The order in which array elements are traversed and passed to a provided callback function is **not** guaranteed to match the order of array elements in an [ndarray][@stdlib/ndarray/ctor] view. Accordingly, a provided callback should avoid making assumptions regarding the order of provided elements.

</section>

<!-- /.notes -->

<section class="examples">

## Examples

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

```javascript
var discreteUniform = require( '@stdlib/random-array-discrete-uniform' );
var array = require( '@stdlib/ndarray-array' );
var ndarray2array = require( '@stdlib/ndarray-to-array' );
var flattenBy = require( '@stdlib/ndarray-flatten-by' );

function scale( value ) {
    return value * 2.0;
}

var xbuf = discreteUniform( 12, -100, 100, {
    'dtype': 'generic'
});

var x = array( xbuf, {
    'shape': [ 2, 2, 3 ],
    'dtype': 'generic'
});
console.log( ndarray2array( x ) );

var y = flattenBy( x, scale );
console.log( ndarray2array( y ) );
```

</section>

<!-- /.examples -->

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

<section class="related">

</section>

<!-- /.related -->


<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].

</section>

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