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

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# flattenFromBy

[![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 starting from a specified dimension.

<section class="intro">

</section>

<!-- /.intro -->

<section class="installation">

## Installation

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

</section>

<section class="usage">

## Usage

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

#### flattenFromBy( x, dim\[, options], fcn\[, thisArg] )

Flattens an [ndarray][@stdlib/ndarray/ctor] according to a callback function starting from a specified dimension.

```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 = flattenFromBy( x, 1, 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]. Must have one or more dimensions.
-   **dim**: dimension to start flattening from. If provided an integer less than zero, the dimension index is resolved relative to the last dimension, with the last dimension corresponding to the value `-1`.
-   **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'`.

-   **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 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 = flattenFromBy( x, 0, 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 = flattenFromBy( x, 0, 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 = flattenFromBy( x, 1, 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 flattenFromBy = require( '@stdlib/ndarray-flatten-from-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 = flattenFromBy( x, 1, 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>

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<section class="main-repo" >

* * *

## Notice

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## License

See [LICENSE][stdlib-license].


## Copyright

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

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