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# flattenBy

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

<section class="intro">

</section>

<!-- /.intro -->

<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="links">

[@stdlib/ndarray/ctor]: https://github.com/stdlib-js/ndarray/tree/main/ctor

[@stdlib/ndarray/dtypes]: https://github.com/stdlib-js/ndarray/tree/main/dtypes

[@stdlib/ndarray/orders]: https://github.com/stdlib-js/ndarray/tree/main/orders

</section>

<!-- /.links -->
