npm package: @stdlib/ndarray-base-unary-reduce-subarray-by
Description: Perform a reduction over a list of specified dimensions in an input ndarray according to a callback function and assign results to a provided output ndarray.
Author: The Stdlib Authors
Latest version: 0.1.1

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# unaryReduceSubarrayBy

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

> Perform a reduction over a list of specified dimensions in an input ndarray according to a callback function and assign results to a provided output ndarray.

<section class="intro">

</section>

<!-- /.intro -->

<section class="installation">

## Installation

```bash
npm install @stdlib/ndarray-base-unary-reduce-subarray-by
```

</section>

<section class="usage">

## Usage

```javascript
var unaryReduceSubarrayBy = require( '@stdlib/ndarray-base-unary-reduce-subarray-by' );
```

#### unaryReduceSubarrayBy( fcn, arrays, dims\[, options], clbk\[, thisArg] )

Performs a reduction over a list of specified dimensions in an input ndarray according to a callback function and assigns results to a provided output ndarray.

<!-- eslint-disable max-len -->

```javascript
var Float64Array = require( '@stdlib/array-float64' );
var filled = require( '@stdlib/array-base-filled' );
var ndarray2array = require( '@stdlib/ndarray-base-to-array' );
var everyBy = require( '@stdlib/ndarray-base-every-by' );

function clbk( value ) {
    return value > 0.0;
}

// Create data buffers:
var xbuf = new Float64Array( [ 1.0, 2.0, 3.0, 4.0, 5.0, 0.0, 7.0, 8.0, 9.0, 10.0, 11.0, 12.0 ] );
var ybuf = filled( false, 3 );

// Define the array shapes:
var xsh = [ 1, 3, 2, 2 ];
var ysh = [ 1, 3 ];

// Define the array strides:
var sx = [ 12, 4, 2, 1 ];
var sy = [ 3, 1 ];

// Define the index offsets:
var ox = 0;
var oy = 0;

// Create an input ndarray-like object:
var x = {
    'dtype': 'float64',
    'data': xbuf,
    'shape': xsh,
    'strides': sx,
    'offset': ox,
    'order': 'row-major'
};

// Create an output ndarray-like object:
var y = {
    'dtype': 'generic',
    'data': ybuf,
    'shape': ysh,
    'strides': sy,
    'offset': oy,
    'order': 'row-major'
};

// Perform a reduction:
unaryReduceSubarrayBy( everyBy, [ x, y ], [ 2, 3 ], clbk );

var arr = ndarray2array( y.data, y.shape, y.strides, y.offset, y.order );
// returns [ [ true, false, true ] ]
```

The function accepts the following arguments:

-   **fcn**: function which will be applied to a subarray and should reduce the subarray to a single scalar value.
-   **arrays**: array-like object containing one input ndarray and one output ndarray, followed by any additional ndarray arguments.
-   **dims**: list of dimensions over which to perform a reduction.
-   **options**: function options which are passed through to `fcn` (_optional_).
-   **clbk**: callback function.
-   **thisArg**: callback execution context (_optional_).

Each provided ndarray should be an object with the following properties:

-   **dtype**: data type.
-   **data**: data buffer.
-   **shape**: dimensions.
-   **strides**: stride lengths.
-   **offset**: index offset.
-   **order**: specifies whether an ndarray is row-major (C-style) or column major (Fortran-style).

The invoked callback function is provided the following arguments:

-   **value**: input array element.
-   **indices**: current array element indices.
-   **arr**: the input ndarray.

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

<!-- eslint-disable max-len -->

```javascript
var Float64Array = require( '@stdlib/array-float64' );
var filled = require( '@stdlib/array-base-filled' );
var ndarray2array = require( '@stdlib/ndarray-base-to-array' );
var everyBy = require( '@stdlib/ndarray-base-every-by' );

function clbk( value ) {
    this.count += 1;
    return value > 0.0;
}

// Create data buffers:
var xbuf = new Float64Array( [ 1.0, 2.0, 3.0, 4.0, 5.0, 0.0, 7.0, 8.0, 9.0, 10.0, 11.0, 12.0 ] );
var ybuf = filled( false, 6 );

// Define the array shapes:
var xsh = [ 3, 2, 2 ];
var ysh = [ 3, 2 ];

// Define the array strides:
var sx = [ 4, 2, 1 ];
var sy = [ 2, 1 ];

// Define the index offsets:
var ox = 0;
var oy = 0;

// Create an input ndarray-like object:
var x = {
    'dtype': 'float64',
    'data': xbuf,
    'shape': xsh,
    'strides': sx,
    'offset': ox,
    'order': 'row-major'
};

// Create an output ndarray-like object:
var y = {
    'dtype': 'generic',
    'data': ybuf,
    'shape': ysh,
    'strides': sy,
    'offset': oy,
    'order': 'row-major'
};

var ctx = {
    'count': 0
};

// Perform a reduction:
unaryReduceSubarrayBy( everyBy, [ x, y ], [ 1 ], clbk, ctx );

var arr = ndarray2array( y.data, y.shape, y.strides, y.offset, y.order );
// returns [ [ true, true ], [ true, false ], [ true, true ] ]

var count = ctx.count;
// returns 11
```

#### TODO: document factory method

</section>

<!-- /.usage -->

<section class="notes">

## Notes

-   The output ndarray and any additional ndarray arguments are expected to have the same dimensions as the non-reduced dimensions of the input ndarray. When calling the reduction function, any additional ndarray arguments are provided as zero-dimensional ndarray-like objects.

-   The reduction function is expected to have the following signature:

    ```text
    fcn( arrays[, options], wrappedCallback )
    ```

    where

    -   **arrays**: array containing a subarray of the input ndarray and any additional ndarray arguments as zero-dimensional ndarrays.
    -   **options**: function options (_optional_).
    -   **wrappedCallback**: callback function. This function is a wrapper around a provided `clbk` argument.

-   For very high-dimensional ndarrays which are non-contiguous, one should consider copying the underlying data to contiguous memory before performing a reduction in order to achieve better performance.

</section>

<!-- /.notes -->

<section class="examples">

## Examples

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

```javascript
var discreteUniform = require( '@stdlib/random-array-discrete-uniform' );
var filled = require( '@stdlib/array-base-filled' );
var ndarray2array = require( '@stdlib/ndarray-base-to-array' );
var everyBy = require( '@stdlib/ndarray-base-every-by' );
var unaryReduceSubarrayBy = require( '@stdlib/ndarray-base-unary-reduce-subarray-by' );

function clbk( value ) {
    return value > -3;
}

var x = {
    'dtype': 'generic',
    'data': discreteUniform( 40, -5, 5, {
        'dtype': 'generic'
    }),
    'shape': [ 2, 5, 2, 2 ],
    'strides': [ 1, 2, 10, 20 ],
    'offset': 0,
    'order': 'column-major'
};
var y = {
    'dtype': 'generic',
    'data': filled( false, 10 ),
    'shape': [ 2, 5 ],
    'strides': [ 1, 2 ],
    'offset': 0,
    'order': 'column-major'
};

unaryReduceSubarrayBy( everyBy, [ x, y ], [ 2, 3 ], clbk );

console.log( ndarray2array( x.data, x.shape, x.strides, x.offset, x.order ) );
console.log( ndarray2array( y.data, y.shape, y.strides, y.offset, y.order ) );
```

</section>

<!-- /.examples -->

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

<section class="related">

</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].

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---

## License

See [LICENSE][stdlib-license].


## Copyright

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

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