npm package: @stdlib/strided-base-map-by2
Description: Apply a binary function to each pair of elements retrieved from strided input arrays according to a callback function and assign results to a strided output array.
Author: The Stdlib Authors
Latest version: 0.2.3

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# mapBy2

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

> Apply a binary function to each pair of elements retrieved from strided input arrays according to a callback function and assign results to a strided output array.

<section class="intro">

</section>

<!-- /.intro -->

<section class="installation">

## Installation

```bash
npm install @stdlib/strided-base-map-by2
```

</section>

<section class="usage">

## Usage

```javascript
var mapBy2 = require( '@stdlib/strided-base-map-by2' );
```

#### mapBy2( N, x, strideX, y, strideY, z, strideZ, fcn, clbk\[, thisArg] )

Applies a binary function to each pair of elements retrieved from strided input arrays according to a callback function and assigns results to a strided output array.

```javascript
var add = require( '@stdlib/number-float64-base-add' );

function accessor( values ) {
    values[ 0 ] *= 2.0;
    values[ 1 ] *= 2.0;
    return values;
}

var x = [ -2.0, 1.0, 3.0, -5.0, 4.0, 0.0, -1.0, -3.0 ];
var y = [ 1.0, 1.0, 2.0, 2.0, 3.0, 3.0, 4.0, 4.0 ];
var z = [ 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0 ];

mapBy2( x.length, x, 1, y, 1, z, 1, add, accessor );
// z => [ -2.0, 4.0, 10.0, -6.0, 14.0, 6.0, 6.0, 2.0 ]
```

The function accepts the following arguments:

-   **N**: number of indexed elements.
-   **x**: input [`Array`][mdn-array], [`typed array`][mdn-typed-array], or an array-like object (excluding strings and functions).
-   **strideX**: index increment for `x`.
-   **y**: input [`Array`][mdn-array], [`typed array`][mdn-typed-array], or an array-like object (excluding strings and functions).
-   **strideY**: index increment for `y`.
-   **z**: output [`Array`][mdn-array], [`typed array`][mdn-typed-array], or an array-like object (excluding strings and functions).
-   **strideZ**: index increment for `z`.
-   **fcn**: binary function to apply to callback return values.
-   **clbk**: callback function which returns an array-like object containing two values.
-   **thisArg**: execution context (_optional_).

The invoked callback function is provided four arguments:

-   **values**: input array element values `[vx, vy]`.
-   **idx**: iteration index (zero-based).
-   **indices**: input and output array strided indices `[ix, iy, iz]` (computed according to `offset + idx*stride`).
-   **arrays**: input and output arrays/collections `[x, y, z]`.

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

```javascript
var add = require( '@stdlib/number-float64-base-add' );

function accessor( values ) {
    this.count += 1;
    values[ 0 ] *= 2.0;
    values[ 1 ] *= 2.0;
    return values;
}

var context = {
    'count': 0
};

var x = [ -2.0, 1.0, 3.0, -5.0, 4.0, 0.0, -1.0, -3.0 ];
var y = [ 1.0, 1.0, 2.0, 2.0, 3.0, 3.0, 4.0, 4.0 ];
var z = [ 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0 ];

mapBy2( x.length, x, 1, y, 1, z, 1, add, accessor, context );
// z => [ -2.0, 4.0, 10.0, -6.0, 14.0, 6.0, 6.0, 2.0 ]

var cnt = context.count;
// returns 8
```

The `N` and `stride` parameters determine which elements in the strided arrays are accessed at runtime. For example, to index every other value in `x` and to index the first `N` elements of `y` in reverse order,

```javascript
var add = require( '@stdlib/number-float64-base-add' );

function accessor( values ) {
    values[ 0 ] *= 2.0;
    values[ 1 ] *= 2.0;
    return values;
}

var x = [ -1.0, -2.0, -3.0, -4.0, -5.0, -6.0 ];
var y = [ 1.0, 1.0, 2.0, 2.0, 3.0, 3.0 ];
var z = [ 0.0, 0.0, 0.0, 0.0, 0.0, 0.0 ];

mapBy2( 3, x, 2, y, -1, z, 1, add, accessor );
// z => [ 2.0, -4.0, -8.0, 0.0, 0.0, 0.0 ]
```

Note that indexing is relative to the first index. To introduce an offset, use [`typed array`][mdn-typed-array] views.

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

function accessor( values ) {
    values[ 0 ] *= 2.0;
    values[ 1 ] *= 2.0;
    return values;
}

// Initial arrays...
var x0 = new Float64Array( [ -1.0, -2.0, -3.0, -4.0, -5.0, -6.0 ] );
var y0 = new Float64Array( [ 1.0, 1.0, 2.0, 2.0, 3.0, 3.0 ] );
var z0 = new Float64Array( [ 0.0, 0.0, 0.0, 0.0, 0.0, 0.0 ] );

// Create offset views...
var x1 = new Float64Array( x0.buffer, x0.BYTES_PER_ELEMENT*1 ); // start at 2nd element
var y1 = new Float64Array( y0.buffer, y0.BYTES_PER_ELEMENT*3 ); // start at 4th element
var z1 = new Float64Array( z0.buffer, z0.BYTES_PER_ELEMENT*2 ); // start at 3rd element

mapBy2( 3, x1, -2, y1, 1, z1, 1, add, accessor );
// z0 => <Float64Array>[ 0.0, 0.0, -8.0, -2.0, 2.0, 0.0 ]
```

#### mapBy2.ndarray( N, x, strideX, offsetX, y, strideY, offsetY, z, strideZ, offsetZ, fcn, clbk\[, thisArg] )

Applies a binary function to each pair of elements retrieved from strided input arrays according to a callback function and assigns results to a strided output array using alternative indexing semantics.

```javascript
var add = require( '@stdlib/number-float64-base-add' );

function accessor( values ) {
    values[ 0 ] *= 2.0;
    values[ 1 ] *= 2.0;
    return values;
}

var x = [ -1.0, -2.0, -3.0, -4.0, -5.0 ];
var y = [ 1.0, 1.0, 2.0, 2.0, 3.0 ];
var z = [ 0.0, 0.0, 0.0, 0.0, 0.0 ];

mapBy2.ndarray( x.length, x, 1, 0, y, 1, 0, z, 1, 0, add, accessor );
// z => [ 0.0, -2.0, -2.0, -4.0, -4.0 ]
```

The function accepts the following additional arguments:

-   **offsetX**: starting index for `x`.
-   **offsetY**: starting index for `y`.
-   **offsetZ**: starting index for `z`.

While [`typed array`][mdn-typed-array] views mandate a view offset based on the underlying `buffer`, the offset parameters support indexing semantics based on starting indices. For example, to index every other value in `x` starting from the second value and to index the last `N` elements in `y` in reverse order,

```javascript
var add = require( '@stdlib/number-float64-base-add' );

function accessor( values ) {
    values[ 0 ] *= 2.0;
    values[ 1 ] *= 2.0;
    return values;
}

var x = [ -1.0, -2.0, -3.0, -4.0, -5.0, -6.0 ];
var y = [ 1.0, 1.0, 2.0, 2.0, 3.0, 3.0 ];
var z = [ 0.0, 0.0, 0.0, 0.0, 0.0, 0.0 ];

mapBy2.ndarray( 3, x, 2, 1, y, -1, y.length-1, z, 1, 3, add, accessor );
// z => [ 0.0, 0.0, 0.0, 2.0, -2.0, -8.0 ]
```

</section>

<!-- /.usage -->

<section class="notes">

## Notes

-   If a provided callback function does not return any value (or equivalently, explicitly returns `undefined`), the value is **ignored**.

    ```javascript
    var add = require( '@stdlib/number-float64-base-add' );

    function accessor() {
        // No-op...
    }

    var x = [ -2.0, 1.0, 3.0, -5.0, 4.0, 0.0, -1.0, -3.0 ];
    var y = [ 1.0, 1.0, 2.0, 2.0, 3.0, 3.0, 4.0, 4.0 ];
    var z = [ 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0 ];

    mapBy2( x.length, x, 1, y, 1, z, 1, add, accessor );
    // z => [ 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0 ]
    ```

</section>

<!-- /.notes -->

<section class="examples">

## Examples

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

```javascript
var discreteUniform = require( '@stdlib/random-base-discrete-uniform' ).factory;
var filledarray = require( '@stdlib/array-filled' );
var filledarrayBy = require( '@stdlib/array-filled-by' );
var add = require( '@stdlib/number-float64-base-add' );
var mapBy2 = require( '@stdlib/strided-base-map-by2' );

function accessor( values, i ) {
    if ( (i%3) === 0 ) {
        // Simulate a "missing" value...
        return;
    }
    return values;
}

var x = filledarrayBy( 10, 'generic', discreteUniform( -100, 100 ) );
console.log( x );

var y = filledarrayBy( 10, 'generic', discreteUniform( -100, 100 ) );
console.log( y );

var z = filledarray( null, 10, 'generic' );
console.log( z );

mapBy2.ndarray( x.length, x, 1, 0, y, -1, y.length-1, z, 1, 0, add, accessor );
console.log( z );
```

</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/strided-base/map-by`][@stdlib/strided/base/map-by]</span><span class="delimiter">: </span><span class="description">apply a unary function to each element retrieved from a strided input array according to a callback function and assign results to a strided output array.</span>
-   <span class="package-name">[`@stdlib/strided-base/binary`][@stdlib/strided/base/binary]</span><span class="delimiter">: </span><span class="description">apply a binary callback to elements in strided input arrays and assign results to elements in a strided output array.</span>

</section>

<!-- /.related -->

<!-- Section for all links. Make sure to keep an empty line after the `section` element and another before the `/section` close. -->


<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>

<!-- /.stdlib -->

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[test-image]: https://github.com/stdlib-js/strided-base-map-by2/actions/workflows/test.yml/badge.svg?branch=v0.2.3
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[esm-url]: https://github.com/stdlib-js/strided-base-map-by2/tree/esm
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[branches-url]: https://github.com/stdlib-js/strided-base-map-by2/blob/main/branches.md

[stdlib-license]: https://raw.githubusercontent.com/stdlib-js/strided-base-map-by2/main/LICENSE

[mdn-array]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array

[mdn-typed-array]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray

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[@stdlib/strided/base/map-by]: https://www.npmjs.com/package/@stdlib/strided-base-map-by

[@stdlib/strided/base/binary]: https://www.npmjs.com/package/@stdlib/strided-base-binary

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</section>

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