npm package: @stdlib/math-strided-ops-mul-by
Description: Element-wise multiplication of two strided arrays via a callback function.
Author: The Stdlib Authors
Latest version: 0.2.3

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# mulBy

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

> Element-wise [multiplication][@stdlib/number/float64/base/mul] of two strided arrays via a callback function.

<section class="intro">

</section>

<!-- /.intro -->

<section class="installation">

## Installation

```bash
npm install @stdlib/math-strided-ops-mul-by
```

</section>

<section class="usage">

## Usage

```javascript
var mulBy = require( '@stdlib/math-strided-ops-mul-by' );
```

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

Performs element-wise [multiplication][@stdlib/number/float64/base/mul] of two strided arrays via a callback function and assigns each result to an element in an output strided array.

```javascript
function accessor( values ) {
    return values;
}

var x = [ 1.0, 2.0, 3.0, 4.0, 5.0 ];
var y = [ 11.0, 12.0, 13.0, 14.0, 15.0 ];
var z = [ 0.0, 0.0, 0.0, 0.0, 0.0 ];

mulBy( x.length, x, 1, y, 1, z, 1, accessor );
// z => [ 11.0, 24.0, 39.0, 56.0, 75.0 ]
```

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`.
-   **clbk**: callback function.
-   **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
function accessor( values ) {
    this.count += 1;
    return values;
}

var context = {
    'count': 0
};

var x = [ 1.0, 2.0, 3.0, 4.0, 5.0 ];
var y = [ 11.0, 12.0, 13.0, 14.0, 15.0 ];
var z = [ 0.0, 0.0, 0.0, 0.0, 0.0 ];

mulBy( x.length, x, 1, y, 1, z, 1, accessor, context );
// z => [ 11.0, 24.0, 39.0, 56.0, 75.0 ]

var cnt = context.count;
// returns 5
```

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
function accessor( values ) {
    return values;
}

var x = [ 1.0, 2.0, 3.0, 4.0, 5.0, 6.0 ];
var y = [ 11.0, 12.0, 13.0, 14.0, 15.0, 16.0 ];
var z = [ 0.0, 0.0, 0.0, 0.0, 0.0, 0.0 ];

mulBy( 3, x, 2, y, -1, z, 1, accessor );
// z => [ 13.0, 36.0, 55.0, 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' );

function accessor( values ) {
    return values;
}

// Initial arrays...
var x0 = new Float64Array( [ 1.0, 2.0, 3.0, 4.0, 5.0, 6.0 ] );
var y0 = new Float64Array( [ 11.0, 12.0, 13.0, 14.0, 15.0, 16.0 ] );
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*3 ); // start at 4th element

mulBy( 3, x1, -2, y1, 1, z1, 1, accessor );
// z0 => <Float64Array>[ 0.0, 0.0, 0.0, 84.0, 60.0, 32.0 ]
```

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

Performs element-wise [multiplication][@stdlib/number/float64/base/mul] of two strided arrays via a callback function and assigns each result to an element in an output strided array using alternative indexing semantics.

```javascript
function accessor( values ) {
    return values;
}

var x = [ 1.0, 2.0, 3.0, 4.0, 5.0 ];
var y = [ 11.0, 12.0, 13.0, 14.0, 15.0 ];
var z = [ 0.0, 0.0, 0.0, 0.0, 0.0 ];

mulBy.ndarray( x.length, x, 1, 0, y, 1, 0, z, 1, 0, accessor );
// z => [ 11.0, 24.0, 39.0, 56.0, 75.0 ]
```

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`,

```javascript
function accessor( values ) {
    return values;
}

var x = [ 1.0, 2.0, 3.0, 4.0, 5.0, 6.0 ];
var y = [ 11.0, 12.0, 13.0, 14.0, 15.0, 16.0 ];
var z = [ 0.0, 0.0, 0.0, 0.0, 0.0, 0.0 ];

mulBy.ndarray( 3, x, 2, 1, y, -1, y.length-1, z, 1, 2, accessor );
// z => [ 0.0, 0.0, 0.0, 84.0, 60.0, 32.0 ]
```

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

    var x = [ 1.0, 2.0, 3.0, 4.0, 5.0 ];
    var y = [ 11.0, 12.0, 13.0, 14.0, 15.0 ];
    var z = [ 0.0, 0.0, 0.0, 0.0, 0.0 ];

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

</section>

<!-- /.notes -->

<section class="examples">

## Examples

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

```javascript
var uniform = require( '@stdlib/random-base-uniform' ).factory;
var filledarray = require( '@stdlib/array-filled' );
var filledarrayBy = require( '@stdlib/array-filled-by' );
var mulBy = require( '@stdlib/math-strided-ops-mul-by' );

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

var x = filledarrayBy( 10, 'generic', uniform( -10.0, 10.0 ) );
console.log( x );

var y = filledarrayBy( x.length, 'generic', uniform( -10.0, 10.0 ) );
console.log( y );

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

mulBy.ndarray( x.length, x, 1, 0, y, -1, y.length-1, z, 1, 0, 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/math-strided/ops/mul`][@stdlib/math/strided/ops/mul]</span><span class="delimiter">: </span><span class="description">element-wise multiplication of two strided arrays.</span>
-   <span class="package-name">[`@stdlib/math-strided/ops/add-by`][@stdlib/math/strided/ops/add-by]</span><span class="delimiter">: </span><span class="description">element-wise addition of two strided arrays via a callback function.</span>
-   <span class="package-name">[`@stdlib/math-strided/ops/sub-by`][@stdlib/math/strided/ops/sub-by]</span><span class="delimiter">: </span><span class="description">element-wise subtraction of two strided arrays via a callback function.</span>

</section>

<!-- /.related -->

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<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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