@stdlib/strided/base/map-by2/README.md

Strided.

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# mapBy2

> 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="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/math/base/ops/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/math/base/ops/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/math/base/ops/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/math/base/ops/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/math/base/ops/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/math/base/ops/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/math/base/ops/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/math/base/ops/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="links">

[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

<!-- <related-links> -->

[@stdlib/strided/base/map-by]: https://github.com/stdlib-js/strided/tree/main/base/map-by

[@stdlib/strided/base/binary]: https://github.com/stdlib-js/strided/tree/main/base/binary

<!-- </related-links> -->

</section>

<!-- /.links -->