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

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# mapBy

> 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.

<section class="intro">

</section>

<!-- /.intro -->

<section class="usage">

## Usage

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

#### mapBy( N, x, strideX, y, strideY, fcn, clbk\[, thisArg] )

Applies a unary function to each element retrieved from a strided input array according to a callback function and assigns results to a strided output array.

```javascript
var abs = require( '@stdlib/math/base/special/abs' );

function accessor( v ) {
    return v * 2.0;
}

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

mapBy( x.length, x, 1, y, 1, abs, accessor );
// y => [ 4.0, 2.0, 6.0, 10.0, 8.0, 0.0, 2.0, 6.0 ]
```

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**: output [`Array`][mdn-array], [`typed array`][mdn-typed-array], or an array-like object (excluding strings and functions).
-   **strideY**: index increment for `y`.
-   **fcn**: unary function to apply to callback return values.
-   **clbk**: callback function.
-   **thisArg**: execution context (_optional_).

The invoked callback function is provided four arguments:

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

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

```javascript
var abs = require( '@stdlib/math/base/special/abs' );

function accessor( v ) {
    this.count += 1;
    return v * 2.0;
}

var context = {
    'count': 0
};

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

mapBy( x.length, x, 1, y, 1, abs, accessor, context );
// y => [ 4.0, 2.0, 6.0, 10.0, 8.0, 0.0, 2.0, 6.0 ]

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

The `N` and `stride` parameters determine which elements in `x` and `y` 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 abs = require( '@stdlib/math/base/special/abs' );

function accessor( v ) {
    return v * 2.0;
}

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

mapBy( 3, x, 2, y, -1, abs, accessor );
// y => [ 10.0, 6.0, 2.0, 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 abs = require( '@stdlib/math/base/special/abs' );

function accessor( v ) {
    return v * 2.0;
}

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

mapBy( 3, x1, -2, y1, 1, abs, accessor );
// y0 => <Float64Array>[ 0.0, 0.0, 0.0, 12.0, 8.0, 4.0 ]
```

#### mapBy.ndarray( N, x, strideX, offsetX, y, strideY, offsetY, fcn, clbk\[, thisArg] )

Applies a unary function to each element retrieved from a strided input array according to a callback function and assigns results to a strided output array using alternative indexing semantics.

```javascript
var abs = require( '@stdlib/math/base/special/abs' );

function accessor( v ) {
    return v * 2.0;
}

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

mapBy.ndarray( x.length, x, 1, 0, y, 1, 0, abs, accessor );
// y => [ 2.0, 4.0, 6.0, 8.0, 10.0 ]
```

The function accepts the following additional arguments:

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

While [`typed array`][mdn-typed-array] views mandate a view offset based on the underlying `buffer`, the `offsetX` and `offsetY` 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 abs = require( '@stdlib/math/base/special/abs' );

function accessor( v ) {
    return v * 2.0;
}

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

mapBy.ndarray( 3, x, 2, 1, y, -1, y.length-1, abs, accessor );
// y => [ 0.0, 0.0, 0.0, 12.0, 8.0, 4.0 ]
```

</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 abs = require( '@stdlib/math/base/special/abs' );

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

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

    mapBy( x.length, x, 1, y, 1, abs, accessor );
    // y => [ 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 abs = require( '@stdlib/math/base/special/abs' );
var mapBy = require( '@stdlib/strided/base/map-by' );

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

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

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

mapBy.ndarray( x.length, x, 1, 0, y, -1, y.length-1, abs, accessor );
console.log( y );
```

</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-by2`][@stdlib/strided/base/map-by2]</span><span class="delimiter">: </span><span class="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.</span>
-   <span class="package-name">[`@stdlib/strided/base/unary`][@stdlib/strided/base/unary]</span><span class="delimiter">: </span><span class="description">apply a unary callback to elements in a strided input array and assign results to elements in a strided output array.</span>

</section>

<!-- /.related -->

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<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-by2]: https://github.com/stdlib-js/strided/tree/main/base/map-by2

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

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</section>

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