@stdlib/utils/map2/README.md

Standard utilities.

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# map2

> Apply a function to elements in two input arrays and assign the results to an output array.

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</section>

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<section class="usage">

## Usage

```javascript
var map2 = require( '@stdlib/utils/map2' );
```

<a name="fcn-map2"></a>

#### map2( x, y, fcn\[, thisArg] )

Applies a function to elements in two input arrays and assigns the results to a new array.

```javascript
var naryFunction = require( '@stdlib/utils/nary-function' );
var add = require( '@stdlib/math/base/ops/add' );

var x = [ 1, 2, 3, 4, 5, 6 ];
var y = [ 1, 1, 1, 1, 1, 1 ];

var out = map2( x, y, naryFunction( add, 2 ) );
// returns [ 2, 3, 4, 5, 6, 7 ]
```

The function accepts both array-like objects and [`ndarray`][@stdlib/ndarray/ctor]-like objects.

```javascript
var naryFunction = require( '@stdlib/utils/nary-function' );
var add = require( '@stdlib/math/base/ops/add' );
var array = require( '@stdlib/ndarray/array' );

var opts = {
    'dtype': 'generic'
};
var x = array( [ [ 1, 2, 3 ], [ 4, 5, 6 ] ], opts );
var y = array( [ [ 1, 1, 1 ], [ 1, 1, 1 ] ], opts );

var out = map2( x, y, naryFunction( add, 2 ) );
// returns <ndarray>

var v = out.get( 1, 1 );
// returns 6
```

The applied function is provided the following arguments:

-   **v1**: element from first input array.
-   **v2**: element from second input array.
-   **idx**: element index.
-   **arrays**: input arrays.

To set the `this` context when invoking the input function, provide a `thisArg`.

<!-- eslint-disable no-invalid-this -->

```javascript
var add = require( '@stdlib/math/base/ops/add' );

function fcn( v1, v2 ) {
    this.count += 1;
    return add( v1, v2 );
}

var x = [ 1, 2, 3, 4, 5, 6 ];
var y = [ 1, 1, 1, 1, 1, 1 ];

var ctx = {
    'count': 0
};

var out = map2( x, y, fcn, ctx );
// returns [ 2, 3, 4, 5, 6, 7 ]

var cnt = ctx.count;
// returns 6
```

<a name="method-map2-assign"></a>

#### map2.assign( x, y, out, fcn\[, thisArg] )

Applies a function to elements in two input arrays and assigns the results to an output array `out`.

```javascript
var naryFunction = require( '@stdlib/utils/nary-function' );
var add = require( '@stdlib/math/base/ops/add' );

var x = [ 1, 2, 3, 4, 5, 6 ];
var y = [ 1, 1, 1, 1, 1, 1 ];
var out = [ 0, 0, 0, 0, 0, 0 ];

map2.assign( x, y, out, naryFunction( add, 2 ) );

console.log( out );
// => [ 2, 3, 4, 5, 6, 7 ]
```

The method accepts both array-like objects and [`ndarray`][@stdlib/ndarray/ctor]-like objects.

```javascript
var naryFunction = require( '@stdlib/utils/nary-function' );
var add = require( '@stdlib/math/base/ops/add' );
var array = require( '@stdlib/ndarray/array' );

var opts = {
    'dtype': 'generic',
    'shape': [ 2, 3 ]
};
var x = array( [ [ 1, 2, 3 ], [ 4, 5, 6 ] ], opts );
var y = array( [ [ 1, 1, 1 ], [ 1, 1, 1 ] ], opts );
var out = array( opts );

map2.assign( x, y, out, naryFunction( add, 2 ) );

var v = out.get( 1, 1 );
// returns 6
```

Input and output arrays must be either all array-like objects or all [`ndarray`][@stdlib/ndarray/ctor]-like objects. If input and output arrays are array-like objects, all arrays **must** have the same number of elements.

If input and output arrays are [`ndarray`][@stdlib/ndarray/ctor]-like objects, the arrays **must** be [broadcast compatible][@stdlib/ndarray/base/broadcast-shapes]. To map from one or more input [`ndarrays`][@stdlib/ndarray/ctor] to an output [`ndarray`][@stdlib/ndarray/ctor] which has the same rank (i.e., dimensionality) and the same number of elements, but which is not [broadcast compatible][@stdlib/ndarray/base/broadcast-shapes], reshape the arrays prior to invocation.

```javascript
var naryFunction = require( '@stdlib/utils/nary-function' );
var add = require( '@stdlib/math/base/ops/add' );
var array = require( '@stdlib/ndarray/array' );

var opts = {
    'dtype': 'generic',
    'shape': [ 2, 3 ]
};
var x = array( [ [ 1, 2, 3 ], [ 4, 5, 6 ] ], opts );
var y = array( [ [ 1, 1, 1 ], [ 1, 1, 1 ] ], opts );

opts = {
    'dtype': 'generic',
    'shape': [ 2, 2, 3 ]  // broadcast compatible shape
};
var out = array( opts );

map2.assign( x, y, out, naryFunction( add, 2 ) );

var v = out.get( 0, 1, 1 );
// returns 6

v = out.get( 1, 1, 1 );
// returns 6
```

In general, avoid providing output [`ndarray`][@stdlib/ndarray/ctor]-like objects which are [non-contiguous][@stdlib/ndarray/base/assert/is-contiguous] views containing one or more elements referring to the **same** memory location. Writing to an overlapping [non-contiguous][@stdlib/ndarray/base/assert/is-contiguous] view is likely to simultaneously affect multiple elements and yield unexpected results.

The applied function is provided the same arguments as with [`map2`](#fcn-map2).

</section>

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<!-- Package usage notes. Make sure to keep an empty line after the `section` element and another before the `/section` close. -->

<section class="notes">

## Notes

-   The [`map2`](#fcn-map2) function **always** returns an output value having a "generic" data type. For example, if provided array-like objects, the function returns a generic `array`. If provided [`ndarrays`][@stdlib/ndarray/ctor], the function returns an [`ndarray`][@stdlib/ndarray/ctor] having a "generic" data type.

    Accordingly, when provided a typed array, the [`map2`](#fcn-map2) function does **not** return a typed array of the same type. To assign results to a typed array, use the [`map2.assign`](#method-map2-assign) method.

-   Both [`map2`](#fcn-map2) and [`map2.assign`](#method-map2-assign) accept array-like objects exposing getters and setters for array element access (e.g., [`Complex64Array`][@stdlib/array/complex64], [`Complex128Array`][@stdlib/array/complex128], etc).

    ```javascript
    var naryFunction = require( '@stdlib/utils/nary-function' );
    var Complex64Array = require( '@stdlib/array/complex64' );
    var Complex64 = require( '@stdlib/complex/float32/ctor' );
    var realf = require( '@stdlib/complex/float32/real' );
    var imagf = require( '@stdlib/complex/float32/imag' );
    var add = require( '@stdlib/complex/float32/base/add' );

    var x = new Complex64Array( [ 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0 ] );
    var y = new Complex64Array( [ 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0 ] );
    var z = new Complex64Array( 4 );

    map2.assign( x, y, z, naryFunction( add, 2 ) );

    var v = z.get( 0 );

    var re = realf( v );
    // returns 2.0

    var im = imagf( v );
    // returns 3.0
    ```

-   When applying a function to [`ndarray`][@stdlib/ndarray/ctor]-like objects, performance will be best for [`ndarray`][@stdlib/ndarray/ctor]-like objects which are single-segment contiguous. For non-contiguous arrays, see [`@stdlib/ndarray/base/binary`][@stdlib/ndarray/base/binary].

-   Both [`map2`](#fcn-map2) and [`map2.assign`](#method-map2-assign) do **not** skip `undefined` elements.

</section>

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<section class="examples">

## Examples

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

```javascript
var filledarrayBy = require( '@stdlib/array/filled-by' );
var discreteUniform = require( '@stdlib/random/base/discrete-uniform' ).factory;
var naryFunction = require( '@stdlib/utils/nary-function' );
var add = require( '@stdlib/math/base/ops/add' );
var array = require( '@stdlib/ndarray/array' );
var map2 = require( '@stdlib/utils/map2' );

function fill( i ) {
    var rand = discreteUniform( -10*(i+1), 10*(i+1) );
    return filledarrayBy( 10, 'generic', rand );
}

// Create two-dimensional ndarrays (i.e., matrices):
var opts = {
    'dtype': 'generic',
    'flatten': true
};
var x = array( filledarrayBy( 10, opts.dtype, fill ), opts );
var y = array( filledarrayBy( 10, opts.dtype, fill ), opts );

// Create an explicit binary function:
var f = naryFunction( add, 2 );

// Compute element-wise sums...
var z = map2( x, y, f );

console.log( 'x:' );
console.log( x.data );

console.log( 'y:' );
console.log( y.data );

console.log( 'z:' );
console.log( z.data );
```

</section>

<!-- /.examples -->

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</section>

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* * *

## See Also

-   <span class="package-name">[`@stdlib/utils/map`][@stdlib/utils/map]</span><span class="delimiter">: </span><span class="description">apply a function to each element in an array and assign the result to an element in an output array.</span>

</section>

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<section class="links">

[@stdlib/ndarray/ctor]: https://www.npmjs.com/package/@stdlib/ndarray-ctor

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

[@stdlib/ndarray/base/broadcast-shapes]: https://www.npmjs.com/package/@stdlib/ndarray-base-broadcast-shapes

[@stdlib/ndarray/base/assert/is-contiguous]: https://www.npmjs.com/package/@stdlib/ndarray-base-assert-is-contiguous

[@stdlib/array/complex64]: https://www.npmjs.com/package/@stdlib/array-complex64

[@stdlib/array/complex128]: https://www.npmjs.com/package/@stdlib/array-complex128

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

[@stdlib/utils/map]: https://github.com/stdlib-js/utils/tree/main/map

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

</section>

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