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# unaryBy

> Apply a unary function to each element in an input ndarray according to a callback function and assign results to elements in an output ndarray.

<section class="intro">

</section>

<!-- /.intro -->

<section class="usage">

## Usage

```javascript
var unaryBy = require( '@stdlib/ndarray/base/unary-by' );
```

#### unaryBy( arrays, fcn, clbk\[, thisArg] )

Applies a unary function to each element retrieved from an input ndarray according to a callback function and assigns results to elements in an output ndarray.

<!-- eslint-disable max-len -->

```javascript
var Float64Array = require( '@stdlib/array/float64' );

function scale( x ) {
    return x * 10.0;
}

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

// Create data buffers:
var xbuf = new Float64Array( [ 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0, 11.0, 12.0 ] );
var ybuf = new Float64Array( 6 );

// Define the shape of the input and output arrays:
var shape = [ 3, 1, 2 ];

// Define the array strides:
var sx = [ 4, 4, 1 ];
var sy = [ 2, 2, 1 ];

// Define the index offsets:
var ox = 1;
var oy = 0;

// Create the input and output ndarray-like objects:
var x = {
    'dtype': 'float64',
    'data': xbuf,
    'shape': shape,
    'strides': sx,
    'offset': ox,
    'order': 'row-major'
};
var y = {
    'dtype': 'float64',
    'data': ybuf,
    'shape': shape,
    'strides': sy,
    'offset': oy,
    'order': 'row-major'
};

// Apply the unary function:
unaryBy( [ x, y ], scale, accessor );

console.log( y.data );
// => <Float64Array>[ 40.0, 60.0, 120.0, 140.0, 200.0, 220.0 ]
```

The function accepts the following arguments:

-   **arrays**: array-like object containing one input ndarray and one output ndarray.
-   **fcn**: unary function to apply.

Each provided ndarray should be an object with the following properties:

-   **dtype**: data type.
-   **data**: data buffer.
-   **shape**: dimensions.
-   **strides**: stride lengths.
-   **offset**: index offset.
-   **order**: specifies whether an ndarray is row-major (C-style) or column major (Fortran-style).

The invoked callback function is provided four arguments:

-   **value**: input array element.
-   **idx**: iteration index (zero-based).
-   **indices**: input and output ndarray data buffer indices `[ix, iy]`.
-   **arrays**: input and output ndarrays `[x, y]`.

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

<!-- eslint-disable max-len -->

```javascript
var Float64Array = require( '@stdlib/array/float64' );

function scale( x ) {
    return x * 10.0;
}

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

// Create data buffers:
var xbuf = new Float64Array( [ 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0, 11.0, 12.0 ] );
var ybuf = new Float64Array( 6 );

// Define the shape of the input and output arrays:
var shape = [ 3, 1, 2 ];

// Define the array strides:
var sx = [ 4, 4, 1 ];
var sy = [ 2, 2, 1 ];

// Define the index offsets:
var ox = 1;
var oy = 0;

// Create the input and output ndarray-like objects:
var x = {
    'dtype': 'float64',
    'data': xbuf,
    'shape': shape,
    'strides': sx,
    'offset': ox,
    'order': 'row-major'
};
var y = {
    'dtype': 'float64',
    'data': ybuf,
    'shape': shape,
    'strides': sy,
    'offset': oy,
    'order': 'row-major'
};

// Apply the unary function:
var context = {
    'count': 0
};
unaryBy( [ x, y ], scale, accessor, context );

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

</section>

<!-- /.usage -->

<section class="notes">

## Notes

-   For very high-dimensional ndarrays which are non-contiguous, one should consider copying the underlying data to contiguous memory before applying a unary function in order to achieve better performance.

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

    <!-- eslint-disable max-len -->

    ```javascript
    var Float64Array = require( '@stdlib/array/float64' );

    function scale( x ) {
        return x * 10.0;
    }

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

    // Create data buffers:
    var xbuf = new Float64Array( [ 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0, 11.0, 12.0 ] );
    var ybuf = new Float64Array( 6 );

    // Define the shape of the input and output arrays:
    var shape = [ 3, 1, 2 ];

    // Define the array strides:
    var sx = [ 4, 4, 1 ];
    var sy = [ 2, 2, 1 ];

    // Define the index offsets:
    var ox = 1;
    var oy = 0;

    // Create the input and output ndarray-like objects:
    var x = {
        'dtype': 'float64',
        'data': xbuf,
        'shape': shape,
        'strides': sx,
        'offset': ox,
        'order': 'row-major'
    };
    var y = {
        'dtype': 'float64',
        'data': ybuf,
        'shape': shape,
        'strides': sy,
        'offset': oy,
        'order': 'row-major'
    };

    // Apply the unary function:
    unaryBy( [ x, y ], scale, accessor );

    console.log( y.data );
    // => <Float64Array>[ 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 sqrt = require( '@stdlib/math/base/special/sqrt' );
var naryFunction = require( '@stdlib/utils/nary-function' );
var shape2strides = require( '@stdlib/ndarray/base/shape2strides' );
var ndarray2array = require( '@stdlib/ndarray/base/to-array' );
var unaryBy = require( '@stdlib/ndarray/base/unary-by' );

var N = 10;
var x = {
    'dtype': 'generic',
    'data': filledarrayBy( N, 'generic', discreteUniform( -100, 100 ) ),
    'shape': [ 5, 2 ],
    'strides': [ 2, 1 ],
    'offset': 0,
    'order': 'row-major'
};
var y = {
    'dtype': 'generic',
    'data': filledarray( 0, N, 'generic' ),
    'shape': x.shape.slice(),
    'strides': shape2strides( x.shape, 'column-major' ),
    'offset': 0,
    'order': 'column-major'
};

unaryBy( [ x, y ], sqrt, naryFunction( abs, 1 ) );
console.log( ndarray2array( x.data, x.shape, x.strides, x.offset, x.order ) );
console.log( ndarray2array( y.data, y.shape, y.strides, y.offset, y.order ) );
```

</section>

<!-- /.examples -->

<!-- Section for related `stdlib` packages. Do not manually edit this section, as it is automatically populated. -->

<section class="related">

</section>

<!-- /.related -->

<section class="links">

</section>

<!-- /.links -->
