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 => [ 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 ]
`