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.

Usage

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.

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, typed array, or an array-like object (excluding strings and functions).
strideX: index increment for x.
y: output Array, 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.

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,

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 views.

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.

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 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,

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 ]

Notes

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

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 ]

Examples

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 );