mapBy2

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.

Usage

var mapBy2 = require( '@stdlib/strided/base/map-by2' );

mapBy2( N, x, strideX, y, strideY, z, strideZ, fcn, clbk[, thisArg] )

Applies a binary function to each pair of elements retrieved from strided input arrays according to a callback function and assigns results to a strided output array.

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

function accessor( vx, vy ) {
    return [ vx*2.0, vy*2.0 ];
}

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

mapBy2( x.length, x, 1, y, 1, z, 1, add, accessor );
// z => [ -2.0, 4.0, 10.0, -6.0, 14.0, 6.0, 6.0, 2.0 ]

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: input Array, typed array, or an array-like object (excluding strings and functions).
  • strideY: index increment for y.
  • z: output Array, typed array, or an array-like object (excluding strings and functions).
  • strideZ: index increment for z.
  • fcn: binary function to apply to callback return values.
  • clbk: callback function which returns an array-like object containing two values.
  • thisArg: execution context (optional).

The invoked callback function is provided nine arguments:

  • vx: input array element.
  • vy: input array element.
  • idx: iteration index (zero-based).
  • ix: input array strided index (offsetX + idx*strideX).
  • iy: input array strided index (offsetY + idx*strideY).
  • iz: output array strided index (offsetZ + idx*strideZ).
  • x: input array/collection.
  • y: input array/collection.
  • z: output array/collection.

To set the callback execution context, provide a thisArg.

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

function accessor( vx, vy ) {
    this.count += 1;
    return [ vx*2.0, vy*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 = [ 1.0, 1.0, 2.0, 2.0, 3.0, 3.0, 4.0, 4.0 ];
var z = [ 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0 ];

mapBy2( x.length, x, 1, y, 1, z, 1, add, accessor, context );
// z => [ -2.0, 4.0, 10.0, -6.0, 14.0, 6.0, 6.0, 2.0 ]

var cnt = context.count;
// returns 8

The N and stride parameters determine which elements in the strided arrays 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 add = require( '@stdlib/math/base/ops/add' );

function accessor( vx, vy ) {
    return [ vx*2.0, vy*2.0 ];
}

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

mapBy2( 3, x, 2, y, -1, z, 1, add, accessor );
// z => [ 2.0, -4.0, -8.0, 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 add = require( '@stdlib/math/base/ops/add' );

function accessor( vx, vy ) {
    return [ vx*2.0, vy*2.0 ];
}

// Initial arrays...
var x0 = new Float64Array( [ -1.0, -2.0, -3.0, -4.0, -5.0, -6.0 ] );
var y0 = new Float64Array( [ 1.0, 1.0, 2.0, 2.0, 3.0, 3.0 ] );
var z0 = 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
var z1 = new Float64Array( z0.buffer, z0.BYTES_PER_ELEMENT*2 ); // start at 3rd element

mapBy2( 3, x1, -2, y1, 1, z1, 1, add, accessor );
// z0 => <Float64Array>[ 0.0, 0.0, -8.0, -2.0, 2.0, 0.0 ]

mapBy2.ndarray( N, x, strideX, offsetX, y, strideY, offsetY, z, strideZ, offsetZ, fcn, clbk[, thisArg] )

Applies a binary function to each pair of elements retrieved from strided input arrays according to a callback function and assigns results to a strided output array using alternative indexing semantics.

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

function accessor( vx, vy ) {
    return [ vx*2.0, vy*2.0 ];
}

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

mapBy2.ndarray( x.length, x, 1, 0, y, 1, 0, z, 1, 0, add, accessor );
// z => [ 0.0, -2.0, -2.0, -4.0, -4.0 ]

The function accepts the following additional arguments:

  • offsetX: starting index for x.
  • offsetY: starting index for y.
  • offsetZ: starting index for z.

While typed array views mandate a view offset based on the underlying buffer, the offset 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 add = require( '@stdlib/math/base/ops/add' );

function accessor( vx, vy ) {
    return [ vx*2.0, vy*2.0 ];
}

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

mapBy2.ndarray( 3, x, 2, 1, y, -1, y.length-1, z, 1, 3, add, accessor );
// z => [ 0.0, 0.0, 0.0, 2.0, -2.0, -8.0 ]

Notes

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

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

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

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

mapBy2( x.length, x, 1, y, 1, z, 1, add, accessor );
// z => [ 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 add = require( '@stdlib/math/base/ops/add' );
var mapBy2 = require( '@stdlib/strided/base/map-by2' );

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

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

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

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

mapBy2.ndarray( x.length, x, 1, 0, y, -1, y.length-1, z, 1, 0, add, accessor );
console.log( z );
Did you find this page helpful?