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( values ) {
    values[ 0 ] *= 2.0;
    values[ 1 ] *= 2.0;
    return values;
}

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 four arguments:

  • values: input array element values [vx, vy].
  • idx: iteration index (zero-based).
  • indices: input and output array strided indices [ix, iy, iz] (computed according to offset + idx*stride).
  • arrays: input and output arrays/collections [x, y, z].

To set the callback execution context, provide a thisArg.

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

function accessor( values ) {
    this.count += 1;
    values[ 0 ] *= 2.0;
    values[ 1 ] *= 2.0;
    return values;
}

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( values ) {
    values[ 0 ] *= 2.0;
    values[ 1 ] *= 2.0;
    return values;
}

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( values ) {
    values[ 0 ] *= 2.0;
    values[ 1 ] *= 2.0;
    return values;
}

// 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( values ) {
    values[ 0 ] *= 2.0;
    values[ 1 ] *= 2.0;
    return values;
}

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( values ) {
    values[ 0 ] *= 2.0;
    values[ 1 ] *= 2.0;
    return values;
}

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( values, i ) {
    if ( (i%3) === 0 ) {
        // Simulate a "missing" value...
        return;
    }
    return values;
}

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 );

See Also

  • @stdlib/strided/base/map-by: 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.
  • @stdlib/strided/base/binary: apply a binary callback to elements in strided input arrays and assign results to elements in a strided output array.
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