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.

Installation

npm install @stdlib/strided-base-map-by

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 );

See Also

• @stdlib/strided-base/map-by2: 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.
• @stdlib/strided-base/unary: apply a unary callback to elements in a strided input array and assign results to elements in a strided output array.

Notice

This package is part of stdlib, a standard library for JavaScript and Node.js, with an emphasis on numerical and scientific computing. The library provides a collection of robust, high performance libraries for mathematics, statistics, streams, utilities, and more.

For more information on the project, filing bug reports and feature requests, and guidance on how to develop stdlib, see the main project repository.

Community

License

See LICENSE.

Copyright

Copyright © 2016-2024. The Stdlib Authors.