unaryBy

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 unaryBy = require( '@stdlib/strided/base/unary-by' );

unaryBy( arrays, shape, strides, 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 ];

unaryBy( [ x, y ], [ x.length ], [ 1, 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:

arrays: array-like object containing one strided input array and one strided output array.
shape: array-like object containing a single element, the number of indexed elements.
strides: array-like object containing the stride lengths for the strided input and output arrays.
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 ];

unaryBy( [ x, y ], [ x.length ], [ 1, 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 shape and stride parameters determine which elements in the input and output 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 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 ];

unaryBy( [ x, y ], [ 3 ], [ 2, -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

unaryBy( [ x1, y1 ], [ 3 ], [ -2, 1 ], abs, accessor );
// y0 => <Float64Array>[ 0.0, 0.0, 0.0, 12.0, 8.0, 4.0 ]

unaryBy.ndarray( arrays, shape, strides, offsets, 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 ];

unaryBy.ndarray( [ x, y ], [ x.length ], [ 1, 1 ], [ 0, 0 ], abs, accessor );
// y => [ 2.0, 4.0, 6.0, 8.0, 10.0 ]

The function accepts the following additional arguments:

offsets: array-like object containing the starting indices (i.e., index offsets) for the strided input and output arrays.

While typed array views mandate a view offset based on the underlying buffer, the offsets parameter supports 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 ];

unaryBy.ndarray( [ x, y ], [ 3 ], [ 2, -1 ], [ 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 ];

unaryBy( [ x, y ], [ x.length ], [ 1, 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 unaryBy = require( '@stdlib/strided/base/unary-by' );

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

var N = 10;

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

var y = filledarray( null, N, 'generic' );
console.log( y );

var shape = [ N ];
var strides = [ 1, -1 ];
var offsets = [ 0, N-1 ];

unaryBy.ndarray( [ x, y ], shape, strides, offsets, abs, accessor );
console.log( y );