reduceRight

Apply a function against an accumulator and each element in an array while iterating from right to left and return the accumulated result.

Usage

var reduceRight = require( '@stdlib/utils/reduce-right' );

reduceRight( arr, initial, reducer[, thisArg ] )

Applies a function against an accumulator and each element in an array while iterating from right to left and returns the accumulated result.

function sum( accumulator, value ) {
    return accumulator + value;
}

var arr = [ 1, 2, 3, 4 ];

var out = reduceRight( arr, 0, sum );
// returns 10

The function accepts both array-like objects and ndarray-like objects.

var array = require( '@stdlib/ndarray/array' );

function sum( accumulator, value ) {
    return accumulator + value;
}

var opts = {
    'dtype': 'generic'
};
var arr = array( [ [ 1, 2, 3 ], [ 4, 5, 6 ] ], opts );

var out = reduceRight( arr, 0, sum );
// returns 21

The applied function is provided the following arguments:

  • accumulator: accumulated value.
  • value: array element.
  • index: element index.
  • arr: input array.

To set the this context when invoking the input function, provide a thisArg.

function sum( accumulator, value ) {
    this.count += 1;
    return accumulator + value;
}

var arr = [ 1, 2, 3, 4 ];

var ctx = {
    'count': 0
};

var out = reduceRight( arr, 0, sum, ctx );
// returns 10

var mean = out / ctx.count;
// returns 2.5

Notes

  • For input arrays, the function differs from Array.prototype.reduceRight in the following ways:

    • The function requires an initial value for the accumulator. The initial value is used during the first invocation of the reducer function.
    • The function does not skip the last array element.
    • The function does not skip undefined elements.
    • The function does not support dynamic array-like objects (i.e., array-like objects whose length changes during execution).

  • The function supports array-like objects exposing getters and setters for array element access (e.g., Complex64Array, Complex128Array, etc).

    var Complex64Array = require( '@stdlib/array/complex64' );
var Complex64 = require( '@stdlib/complex/float32/ctor' );
var realf = require( '@stdlib/complex/float32/real' );
var imagf = require( '@stdlib/complex/float32/imag' );

function sum( acc, z ) {
    var re1 = realf( acc );
    var im1 = imagf( acc );
    var re2 = realf( z );
    var im2 = imagf( z );
    return new Complex64( re1+re2, im1+im2 );
}

var x = new Complex64Array( [ 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0 ] );

var v = reduceRight( x, new Complex64( 0.0, 0.0 ), sum );
// returns <Complex64>

var re = realf( v );
// returns 16.0

var im = imagf( v );
// returns 20.0

  • For ndarray-like objects, the function performs a reduction over the entire input ndarray (i.e., higher-order ndarray dimensions are flattened to a single-dimension).

  • When applying a function to ndarray-like objects, performance will be best for ndarray-like objects which are single-segment contiguous.

Examples

var filledarrayBy = require( '@stdlib/array/filled-by' );
var discreteUniform = require( '@stdlib/random/base/discrete-uniform' ).factory;
var naryFunction = require( '@stdlib/utils/nary-function' );
var add = require( '@stdlib/math/base/ops/add' );
var array = require( '@stdlib/ndarray/array' );
var reduceRight = require( '@stdlib/utils/reduce-right' );

function fill( i ) {
    var rand = discreteUniform( -10*(i+1), 10*(i+1) );
    return filledarrayBy( 10, 'generic', rand );
}

// Create a two-dimensional ndarray (i.e., a matrix):
var x = array( filledarrayBy( 10, 'generic', fill ), {
    'dtype': 'generic',
    'flatten': true
});

// Create an explicit binary function:
var f = naryFunction( add, 2 );

// Compute the sum:
var out = reduceRight( x, 0, f );

console.log( 'x:' );
console.log( x.data );

console.log( 'sum: %d', out );

See Also

  • @stdlib/utils/for-each-right: invoke a function for each element in a collection, iterating from right to left.
  • @stdlib/utils/map-right: apply a function to each element in an array and assign the result to an element in an output array, iterating from right to left.
  • @stdlib/utils/reduce: apply a function against an accumulator and each element in an array and return the accumulated result.
  • @stdlib/utils/async/reduce-right: apply a function against an accumulator and each element in a collection and return the accumulated result, iterating from right to left.
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