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reduceRight

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

Installation

npm install @stdlib/utils-reduce-right

Alternatively,

• To load the package in a website via a script tag without installation and bundlers, use the ES Module available on the esm branch (see README).
• If you are using Deno, visit the deno branch (see README for usage intructions).
• For use in Observable, or in browser/node environments, use the Universal Module Definition (UMD) build available on the umd branch (see README).

The branches.md file summarizes the available branches and displays a diagram illustrating their relationships.

To view installation and usage instructions specific to each branch build, be sure to explicitly navigate to the respective README files on each branch, as linked to above.

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 );

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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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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

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License

See LICENSE.

Copyright

Copyright © 2016-2024. The Stdlib Authors.