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mapReduce
Perform a single-pass map-reduce operation against each element in an array and return the accumulated result.
Installation
npm install @stdlib/utils-map-reduce
Usage
var mapReduce = require( '@stdlib/utils-map-reduce' );
mapReduce( arr, initial, mapper, reducer[, thisArg ] )
Performs a map-reduce operation against each element in an array and returns the accumulated result.
function square( value ) {
return value * value;
}
function sum( accumulator, value ) {
return accumulator + value;
}
var arr = [ 1, 2, 3, 4 ];
var out = mapReduce( arr, 0, square, sum );
// returns 30
The function accepts both array-like objects and ndarray
-like objects.
var array = require( '@stdlib/ndarray-array' );
function square( value ) {
return value * value;
}
function sum( accumulator, value ) {
return accumulator + value;
}
var opts = {
'dtype': 'generic'
};
var arr = array( [ [ 1, 2, 3 ], [ 4, 5, 6 ] ], opts );
var out = mapReduce( arr, 0, square, sum );
// returns 91
The mapping function is provided the following arguments:
- value: array element.
- index: element index.
- arr: input array.
The reducing function is provided the following arguments:
- accumulator: accumulated value.
- value: result of applying the mapping function to the current array element.
- index: element index.
- arr: input array.
To set the this
context when invoking the reducing function, provide a thisArg
.
function square( value ) {
return value * value;
}
function sum( accumulator, value ) {
this.count += 1;
return accumulator + value;
}
var arr = [ 1, 2, 3, 4 ];
var ctx = {
'count': 0
};
var out = mapReduce( arr, 0, square, sum, ctx );
// returns 30
var mean = out / ctx.count;
// returns 7.5
Notes
-
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' ); var cceil = require( '@stdlib/math-base-special-cceil' ); var realf = require( '@stdlib/complex-realf' ); var imagf = require( '@stdlib/complex-imagf' ); 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( [ 1.5, 2.5, 3.5, 4.5, 5.5, 6.5, 7.5, 8.5 ] ); var v = mapReduce( x, new Complex64( 0.0, 0.0 ), cceil, sum ); // returns <Complex64> var re = realf( v ); // returns 20.0 var im = imagf( v ); // returns 24.0
-
For
ndarray
-like objects, the function performs a single-pass map-reduce operation over the entire inputndarray
(i.e., higher-orderndarray
dimensions are flattened to a single-dimension). -
When applying a function to
ndarray
-like objects, performance will be best forndarray
-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 abs = require( '@stdlib/math-base-special-abs' );
var array = require( '@stdlib/ndarray-array' );
var mapReduce = require( '@stdlib/utils-map-reduce' );
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 unary function:
var f1 = naryFunction( abs, 1 );
// Create an explicit binary function:
var f2 = naryFunction( add, 2 );
// Compute the sum of absolute values:
var out = mapReduce( x, 0, f1, f2 );
console.log( 'x:' );
console.log( x.data );
console.log( 'sum: %d', out );
See Also
-
@stdlib/utils-map
: apply a function to each element in an array and assign the result to an element in an output array. -
@stdlib/utils-map-reduce-right
: perform a single-pass map-reduce operation against each element in an array while iterating from right to left and return the accumulated result. -
@stdlib/utils-reduce
: apply a function against an accumulator and each element in an array and return the accumulated result.
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.