Using only `.reduce()` to swap 2 elements in the array in JavaScript?

Question

I would like to find out if swapping elements can be done using only .reduce in JavaScript. If not, what else should be used from the functional programming land?

This is not for sorting an array. I wanted to find all the permutations of the array element using .reduce which required the swap step as per this method.

do you have an example of the array and why you use reduce for it? — Nina Scholz, Feb 16 '19 at 17:43
You usually change the order of the elements of an array using `.sort()`. — Titus, Feb 16 '19 at 17:43
Do you want to create a new array instance, with two values swapped, or just (mutably) swap to elements of an array? — Bergi, Feb 16 '19 at 18:43

Nina Scholz · Answer 1 · 2019-02-16T18:02:25.487

4

You could take a function which takes an array and two indices and uses a destructuring assignment.

const swap = (array, i, j) => [array[i], array[j]] = [array[j], array[i]];

var array = [1, 2, 3];

swap(array, 0, 1)
console.log(array);

A version with reduce by taking an array of indices and swap all pairs from start to end.

const
    swap = (array, ...indices) => 
        indices.reduce((a, b) => ([array[a], array[b]] = [array[b], array[a]], b));

var array = [1, 2, 3];

swap(array, 0, 1)
console.log(array);

edited Feb 16 '19 at 18:02

answered Feb 16 '19 at 17:46

Nina Scholz

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@vsemozhetbyt It works, it mutates the array, you are not supposed to use the return value of the function. – Titus Feb 16 '19 at 18:01
1

for using it with a fluent interface, you could return the array after swapping. – Nina Scholz Feb 16 '19 at 18:03
Oh, sorry. I was accustomed to side-effect arrow functions not using contracted syntax with implicit return) – vsemozhebuty Feb 16 '19 at 18:07

score 4 · Answer 2 · edited Feb 16 '19 at 19:17

In es6, the idiomatic way to swap array elements is:

;[a[i], a[j]] = [a[j], a[i]]

Using .reduce is not appropriate for this task. You could technically do something like this:

a = a.reduce((acc, element, idx) => {
    acc.push(idx === i ? a[j] : idx === j ? a[i] : a[idx])
    return acc
}, [])

but it would result in convoluted code.

If your goal is to avoid mutating the original array, you can use Object.assign:

b = Object.assign([], a, {[i]: a[j], [j]: a[i]})

score 0 · Answer 3 · answered Feb 16 '19 at 18:05

The reduce function reduces the array to a value of an object, as defined by the accumulator.

let array1 = [2, 5, 8, 0, 10];
let array2 = [1, 4, 9, 7, 6];

const reducer = (accumulator, currentValue) => accumulator + currentValue;

// 1 + 2 + 3 + 4
console.log(array1.reduce(reducer));
// expected output: 10

// 5 + 1 + 2 + 3 + 4
console.log(array1.reduce(reducer, 5));
// expected output: 15

The sort() method sorts the elements of an array in place and returns the array. The default sort order is built upon converting the elements into strings, then comparing their sequences of UTF-16 code units values.

  

var months = ['March', 'Jan', 'Feb', 'Dec'];
months.sort();
console.log(months);
// expected output: Array ["Dec", "Feb", "Jan", "March"]

const sortingAccending = (a, b) => a - b

let numbers = [4, 2, 5, 1, 3];
numbers.sort(sortingAccending);
console.log(numbers);
// expected output: Array [1, 100000, 21, 30, 4]

And you answer your question, reduce can't be used for swapping elements. You will have to either use sort for write your custom sort function

Using only `.reduce()` to swap 2 elements in the array in JavaScript?

3 Answers3