Rotate an array at a specific index in JavaScript

Question

I have a a position and an array of users

const position = 2
const users = ["John", "Mary", "Daniel", "Michael"]

I want to generate a new array (or reorder it) starting from the position.

In the case of position = 2 the generated array should be

users = ["Daniel", "Michael", "John", "Mary"]

In the case of position = 3 the generated array should be

users = ["Michael", "John", "Mary", "Daniel"]

In the case of position 0 (no changes) the generated array should left intact

const users = ["John", "Mary", "Daniel", "Michael"]

how can I achieve this?

What have you tried? Hint: [`Array#slice`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/slice). — ggorlen, Mar 20 '20 at 23:57
Does this answer your question? [Rotate the elements in an array in JavaScript](https://stackoverflow.com/questions/1985260/rotate-the-elements-in-an-array-in-javascript) — ggorlen, Mar 21 '20 at 00:23

score 2 · Answer 1 · answered Mar 21 '20 at 00:03

2

You could use map() or Array.from() to make a new array from the index + your offset using the modulus operator to wrap around:

const position = 2
const users = ["John", "Mary", "Daniel", "Michael"]

const rotate = (arr, position) => 
   Array.from(arr, (_, index) => arr[(index + position) % arr.length])


console.log(rotate(users, 0))
console.log(rotate(users, 1))
console.log(rotate(users, 2))
console.log(rotate(users, 3))

answered Mar 21 '20 at 00:03

Mark

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@ggorlen I don't think so, however, I felt like `map()` was (only a little) semantically wrong. – Mark Mar 21 '20 at 00:17
Gotcha. I forgot that `Array.from` does mapping. – ggorlen Mar 21 '20 at 00:18

ggorlen · Answer 2 · 2020-03-21T00:24:48.023

2

You can use Array#slice from 0..pos and from pos..length to divide the array into two chunks at the split index. Then swap the chunks and either spread or concat the two subarrays back together. Take the mod of the position to keep the rotation in-bounds.

const rotate = (arr, i) => {
  i %= arr.length;
  return [...arr.slice(i), ...arr.slice(0, i)];
};

for (let i = -1; i < 5; i++) {
  console.log(rotate(["John", "Mary", "Daniel", "Michael"], i));
}

edited Mar 21 '20 at 00:24

answered Mar 21 '20 at 00:17

ggorlen

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I like that approach. – Mark Mar 21 '20 at 00:17

Dum · Answer 3 · 2020-03-21T00:07:27.283

0

Simple way to do that.

function reorder(arr, pos){
   var output = [];
   for(var i = pos;i<arr.length;i++){
      output.push(arr[i]);
   }

   for(var i = 0;i<pos;i++){
      output.push(arr[i]);
   }

   return output;
}

Not sure how efficient is this. There must be better ways to do this.

edited Mar 21 '20 at 00:07

answered Mar 21 '20 at 00:04

Dum

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Wax · Answer 4 · 2020-03-21T02:29:56.423

You can use a shift() + push() combinaison

users.push(users.shift());

...and then loop it by the position number.

const users = ["John", "Mary", "Daniel", "Michael"];
let position;

function rotate(array, index){
  for(let i = 0; i < index; i++){
    array.push(array.shift());
  }
  console.log(`Position: ${index}`);
  console.log(array);
}

// Test Zone
position = 0;
rotate(users, position);
position = 1;
rotate(users, position);
position = 2;
rotate(users, position);
position = 3;
rotate(users, position);

Rotate an array at a specific index in JavaScript

4 Answers4