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?

Does this answer your question? Rotate the elements in an array in JavaScript — ggorlen
– ggorlen, Commented Mar 21, 2020 at 0:23

Mark · Accepted Answer · 2020-03-21 00:03:26Z

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, 2020 at 0:03

Mark

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2 Comments

Mark Over a year ago

@ggorlen I don't think so, however, I felt like map() was (only a little) semantically wrong.

ggorlen Over a year ago

Gotcha. I forgot that Array.from does mapping.

ggorlen · Accepted Answer · 2020-03-21 00:24:48Z

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, 2020 at 0:24

answered Mar 21, 2020 at 0:17

ggorlen

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1 Comment

Mark Over a year ago

I like that approach.

Dum · Accepted Answer · 2020-03-21 00:07:27Z

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, 2020 at 0:07

answered Mar 21, 2020 at 0:04

Dum

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Comments

Wax · Accepted Answer · 2020-03-21 02:29:56Z

0

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);

edited Mar 21, 2020 at 2:29

answered Mar 21, 2020 at 2:23

Wax

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