Sorting an array by chunks of 3

Question

Assume that you have an array and want to divide it by chunks of 3. If the array is..

let arr = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13];

...the new array would be

let newArr = [1, 4, 7, 10, 13,   2, 5, 8, 11,   3, 6, 9, 12]

// In other words:
1  2  3
4  5  6
7  8  9
10 11 12
13

The chunking part should be this code (if sorted like newArr):

let chunkedArr = _.chunk(_.toArray(newArr), 3);

...however I couldn't figure out how to sort the arr to newArr to be able to chunk in the right order. What is the proper way of handling such case?

_{Please note that the integers are just pseudo and I will use proper objects of array.}

Eddie · Answer 1 · 2018-05-26T04:06:01.090

5

One option is to use ES6 reduce to group the array into a multidimensional array. Use concat to flatten the multidimensional array.

[].concat(...) - basically flattens the multidimensional array. Starting with an empty array [], you concat each secondary array. Use the spread operator (...) to reiterate each secondary array and concat each.

let arr = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13];
let groups = 3;

let newArr = [].concat(...arr.reduce((c, v, i) => {
  let k = i % groups;
  c[k] = c[k] || [];
  c[k].push(v);
  return c;
}, []));

console.log(newArr);

edited May 26 '18 at 04:06

answered May 26 '18 at 03:57

Eddie

26,593
6
36
58

1

Could you explain what `[].concat(...)` is for? EDIT: Nvm, just saw the bit about "flatten the multidimensional array". – Asad Saeeduddin May 26 '18 at 04:01
@AsadSaeeduddin I added a simple explanation about that :) – Eddie May 26 '18 at 04:06

Mikaal Anwar · Accepted Answer · 2018-05-26T05:14:05.153

3

Please try the following (jsfiddle):

//This version works for variable chunk sizes as well.
let arr = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13];
alert(chunks(arr, 3)); //You can update chunk size here

function chunks(arr, chunkSize) {
  var result = [];
  var index = 0;
  for (var i = 0; i < chunkSize; i++) {
    for (var j = 0; j < arr.length / chunkSize; j++) {
      if (arr[i + (chunkSize * j)] != null)
        result[index++] = arr[i + (chunkSize * j)];
    }
  }
  return result;
}

//This version only works for chunks of size 3.
let arr = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13];
let subArr1 = [];
let subArr2 = [];
let subArr3 = [];
var result = [];
var i = 0, j = 0, k = 0;

for (var index = 0; index < arr.length; index++) {
  if (index % 3 == 0) {
    subArr1[i++] = arr[index];
  }
  if (index % 3 == 1) {
    subArr2[j++] = arr[index];
  }
  if (index % 3 == 2) {
    subArr3[k++] = arr[index];
  }
}

result.push(subArr1, subArr2, subArr3);
alert(result);

edited May 26 '18 at 05:14

answered May 26 '18 at 03:52

Mikaal Anwar

1,720
10
21

This returns `[1, 4, 7, 10, 13, 2, 5, 8, 11, 14, 2, 5, 8, 11, 14]` though, so it has duplicate values :/ _Please keep in mind that 13 is not explicitly the final index and it should work consistently for infinite elements_ – senty May 26 '18 at 03:55
Please recheck. It was a minor typo in the second last line. I was appending the second sub-array twice. And yes, this would work for any given length of the array. – Mikaal Anwar May 26 '18 at 03:59
1

This is also very nice! – senty May 26 '18 at 04:05
1

**Additionally:** `result.push(subArr1, subArr2, subArr3);` would actually work pretty efficiently for dividing stuff into chunk of 3 – senty May 26 '18 at 04:08
1

Thanks! I have made yet another version with plain javascript that can work for variable chunk sizes as well. Please check out: https://jsfiddle.net/ke1c8sa5/2/ – Mikaal Anwar May 26 '18 at 04:50

score 0 · Answer 3 · answered May 26 '18 at 03:50

Please check this it may help you. I also take a reference from here. Please check and let us know. any thing else you need.

Thanks

Here is the sample code.

var i,j,resArr,chunkSize = 10; for (i=0,j=array.length; i<j; i+=chunk) { resArr = array.slice(i,i+chunk); }

score 0 · Answer 4 · answered May 26 '18 at 04:22

const original = [1,2,3,4,5,6,7,8,9,10,11,12,13]
const chunks = 3

function getChunckedArr(arr, n) {
  let sub = new Array(n);
  let result = [];

  for (let i=0; i<sub.length; i++)
    sub[i] = []

  arr.forEach(function (val, index){
    let o = (index % n);
    sub[o][sub[o].length] = val;
  });

  for (let i=0; i<sub.length; i++)
    result.push.apply(result, sub[i]);

  return result;
}

const chunked = getChunckedArr(original, chunks);

Sorting an array by chunks of 3

4 Answers4