# Splitting a JS array into N arrays

Imagine I have an JS array like this:

``````var a = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11];
``````

What I want is to split that array into N smaller arrays. For instance:

``````split_list_in_n(a, 2)
[[1, 2, 3, 4, 5, 6], [7, 8, 9, 10, 11]]

For N = 3:
[[1, 2, 3, 4], [5, 6, 7, 8], [9, 10, 11]]

For N = 4:
[[1, 2, 3], [4, 5, 6], [7, 8, 9], [10, 11]]

For N = 5:
[[1, 2, 3], [4, 5], [6, 7], [8, 9], [10, 11]]
``````

For Python, I have this:

``````def split_list_in_n(l, cols):
""" Split up a list in n lists evenly size chuncks """
start = 0
for i in xrange(cols):
stop = start + len(l[i::cols])
yield l[start:stop]
start = stop
``````

For JS, the best right solution that I could come up with is a recursive function, but I don't like it because it's complicated and ugly. This inner function returns an array like this [1, 2, 3, null, 4, 5, 6, null, 7, 8], and then I have to loop it again and split it manually. (My first attempt was returning this: [1, 2, 3, [4, 5, 6, [7, 8, 9]]], and I decided to do it with the null separator).

``````function split(array, cols) {
if (cols==1) return array;
var size = Math.ceil(array.length / cols);
return array.slice(0, size).concat([null]).concat(split(array.slice(size), cols-1));
}
``````

Here's a jsfiddle of that: http://jsfiddle.net/uduhH/

How would you do that? Thanks! You can make the slices "balanced" (subarrays' lengths differ as less as possible) or "even" (all subarrays but the last have the same length):

``````function chunkify(a, n, balanced) {

if (n < 2)
return [a];

var len = a.length,
out = [],
i = 0,
size;

if (len % n === 0) {
size = Math.floor(len / n);
while (i < len) {
out.push(a.slice(i, i += size));
}
}

else if (balanced) {
while (i < len) {
size = Math.ceil((len - i) / n--);
out.push(a.slice(i, i += size));
}
}

else {

n--;
size = Math.floor(len / n);
if (len % size === 0)
size--;
while (i < size * n) {
out.push(a.slice(i, i += size));
}
out.push(a.slice(size * n));

}

return out;
}

///////////////////////

function \$(x) {
return document.getElementById(x);
}

function calc() {
var s = +\$('s').value, a = [];
while (s--)
a.unshift(s);
var n = +\$('n').value;
\$('b').textContent = JSON.stringify(chunkify(a, n, true))
\$('e').textContent = JSON.stringify(chunkify(a, n, false))
}

calc();
}``````
``````<p>slice <input type="number" value="20" id="s"> items into
<input type="number" value="6" id="n"> chunks:</p>
<pre id="b"></pre>
<pre id="e"></pre>`````` I just made an iterative implementation of the algorithm: http://jsfiddle.net/ht22q/. It passes your test cases.

``````function splitUp(arr, n) {
var rest = arr.length % n, // how much to divide
restUsed = rest, // to keep track of the division over the elements
partLength = Math.floor(arr.length / n),
result = [];

for(var i = 0; i < arr.length; i += partLength) {
var end = partLength + i,

if(rest !== 0 && restUsed) { // should add one element for the division
end++;
restUsed--; // we've used one division element now
}

result.push(arr.slice(i, end)); // part of the array

i++; // also increment i in the case we added an extra element for division
}
}

return result;
}
`````` I think this way using splice is the cleanest:

``````splitToChunks(array, parts) {
let result = [];
for (let i = parts; i > 0; i--) {
result.push(array.splice(0, Math.ceil(array.length / i)));
}
return result;
}
``````

For example, for `parts = 3`, you would take 1/3, then 1/2 of the remaining part, then the rest of the array. `Math.ceil` ensures that in case of uneven number of elements they will go to the earliest chunks.

(Note: this destroys the initial array.) You can reduce it into a matrix. The example below split the array (`arr`) into a matrix of two-positions arrays. If you want other sizes just change the 2 value on the second line:

``````target.reduce((memo, value, index) => {
if (index % 2 === 0 && index !== 0) memo.push([])
memo[memo.length - 1].push(value)
return memo
}, [[]])
``````

Hope it helps!

EDIT: Because some people is still commenting this doesn't answer the question since I was fixing the size of each chunk instead of the number of chunks I want. Here it comes the code explaining what I'm trying to explain in the comments section: Using the `target.length`.

``````// Chunk function

const chunk = (target, size) => {
return target.reduce((memo, value, index) => {
// Here it comes the only difference
if (index % (target.length / size) == 0 && index !== 0) memo.push([])
memo[memo.length - 1].push(value)
return memo
}, [[]])
}

// Usage

write(chunk([1, 2, 3, 4], 2))
write(chunk([1, 2, 3, 4], 4))

// For rendering pruposes. Ignore
function write (content) { document.write(JSON.stringify(content), '</br>') }`````` Recursive approach, not tested.

``````function splitArray(array, parts, out) {
var
len = array.length
, partLen

if (parts < len) {
partLen = Math.ceil(len / parts);
out.push(array.slice(0, partLen));
if (parts > 1) {
splitArray(array.slice(partLen), parts - 1, out);
}
} else {
out.push(array);
}
}
`````` Probably the cleaner approach would be the following (without using any other library) :

``````var myArray = [];
for(var i=0; i<100; i++){
myArray.push(i+1);
}
console.log(myArray);

function chunk(arr, size){
var chunkedArr = [];
var noOfChunks = Math.ceil(arr.length/size);
console.log(noOfChunks);
for(var i=0; i<noOfChunks; i++){
chunkedArr.push(arr.slice(i*size, (i+1)*size));
}
return chunkedArr;
}

var chunkedArr = chunk(myArray, 3);
console.log(chunkedArr);
``````

I have created my own array which is to be chunked. You can find the code here

Also we have a method "chunk" in the lodash library which is of great use. Hope that helps If you happen to know the size of the chunks you want beforehand, there's a pretty elegant ES6 way of doing this:

``````const groupsOfFour = ([a,b,c,d, ...etc]) =>
etc.length? [[a,b,c,d], groupsOfFour(etc)] : [[a,b,c,d]];

console.log(groupsOfFour([1,2,3,4,1,2,3,4,1,2,3,4]));``````

I find this notation pretty useful for, for example parsing RGBA out of a `Uint8ClampedArray`. I made it this way, it works...

``````function splitArray(array, parts) {
if (parts< array.length && array.length > 1 && array != null) {
var newArray = [];
var counter1 = 0;
var counter2 = 0;

while (counter1 < parts) {
newArray.push([]);
counter1 += 1;
}

for (var i = 0; i < array.length; i++) {
newArray[counter2++].push(array[i]);
if (counter2 > parts - 1)
counter2 = 0;
}

return newArray;
} else
return array;
}
`````` check my version of this array split

``````// divide array
Array.prototype.divideIt = function(d){
if(this.length <= d) return this;
var arr = this,
hold = [],
ref = -1;
for(var i = 0; i < arr.length; i++){
if(i % d === 0){
ref++;
}
if(typeof hold[ref] === 'undefined'){
hold[ref] = [];
}
hold[ref].push(arr[i]);
}

return hold;
};
`````` if you know wanna set child_arrays.length then i think this solution best:

``````function sp(size, arr){ //size - child_array.length
var out = [],i = 0, n= Math.ceil((arr.length)/size);
while(i < n) { out.push(arr.splice(0, (i==n-1) && size < arr.length ? arr.length: size));  i++;}
return out;
}
``````

call fn: sp(2, [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]) //2 - child_arrat.length

answer: [1, 2], [3, 4], [5, 6], [7, 8], [9, 10], Just use lodash' chunk function to split the array into smaller arrays https://lodash.com/docs#chunk No need to fiddle with the loops anymore! all above might work fine, but what if you have `associative` array with strings as keys?

``````objectKeys = Object.keys;

arraySplit(arr, n) {
let counter = 0;
for (const a of this.objectKeys(arr)) {
this.arr[(counter%n)][a] = arr[a];
counter++;
}
}
`````` I have one that doesn't alter original array

``````function splitArray(array = [], nPieces = 1){
const splitArray = [];
let atArrPos = 0;
for(let i = 0; i < nPieces; i++){
const splitArrayLength  = Math.ceil((array.length - atArrPos)/ (nPieces - i));
splitArray.push([]);
splitArray[i] = array.slice(atArrPos, splitArrayLength + atArrPos);
atArrPos += splitArrayLength;
}
return  splitArray
}`````` ``````splitToChunks(arrayvar, parts) {
let result = [];
for (let i = parts; i > 0; i--) {
result.push(arrayvar.splice(0, Math.ceil(arrayvar.length / i)));
}
return result;
}
`````` lodash `chunk(array, Math.round(array.length / n))` ``````function split(arr, n) {
var res = [];
while (arr.length) {
res.push(arr.splice(0, n));
}
return res;
}`````` Old question, but since vanillaJS is not a requirement and so many are trying to solve this with lodash/chunk, and without mistaking what `_.chunk` actually does, here's a concise + accurate solution using `lodash`:

(Unlike the accepted answer, this also guarantees n columns even if `originalArray.length` < `numCols`)

``````import _chunk from 'lodash/chunk'

/**
* Split an array into n subarrays (or columns)
* @param  {Array} flatArray Doesn't necessarily have to be flat, but this func only works 1 level deep
* @param  {Number} numCols   The desired number of columns
* @return {Array}
*/
export function splitArray(flatArray, numCols){
const maxColLength = Math.ceil(flatArray.length/numCols)
const nestedArray = _chunk(flatArray, maxColLength)
let newArray = []
for (var i = 0; i < numCols; i++) {
newArray[i] = nestedArray[i] || []
}
return newArray
}
``````

The `for` loop at the end is what guarantees the desired number of "columns". Another recursive works quite well, it is less ugly

``````function nSmaller(num, arr, sliced) {

var mySliced = sliced || [];
if(num === 0) {
return sliced;
}

var len = arr.length,
point = Math.ceil(len/num),
nextArr = arr.slice(point);

mySliced.push(arr.slice(0, point));
nSmaller(num-1, nextArr, mySliced);

return(mySliced);
}
`````` ``````function parseToPages(elements, pageSize = 8) {
var result = [];
while (elements.length) {
result.push(elements.splice(0, pageSize));
}
return result;
}
`````` ``````function splitArray(arr, numOfParts = 10){
const splitedArray = []
for (let i = 0; i < numOfParts;i++) {
const numOfItemsToSplice = arr.length / 10;
splitedArray.push(arr.splice(0, numOfItemsToSplice))
}
return splitedArray;
}
`````` You can use a simple recursive function

``````const chunkify = (limit, completeArray, finalArray = [])=>{
if(!completeArray.length) return finalArray
const a = completeArray.splice(0,limit);
return chunkify(limit, completeArray, [...finalArray,a])
}`````` If you can use `lodash` and would like a functional programming approach, here is what I come up with:

``````const _ = require('lodash')

function splitArray(array, numChunks) {
return _.reduce(_.range(numChunks), ({array, result, numChunks}, chunkIndex) => {
const numItems = Math.ceil(array.length / numChunks)
const items = _.take(array, numItems)
result.push(items)
return {
array: _.drop(array, numItems),
result,
numChunks: numChunks - 1
}
}, {
array,
result: [],
numChunks
}).result
}
`````` If you are using lodash, you can achieve it fairly easily like below:

``````import {chunk} from 'lodash';
// divides the array into 2 sections
chunk([1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11], 2); // => [[1,2,3,4,5,6], [7,8,9,10,11]]
``````