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;
}


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

onload = function () {
    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))
    }

    $('s').addEventListener('input', calc);
    $('n').addEventListener('input', calc);
    calc();
}

slice items into chunks:

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.)

function split(array, n) {
  let [...arr]  = array;
  var res = [];
  while (arr.length) {
    res.push(arr.splice(0, n));
  }
  return res;
}

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,
            add = false;
        
        if(rest !== 0 && restUsed) { // should add one element for the division
            end++;
            restUsed--; // we've used one division element now
            add = true;
        }
        
        result.push(arr.slice(i, end)); // part of the array
        
        if(add) {
            i++; // also increment i in the case we added an extra element for division
        }
    }
    
    return result;
}

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>') }

Update: 7/21/2020

The answer I've given a few years ago only works if originalArray.length <= numCols. You could alternatively use something like this function below, but that will create a layout that doesn't quite match the question at hand (horizontal sorting rather than vertical sorting). AKA: [1,2,3,4] -> [[1,4],[2],[3]]. I understand this might still provide value so I'll leave this here, but I recommend Senthe's answer.

function splitArray(flatArray, numCols){
  const newArray = []
  for (let c = 0; c < numCols; c++) {
    newArray.push([])
  }
  for (let i = 0; i < flatArray.length; i++) {
    const mod = i % numCols
    newArray[mod].push(flatArray[i])
  }
  return newArray
}

Original Answer from 2017:

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".

Mutation is, generally speaking, a Bad Thing™.

This is nice, clean, and idempotent.

function partition(list = [], n = 1) {
  const isPositiveInteger = Number.isSafeInteger(n) && n > 0;
  if (!isPositiveInteger) {
    throw new RangeError('n must be a positive integer');
  }

  const partitions = [];
  const partitionLength = Math.ceil(list.length / n);

  for (let i = 0; i < list.length; i += partitionLength) {
    const partition = list.slice(i, i+partitionLength);
    partitions.push( partition );
  }

  return partitions;
}

[Edited to add]

Here's another flavor where the caller specifies the partition size rather than the number of partitions to be created:

function partition(list = [], n = 1) {
  const isPositiveInteger = Number.isSafeInteger(n) && n > 0;
  if (!isPositiveInteger) {
    throw new RangeError('n must be a positive integer');
  }

  const partitions = [];

  for (let i = 0; i < list.length; i += n) {
    const partition = list.slice(i, i+n);
    partitions.push( partition );
  }

  return partitions;
}

And if you want that to be "balanced" such that the individual chunks will differ in length by no more than 1, that only requires a little math.

To distribute, say M things into N buckets in that manner, we need to first determine the quotient Q and remainder R of M / N.

Let Q denote the basic partition length. R will always be less than N, and is the number of excess items that need to be distributed across all the partitions. Ergo, the first R partitions will contain Q+1 items and the remaining partitions will contain Q items.

For example, to partition a list of 100 items into 8 buckets, we get:

M = 10 N = 8 Q = 12 R = 4

So we will get:

• 4 (R) buckets of Q+1 (13) items, and
• 4 (N-R) buckets of Q (12) items

And 4 * 13 + 4 * 12 reduces to 52+48, or 100.

That leads us to this:

function partition(list = [], n = 1) {
  const isPositiveInteger = Number.isSafeInteger(n) && n > 0;
  if (!isPositiveInteger) {
    throw new RangeError('n must be a positive integer');
  }

  const q = Math.floor( list.length / n );
  const r = list.length % n;

  let i   ; // denotes the offset of the start of the slice
  let j   ; // denotes the zero-relative partition number
  let len ; // denotes the computed length of the slice

  const partitions = [];
  for ( i=0, j=0, len=0; i < list.length; i+=len, ++j ) {
    len = j < r ? q+1 : q ;
    const partition = list.slice( i, i+len ) ; 
    partitions.push( partition ) ;
  }

  return partitions;
}

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.

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);
    }
}

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 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;
    }

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

Partition

const partition = (x,n) => {
  const p=x.length%n, q=Math.ceil(x.length/n), r=Math.floor(x.length/n);
  return [...Array(n)].reduce((a,_,i)=>(a[0].push(x.slice(a[1],(a[1]+=i<p?q:r))),a),[[],0])[0];
};

DEMO

// to make it consistent to filter pass index and array as arguments
const partition = (x,n) => {
    const p = x.length % n,q = Math.ceil(x.length / n),r = Math.floor(x.length / n);
    return [...Array(n)].reduce((a,_,i)=>(a[0].push(x.slice(a[1],(a[1]+=i<p?q:r))),a),[[],0])[0];
};

console.log(partition([], 3))
console.log(partition([1, 2], 3))
console.log(partition([1, 2, 3, 4, 5, 6, 7, 8, 9], 3))
console.log(partition([1, 2, 3, 4, 5, 6, 7, 8, 9, 10], 3))

For Typescript

const partition = <T>(x: T[], n: number) => {
    const p = x.length % n, q = Math.ceil(x.length / n), r = Math.floor(x.length / n);
    return [...Array(n) as never[]].reduce((a, _, i) =>
        (a[0].push(x.slice(a[1], a[1] += i < p ? q : r)), a)
        , [[], 0] as [T[][], number])[0]
}

ONE-LINER Partition (but different order)

const part=(x,n)=>x.reduce((a,v,i)=>(a[i%n].push(v),a),[...Array(n)].map(()=>[]));

DEMO

// to make it consistent to filter pass index and array as arguments
const part=(x,n)=>x.reduce((a,v,i)=>(a[i%n].push(v),a),[...Array(n)].map(()=>[]));

console.log(part([1, 2, 3, 4, 5], 3));
console.log(part([1, 2, 3, 4, 5, 6], 3));
console.log(part([1, 2], 3));

For Typescript

const part = <T>(array: T[], parts: number) =>
  array.reduce(
    (acc, value, i) => (acc[i % parts].push(value), acc),
    [...Array(parts)].map(() => []) as T[][]
  );

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], [11]

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!

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
} 

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
}

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])
}

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