JavaScript quicksort

Question

I have been looking around the web for a while and I am wondering if there is a 'stable' defacto implementation of quicksort that is generally used? I can write my own but why reinvent the wheel...

Answer 1

Quicksort (recursive)

function quicksort(array) {
  if (array.length <= 1) {
    return array;
  }

  var pivot = array[0];
  
  var left = []; 
  var right = [];

  for (var i = 1; i < array.length; i++) {
    array[i] < pivot ? left.push(array[i]) : right.push(array[i]);
  }

  return quicksort(left).concat(pivot, quicksort(right));
};

var unsorted = [23, 45, 16, 37, 3, 99, 22];
var sorted = quicksort(unsorted);

console.log('Sorted array', sorted);

Answer 2

1. Put your objects into an array.

2. Call Array.sort(). It's very fast.

   var array = [3,7,2,8,2,782,7,29,1,3,0,34];
   array.sort();
   console.log(array); // prints [0, 1, 2, 2, 29, 3, 3, 34, 7, 7, 782, 8]
   

Why does that print in lexicographic order? That's how array.sort() works by default, e.g. if you don't provide a comparator function. Let's fix this.

    var array = [3,7,2,8,2,782,7,29,1,3,0,34];
    array.sort(function (a, b)
    {
        return a-b;
    });
    console.log(array); // prints [0, 1, 2, 2, 3, 3, 7, 7, 8, 29, 34, 782]

Answer 3

You can easily "stabilize" an unstable sort using a decorate-sort-undecorate pattern

function stableSort(v, f)
{
    if (f === undefined) {
        f = function(a, b) {
            a = ""+a; b = ""+b;
            return a < b ? -1 : (a > b ? 1 : 0);
        }
    }
    var dv = [];
    for (var i=0; i<v.length; i++) {
        dv[i] = [v[i], i];
    }
    dv.sort(function(a, b){
              return f(a[0], b[0]) || (a[1] - b[1]);
            });
    for (var i=0; i<v.length; i++) {
        v[i] = dv[i][0];
    }
}

the idea is to add the index as last sorting term so that no two elements are now "the same" and if everything else is the same the original index will be the discriminating factor.

Answer 4

Quick Sort (ES6)

function quickSort(arr) {
  if (arr.length < 2) {
    return arr;
  }
  const pivot = arr[Math.floor(Math.random() * arr.length)];

  let left = [];
  let right = [];
  let equal = [];

  for (let val of arr) {
    if (val < pivot) {
      left.push(val);
    } else if (val > pivot) {
      right.push(val);
    } else {
      equal.push(val);
    }
  }
  return [
    ...quickSort(left),
    ...equal,
    ...quickSort(right)
  ];
}

Few notes:

• A random pivot keeps the algorithm efficient even when the data is sorted.
• As much as it nice to use Array.filter instead of using for of loop, like some of the answers here, it will increase time complexity (Array.reduce can be used instead though).

Answer 5

A Functional equivalent

In celebration of Functional Javascript, which appears to be the in thing

at the moment, especially given ES6+ wonderful syntactic sugar additions. Arrow functions and destructuring I propose a very clean, short functional equivalent of the quicksort function. I have not tested it for performance or compared it to the built-in quicksort function but it might help those who are struggling to understand the practical use of a quicksort. Given its declarative nature it is very easy to see what is happening as oppose to how it works.

Here is a JSBin version without comments https://jsbin.com/zenajud/edit?js,console

function quickSortF(arr) {
    // Base case
    if (!arr.length) return []

    // This is a ES6 addition, it uses destructuring to pull out the 
    // first value and the rest, similar to how other functional languages
    // such as Haskell, Scala do it. You can then use the variables as 
    // normal below
    const [head, ...tail] = arr,
          // here we are using the arrow functions, and taking full 
          // advantage of the concise syntax, the verbose version of
          // function(e) => { return e < head } is the same thing
          // so we end up with the partition part, two arrays,
          // one smaller than the pivot and one bigger than the 
          // pivot, in this case is the head variable
          left = tail.filter( e => e < head),
          right = tail.filter( e => e >= head)

       // this is the conquer bit of divide-and-conquer
       // recursively run through each left and right array
       // until we hit the if condition which returns an empty
       // array. These results are all connected using concat,
       // and we get our sorted array.
       return quickSortF(left).concat(head, quickSortF(right))           

}

const q7 = quickSortF([11,8,14,3,6,2,7]) 
//[2, 3, 6, 7, 8, 11, 14]
const q8 =  quickSortF([11,8,14,3,6,2,1, 7])
//[1, 2, 3, 6, 7, 8, 11, 14]
const q9 = quickSortF([16,11,9,7,6,5,3, 2])
//[2, 3, 5, 6, 7, 9, 11, 16]

console.log(q7,q8,q9)

The comments should provide enough if it is already not clear what is happening. The actual code is very short without comments, and you may have noticed I am not a fan of the semicolon. :)

Answer 6

In this blog http://www.nczonline.net/blog/2012/11/27/computer-science-in-javascript-quicksort/ which has pointed out that Array.sort is implemented in quicksort or merge sort internaly.

Quicksort is generally considered to be efficient and fast and so is used by V8 as the implementation for Array.prototype.sort() on arrays with more than 23 items. For less than 23 items, V8 uses insertion sort[2]. Merge sort is a competitor of quicksort as it is also efficient and fast but has the added benefit of being stable. This is why Mozilla and Safari use it for their implementation of Array.prototype.sort().

and when using Array.sort,you should return -1 0 1 instead of true or false in Chrome.

arr.sort(function(a,b){
  return a<b;
});
// maybe--> [21, 0, 3, 11, 4, 5, 6, 7, 8, 9, 10, 1, 2, 12, 13, 14, 15, 16, 17, 18, 19, 20, 22]
arr.sort(function(a,b){
  return a > b ? -1 : a < b ? 1 : 0;
});
// --> [22, 21, 20, 19, 18, 17, 16, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0]

Answer 7

var array = [8, 2, 5, 7, 4, 3, 12, 6, 19, 11, 10, 13, 9];
quickSort(array, 0, array.length -1);
document.write(array);


function  quickSort(arr, left, right)
{
 var i = left;
 var j = right;
 var tmp;
 pivotidx = (left + right) / 2; 
 var pivot = parseInt(arr[pivotidx.toFixed()]);  
 /* partition */
 while (i <= j)
 {
  while (parseInt(arr[i]) < pivot)
  i++;
  while (parseInt(arr[j]) > pivot)
   j--;
  if (i <= j)
  {
   tmp = arr[i];
   arr[i] = arr[j];
   arr[j] = tmp;
   i++;
   j--;
  }
 }

 /* recursion */
 if (left < j)
  quickSort(arr, left, j);
 if (i < right)
  quickSort(arr, i, right);
 return arr;
}

Answer 8

Using ES6 rest, spread:

smaller = (a, list) => list.filter(x => x <= a)
larger = (a, list) => list.filter(x => x > a)
qsort = ([x, ...list]) => (!isNaN(x))
    ? [...qsort(smaller(x, list)), x, ...qsort(larger(x, list))]
    : []

Answer 9

This algorithm work almost as fast as the default implementation of Array.prototype.sort in chrome.

function quickSort(t){
    _quickSort(t,0,t.length-1,0,t.length-1);
}

function _quickSort(t, s, e, sp, ep){   
    if( s>=e )  return;
    while( sp<ep && t[sp]<t[e] ) sp++;  
    if( sp==e )
        _quickSort(t,s,e-1,s,e-1);  
    else{
        while(t[ep]>=t[e] && sp<ep ) ep--;      
        if( sp==ep ){
            var temp = t[sp];
            t[sp] = t[e];
            t[e] = temp;
            if( s!=sp ){
                _quickSort(t,s,sp-1,s,sp-1);
            }
            _quickSort(t,sp+1,e,sp+1,e);            
        }else{
            var temp = t[sp];
            t[sp] = t[ep];
            t[ep] = temp;
            _quickSort(t,s,e,sp+1,ep);
        }
    }
}

quickSort time (ms): 738
javaScriptSort time (ms): 603

var m = randTxT(5000,500,-1000,1000);
VS(m);

function VS(M){
    var t;
    t = Date.now();
    for(var i=0;i<M.length;i++){
        quickSort(M[i].slice());
    }console.log("quickSort time (ms): "+(Date.now()-t));

    t = Date.now();
    for(var i=0;i<M.length;i++){
        M[i].slice().sort(compare);
    }console.log("javaScriptSort time (ms): "+(Date.now()-t));
}

function compare(a, b) {
    if( a<b )
        return -1;
    if( a==b )
        return 0;
    return 1;
}

function randT(n,min,max){
    var res = [], i=0;
    while( i<n ){
        res.push( Math.floor(Math.random()*(max-min+1)+min) );
        i++;
    }
    return res; 
}
function randTxT(n,m,min,max){
    var res = [], i=0;
    while( i<n ){
        res.push( randT(m,min,max) );
        i++;
    }
    return res; 
}

Answer 10

Yet another quick sort demonstration, which takes middle of the array as pivot for no specific reason.

const QuickSort = function (A, start, end) {
    // 
    if (start >= end) {
        return;
    }
    // return index of the pivot
    var pIndex = Partition(A, start, end);
    // partition left side
    QuickSort(A, start, pIndex - 1);
    // partition right side
    QuickSort(A, pIndex + 1, end);
}

const Partition = function (A, start, end) {
    if (A.length > 1 == false) {
        return 0;
    }
    let pivotIndex = Math.ceil((start + end) / 2);
    let pivotValue = A[pivotIndex];
    for (var i = 0; i < A.length; i++) {
        var leftValue = A[i];
        // 
        if (i < pivotIndex) {
            if (leftValue > pivotValue) {
                A[pivotIndex] = leftValue;
                A[i] = pivotValue;
                pivotIndex = i;
            }
        }
        else if (i > pivotIndex) {
            if (leftValue < pivotValue) {
                A[pivotIndex] = leftValue;
                A[i] = pivotValue;
                pivotIndex = i;
            }
        }
    }
    return pivotIndex;

}

const QuickSortTest = function () {
    const arrTest = [3, 5, 6, 22, 7, 1, 8, 9];
    QuickSort(arrTest, 0, arrTest.length - 1);
    console.log("arrTest", arrTest);
}
// 
QuickSortTest();

Answer 11

I really thought about this question. So first I found the normal search mode and wrote.

let QuickSort = (arr, low, high) => {
    if (low < high) {
        p = Partition(arr, low, high);
        QuickSort(arr, low, p - 1);
        QuickSort(arr, p + 1, high);
    }
    return arr.A;
}

let Partition = (arr, low, high) => {
    let pivot = arr.A[high];
    let i = low;
    for (let j = low; j <= high; j++) {
        if (arr.A[j] < pivot) {
            [arr.A[i], arr.A[j]] = [arr.A[j], arr.A[i]];
            i++;
        }
    }
    [arr.A[i], arr.A[high]] = [arr.A[high], arr.A[i]];
    return i;
}

let arr = { A/* POINTER */: [33, 22, 88, 23, 45, 0, 44, 11] };
let res = QuickSort(arr, 0, arr.A.length - 1);
console.log(res);

Result is [0, 11, 22, 23, 33, 44, 45, 88] But its not stable; so I checked the other answers and the Idea of @6502 was interesting to me that "two items do not have to be the same" to be distinguishable. Well, I have a solution in my mind, but it is not optimal. We can keep the indexes of the items in a separate array. Memory consumption will almost double in this idea.

arr.A => Array of numbers

arr.I => Indexes related to each item of A

influencer => This should be a very very small number; I want to use this as a factor to be able to distinguish between similar items.

So we can change the partition like this:

let Partition = (arr, low, high) => {
    let pivot = arr.A[high];
    let index = arr.I[high];
    let i = low;
    for (let j = low; j <= high; j++) {
        if (arr.A[j] + (arr.I[j] * influencer) < pivot + (index * influencer)) {
            [arr.A[i], arr.A[j]] = [arr.A[j], arr.A[i]];
            [arr.I[i], arr.I[j]] = [arr.I[j], arr.I[i]];
            i++;
        }
    }
    [arr.A[i], arr.A[high]] = [arr.A[high], arr.A[i]];
    [arr.I[i], arr.I[high]] = [arr.I[high], arr.I[i]];
    return i;
}

let influencer = 0.0000001;

let arr = {
    I/* INDEXES */: [10, 11, 12, 13, 14, 15, 16, 17, 18, 19],
    A/* POINTER */: [33, 22, 88, 33, 23, 45, 33, 89, 44, 11]
};
let res = QuickSort(arr, 0, arr.A.length - 1);
console.log(res);

Result:

I: [19, 11, 14, 10, 13, 16, 18, 15, 12, 17],
A: [11, 22, 23, 33, 33, 33, 44, 45, 88, 89]

Answer 12

More compact and easy to understand quicksort implementation

const quicksort = arr =>
  arr.length <= 1
    ? arr
    : [
        ...quicksort(arr.slice(1).filter((el) => el < arr[0])),
        arr[0],
        ...quicksort(arr.slice(1).filter((el) => el >= arr[0])),
      ];

Answer 13

try my solution

const quickSort = (arr) => {

    // base case
    if(arr.length < 2) return arr;

    // recurisve case

    // pick a random pivot
    let pivotIndex = Math.floor(Math.random() * arr.length);
    let pivot = arr[pivotIndex];
    let left = [];
    let right = [];

    // make  array of the elements less than pivot and greater than it
    for(let i = 0; i < arr.length; i++) {
        if(i === pivotIndex) {
            continue;
        }

        if(arr[i] < pivot) {
            left.push(arr[i])
        } else {
            right.push(arr[i])
        }
    }
    
    // call the recursive case again
    return quickSort(left).concat([pivot], quickSort(right));
} 

when testing this

quickSort([7, 5, 3, 2, 8, 1, 5])   // returns [[1, 2, 3, 5, 5, 7, 8]]

Answer 14

This is it !!!

function typeCheck(a, b){
  if(typeof a === typeof b){
    return true;
  }else{
    return false;
  }
}

function qSort(arr){
  if(arr.length === 0){
    return [];
  }

  var leftArr = [];
  var rightArr = [];
  var pivot = arr[0];

  for(var i = 1; i < arr.length; i++){
    if(typeCheck(arr[i], parseInt(0))){
      if(arr[i] < pivot){
        leftArr.push(arr[i]);
      }else { rightArr.push(arr[i]) } 
    }else{
      throw new Error("All must be integers");
    }
  }

  return qSort(leftArr).concat(pivot, qSort(rightArr));

}

var test = [];

for(var i = 0; i < 10; i++){
  test[i] = Math.floor(Math.random() * 100 + 2);
}

console.log(test);
console.log(qSort(test));

Answer 15

Slim version:

function swap(arr,a,b){
    let temp = arr[a]
    arr[a] = arr[b]
    arr[b] = temp
    return 1
}

function qS(arr, first, last){
    if(first > last) return

    let p = first
    for(let i = p; i < last; i++)
        if(arr[i] < arr[last]) 
            p += swap(arr, i, p)

    swap(arr, p, last)

    qS(arr, first, p - 1)
    qS(arr, p + 1, last)
}

Tested with random values Arrays, and seems to be always faster than Array.sort()

Answer 16

quickSort = (array, left, right) => {
    if (left >= right) {
        return;
    }
    const pivot = array[Math.trunc((left + right) / 2)];
    const index = partition(array, left, right, pivot);
    quickSort(array, left, index - 1);
    quickSort(array, index, right);
}

partition = (array, left, right, pivot) => {
    while (left <= right) {
        while (array[left] < pivot) {
            left++;
        }
        while (array[right] > pivot) {
            right--;
        }
        if (left <= right) {
            swap(array, left, right);
            left++;
            right--;
        }
    }
    return left;
}

swap = (array, left, right) => {
    let temp = array[left];
    array[left] = array[right];
    array[right] = temp;
}
let array = [1, 5, 2, 3, 5, 766, 64, 7678, 21, 567];
quickSort(array, 0, array.length - 1);
console.log('final Array: ', array);

Answer 17

A Fastest implementation

const quickSort = array =>
  (function qsort(arr, start, end) {
    if (start >= end) return arr;
    let swapPos = start;

    for (let i = start; i <= end; i++) {
      if (arr[i] <= arr[end]) {
        [arr[swapPos], arr[i]] = [arr[i], arr[swapPos]];
        swapPos++;
      }
    }
    qsort(arr, start, --swapPos - 1);
    qsort(arr, swapPos + 1, end);

    return arr;
  })(Array.from(array), 0, array.length - 1);

Answer 18

Quicksort using ES6, filter and spread operation.

We establish a base case that 0 or 1 elements in an array are already sorted. Then we establish an inductive case that if quicksort works for 0 or 1 elements, it can work for an array of size 2. We then divide and conquer until and recursively call our function until we reach our base case in the call stack to get our desired result.

O(n log n)

const quick_sort = array => {
  if (array.length < 2) return array; // base case: arrays with 0 or 1 elements are already "sorted"
  const pivot = array[0]; // recursive case;
  const slicedArr = array.slice(1); 
  const left = slicedArr.filter(val => val <= pivot); // sub array of all elements less than pivot
  const right = slicedArr.filter(val => val > pivot); // sub array of all elements greater than pivot
  return [...quick_sort(left), pivot, ...quick_sort(right)];
 }

Answer 19

const quicksort = (arr)=>{

  const length = Math.ceil(arr.length/2);
  const pivot = arr[length];
  let newcondition=false;
  for(i=0;i<length;i++){
    if(arr[i]>arr[i+1]){  
      [arr[i], arr[i+1]] = [arr[i+1], arr[i]]
      newcondition =true
    }
  }
  
  for(i=arr.length;i>length-1;i--){
     if(arr[i]>arr[i+1]){
      [arr[i], arr[i+1]] = [arr[i+1], arr[i]]
      newcondition =true
    }
  }
  return newcondition? quicksort(arr) :arr
 
}
const t1 = performance.now();
const t2 = performance.now();
console.log(t2-t1);
console.log(quicksort([3, 2, 4, 9, 1, 0, 8, 7]));

Answer 20

const quicksort = (arr)=>{
  if (arr.length < 2) return arr;
  const pivot = arr[0];
  const left = [];
  const right = [];
  arr.shift();   
  arr.forEach(number => {
    (number<pivot) ? left.push(number) : right.push(number);
  });
 return ([...quicksort(left), pivot, ...quicksort(right)]);
}
console.log(quicksort([6, 23, 29, 4, 12, 3, 0, 97]));

Answer 21

TypeScript version. O(nlogn)

type NumArr = Array<number>;
function quikSort(arr: NumArr): NumArr {
  if (arr.length < 2) {
    return arr;
  }
  const anchorIndex: number = Math.floor((arr.length - 1) / 2);
  const anchor: number = arr[anchorIndex];
  const greater: NumArr = [];
  const lesser: NumArr = [];
  anchor
  for (let index = 0; index < arr.length; index++) {
    const element = arr[index];
    if (element !== anchor) {
      if (element > anchor) {
        greater.push(element);
      } else {
        lesser.push(element);
      }
    }
  }
  return [...quikSort(lesser), anchor, ...quikSort(greater)];
}
const arrNum: NumArr = [2, 8, 1, 0, 25];
const answer: NumArr = quikSort(arrNum);

In anchorIndex value, you can also use Math.random(). But i like always take middle as an index.

Answer 22

How about this non-mutating functional QuickSort:

const quicksort = (arr, comp, iArr = arr) => {
  if (arr.length < 2) {
    return arr;
  }
  const isInitial = arr.length === iArr.length;
  const arrIndexes = isInitial ? Object.keys(arr) : arr;
  const compF = typeof comp === 'function'
  ? comp : (left, right) => left < right ? -1 : right < left ? 1 : 0;
  const [pivotIndex, ...indexesSansPivot] = arrIndexes;
  const indexSortReducer = isLeftOfPivot => [
    (acc, index) => isLeftOfPivot === (compF(iArr[index], iArr[pivotIndex]) === -1)
    ? acc.concat(index) : acc,
    []
  ];
  const ret = quicksort(indexesSansPivot.reduce(...indexSortReducer(true)), compF, iArr)
  .concat(pivotIndex)
  .concat(quicksort(indexesSansPivot.reduce(...indexSortReducer(false)), compF, iArr));
  return isInitial ? ret.reduce((acc, index) => acc.concat([arr[index]]), []) : ret;
};

As a bonus, it supports optional comparing function which enables sorting of array of objects per property/properties, and doesn't get slower if dealing with larger values/objects.

First quick sorts original array keys, then returns sorted copy of original array.