How to edit the indices of an argument array in JavaScript?

Question

I'm trying to write a quicksort function in JavaScript. My code is below:

function test_quicksort(){
  var arr = [0, 9, 8, 7, 6, 5, 4, 3, 2, 1];
  arr = quicksort_by_percent_filled(arr);
  Logger.log(arr);
}


function quicksort_setup(arr){
  var high = arr.size - 1;
  arr = quicksort_by_percent_filled(arr, 0, high);
  return arr
}


function quicksort_by_percent_filled(arr, low, high){
  if (low < high){
    var pi = partition(arr, low, high);
    quicksort_by_percent_filled(arr, low, pi - 1);
    quicksort_by_percent_filled(arr, pi + 1, high);
  }
  return arr;
}


function partition(arr, low, high){
    var pivot = arr[high];
    var smaller_boundary = low - 1;
    var curr_elem = low;
    for (; curr_elem <= high; curr_elem++){
      if (ar[curr_elem] < pivot){
            smaller_boundary++;
            swap(arr, curr_elem, smaller_boundary);
      }
    }
    swap(arr, high, smaller_boundary + 1);
    return smaller_boundary + 1;
}


function swap(arr, a, b){
  var temp = arr[a];
  arr[a] = arr[b];
  arr[b] = temp;
  Logger.log(arr);
}

Assuming Logger.log(arr) is a function that prints out the contents of the array, it should print out the array properly sorted. However, whenever I run test_quicksort, the [0, 9, 8, 7, 6, 5, 4, 3, 2, 1] is printed out instead. It seems to me that I am somehow unable to edit the array arr when it is passed as an argument. How can I work around this issue?

"However, whenever I run test_quicksort, the [0, 9, 8, 7, 6, 5, 4, 3, 2, 1] is printed out instead. It seems to me that I am somehow unable to edit the array arr when it is passed as an argument." No, that's not why. There's no problem changing the contents of an array passed to you as an argument. There's just a bug of some kind in your code. Step through it in a debugger (there's one built into your browser and your IDE) statement by statement, viewing the contents of variables and watching the logic. — T.J. Crowder
– T.J. Crowder, Commented Mar 21, 2018 at 7:22
@JonasW.: I assumed that's what swap did, but since the code isn't in the question... :-| — T.J. Crowder
– T.J. Crowder, Commented Mar 21, 2018 at 7:25

Faly · Accepted Answer · 2018-03-21 07:41:16Z

1

If you want to mutate the array passed to test_quicksort, you souldn't reassign a new array to it to avoid loosing it's reference (avoid arr = newArr;). Instead of reassigning, you can empty the array with arr.prototype.splice then push all elements of the sorted array into it:

function test_quicksort(arr){
    var newArr = quicksort_by_percent_filled(arr);
    arr.splice(0, arr.length);
    arr.push(...newArr);	
}


function quicksort_by_percent_filled() {
  return [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 0];
}

var arr = [0, 9, 8, 7, 6, 5, 4, 3, 2, 1];
test_quicksort(arr);
console.log(arr);

edited Mar 21, 2018 at 7:41

answered Mar 21, 2018 at 7:32

Faly

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1 Comment

Jonas Wilms Over a year ago

Basically you are right, however completely rewriting the array seems to be a bit unoptimal. Probably swap should directly do [arr[one], arr[two]] = [arr[two], arr[one]] however we cantknow for sure as its not part of the ops code

Nina Scholz · Accepted Answer · 2018-03-21 08:22:06Z

You could

add a swap function,
take quicksort_setup instead of calling quicksort_by_percent_filled directly,
take the length instead of size,
skip call of get_percent_filled(), which is missing and the return value is never used,
use arr instead of ar in for loop.

function swap(array, i, j) { // 1
    var temp = array[i];
    array[i] = array[j];
    array[j] = temp;
}

function test_quicksort() {
    var arr = [0, 9, 8, 7, 6, 5, 4, 3, 2, 1];
    arr = quicksort_setup(arr); // 2
    console.log(arr);
}

function quicksort_setup(arr) {
    var high = arr.length - 1; // 3
    var percent_filled; // = get_percent_filled(); // 4
    arr = quicksort_by_percent_filled(arr, 0, high, percent_filled);
    return arr
}

function quicksort_by_percent_filled(arr, low, high, percent_filled) {
    if (low < high) {
        var pi = partition(arr, low, high, percent_filled);
        quicksort_by_percent_filled(arr, low, pi - 1, percent_filled);
        quicksort_by_percent_filled(arr, pi + 1, high, percent_filled);
    }
    return arr;
}

function partition(arr, low, high, percent_filled) {
    var pivot = arr[high];
    var smaller_boundary = low - 1;
    var curr_elem = low;
    for (; curr_elem <= high; curr_elem++) {
        if (arr[curr_elem] < pivot) { // 5
        //if (percent_filled[arr[curr_elem]] < percent_filled[pivot]){
            smaller_boundary++;
            swap(arr, curr_elem, smaller_boundary);
        }
    }
    swap(arr, high, smaller_boundary + 1);
   return smaller_boundary + 1;
}

test_quicksort();

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