Quicksort infinite loop if there are duplicate values

Question

I have a quicksort program that works fine until I try to sort an array with a duplicate number. The program gets stuck in an endless loop. I believe this happens in a block of code While(lower < upper).

void quickSort(int array[], int size){
  if(size < 2) return;

  int pivot, lower, upper, temp;

  //Set the indeces for the first and last elements                                 
  lower = 0;
  upper = size - 1;

  //Select pivot element randomly                                                   
  pivot = array[rand() % (size)];                                                                                                     

  while(lower < upper){
    //Lower must be a number < than pivot and upper a number >= pivot               
    while(array[lower] < pivot){
      lower++;
    }
    while(array[upper] > pivot){
      upper--;
    }

    //Swap upper and lower                                                          
    temp = array[lower];
    array[lower] = array[upper];
    array[upper] = temp;
  }

  //Repeat the past actions on the two partitions of the array recursively          
  quickSort(array, lower);
  quickSort(&array[lower+1], size-lower-1);
}

Answer 1

EDIT: code added.

From Wikipedia , the pseudo code for quick sorting in place is as follows:
(Not to mention that they should be trusted blindly)

function quicksort(array)
    if length(array) > 1
        pivot := select any element of array
        left := first index of array
        right := last index of array
        while left ≤ right
            while array[left] < pivot
                left := left + 1
            while array[right] > pivot
                right := right - 1
            if left ≤ right
                swap array[left] with array[right]
                left := left + 1
                right := right - 1
        quicksort(array from first index to right)
        quicksort(array from left to last index)

So you see that it is very similar to your algorithm with a few changes.

while(lower <= upper){

You also need to swap only if lower <= upper, and then update the indexes.

:

quicksort(array from first index to right) {array[0] to array[upper]}
quicksort(array from left to last index) {array[lower] to array[size-1]}

, , while, , .

:

#include <iostream>
#include <cstdlib>
using namespace std;

void quickSort(int array[], int size){
  if(size < 2) return;

  int pivot, lower, upper, temp;

  //Set the indeces for the first and last elements                                 
  lower = 0;
  upper = size - 1;

  //Select pivot element randomly                                                   
  pivot = array[rand() % (size)];                                                                                                     

  while(lower <= upper){
    //Lower must be a number < than pivot and upper a number >= pivot               
    while(array[lower] < pivot){
      lower++;
    }
    while(array[upper] > pivot){
      upper--;
    }

    //Swap upper and lower                                                          
    if ( lower <= upper ) {
        temp = array[lower];
        array[lower] = array[upper];
        array[upper] = temp;
        lower++;
        upper--;
    }
  }

  //Repeat the past actions on the two partitions of the array recursively          
  quickSort(array, upper+1);
  quickSort(&array[lower], size-lower);
}

int main() {
    // your code goes here
    int myArray[] = { 10, 9, 8, 7, 7, 7, 7, 3, 2, 1};
    quickSort( myArray, 10 );
    for ( int i = 0; i < 10; i++ )
        cout << myArray[i] << " ";
    return 0;
}

:

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