Question

I am trying to write the code for quicksort but it keeps on going in infinte loop , I have tried hard to find the error in following code ,can anybody help please.

#include<stdio.h>
#include<stdlib.h>

int arr[100];

void quicksort(int low,int high)
{
    int i = low,j = high,pivotIndex,pivot;
    pivotIndex = rand()%20;
    pivot = arr[pivotIndex];

    if(i>=j)
        return;

    while(i<j)
    {

        while(arr[i]<pivot && i<j)
        {
            i++;
        }

        while(arr[j]>pivot && j>i)
        {
            j--;
        }

        if(i<j)
        {
            int temp = arr[i];
            arr[i] = arr[j];
            arr[j] = temp;
        }

    }
    quicksort(low,i);
    quicksort(i+1,high);
}

int main()
{
    int i,j,temp;

    for(i=0;i<20;i++)
        arr[i] = rand()%21;

    for(i=0;i<20;i++)
        printf("%d ",arr[i]);
    printf("\n");

    quicksort(0,19);

    for(i=0;i<20;i++)
            printf("%d ",arr[i]);
    printf("\n");

    return 0;
}

Answer 1

Your pivot selection is wrong, you should chose a pivot in (index) range [low,high), while you chose a pivot in range [0,20):

pivotIndex = rand()%20;

It will later make the partition not go well, and the value of i, which you later use to recurse – will be wrong.

---

EDIT:

One more issue is with the partitioning, it should also consider what to do with the pivot element and its duplicates – there should be some kind of “tie breaker” – the pivot should go to one side of the array (or some alternative).

For example, assume the pivot is 5 and the array is [5,3,8,5]:

You will just infinitely swap the 5’s over and over, which is definetly wrong.