A peculiar case of quicksort pivot

April 24, 2022

I implemented quicksort in C which runs perfectly fine. Then I started to play with the pivot element, now I am stuck in a bizzare situation. What I have implemented runs fine some times but doesnt run at all the other times(shows no output) and I am unable to pinpoint why is this happening. Here is my code.

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


void swap(int *x, int* y)
{
    int temp = *x;
    *x = *y;
    *y = temp;
}

void displayArray(int arr[], size_t size)
{
    for(int i = 0; i < size; ++i)
        printf("%d\t",arr[i]);
    printf("\n");
}

unsigned int getNMinNMax(int arr[], unsigned int lb, unsigned int ub)
{
    unsigned int a = rand()%(ub-lb +1) + lb, b =rand()%(ub-lb +1) + lb,c = rand()%(ub-lb +1) + lb;
    // printf("%d %d %d \n", a,b,c);
    // getchar();
    // inefficient comparisons incoming, brace yourselves
    if(arr[a] >= arr[b] && arr[a] < arr[c]) return a;
    else if(arr[b] >= arr[a] && arr[b] < arr[c]) return b;
    else return c;
}

unsigned int partition(int arr[], unsigned int lb, unsigned int ub)
{
    // pivot selection mania(select necessarily from array elements)****{needs more testing}
    // 1)middle element
    // swap(&arr[lb + (ub - lb)/2], &arr[lb]);
    // 2)neither smallest nor largest
    // swap(&arr[getNMinNMax(arr,lb,ub)], &arr[lb]); (problem here)
    // 3)random
    // swap(&arr[rand()%(ub-lb +1) + lb], &arr[lb]); (problem here)
    // 4)1st element(no optimisation)
    int pivot = arr[lb];
    unsigned int down = lb + 1, up = ub;
    while(down <= up)
    {
        while(arr[down] <= pivot)
            down++;
        while(arr[up] > pivot)
            up--;
        if(down < up)
            swap(&arr[down], &arr[up]);
    }
    arr[lb] = arr[up];
    arr[up] = pivot;
    return up;
}

void quickSort(int arr[], unsigned int lb, unsigned int ub)
{
    while(lb < ub)
    {
        unsigned int pivot = partition(arr, lb, ub);
        if (pivot - lb < ub - pivot)
        {
            quickSort(arr, lb, pivot - 1);
            lb = pivot + 1;
        }
        else
        {
            quickSort(arr, pivot + 1, ub);
            ub = pivot - 1;
        }
    }
}

int main()
{
    int arr[] = {1,2,3,5,0,-1,-2,-3};
    srand(time(NULL));
    quickSort(arr, 0, sizeof(arr)/sizeof(int)-1);
    displayArray(arr,sizeof(arr)/sizeof(int));
    return 0;
}

I have commented which lines cause the output to disappear. I am pretty sure my implementation works in other cases since I have not faced the output disappearacne in those, but feel free to point out any other mistakes. The compiler I used is Onlinegdb C compiler(which is gcc afaik).

PS: I have added my full code because I found it weird for my display function to not work when I mess with my partition function. I tried debugging that as well but no luck.

>Solution :

The problem is caused by:

while(lb < ub)

With the pivot adjustment, ub can reach -1, but the type of ub is unsigned int, so ub looks like a large positive number and the while loop will continue.

Changing this line to: