Question

I made a quicksort algorithm out of the visual presentation of the algorithm in youtube that I watched, but my recursion does not work at all. 🙁 If I commented out these 2 lines,

quicksort(array,0,start-1);
quicksort(array,start+1,temp);

.. The program does not crash and the output becomes 2,1,3,5,4 which is partly correct.. But it crashes when it enters the recursion. After the whole while loop, the start becomes the same as the end..

#include <stdio.h>
#include <conio.h>

void swap(int *a, int *b){

int temp = *a;
*a = *b;
*b = temp;
}

void quicksort(int *array, int start, int end){

int pivot = start;
int temp = end;     
while(start != end){

if(pivot==start){
    if(array[pivot] > array[end]){
    swap(&array[end],&array[pivot]);
    pivot = end;
    start++;
    }
    else
    end--;
}
else{
if(array[pivot] < array[start]){
    swap(&array[start],&array[pivot]);
    pivot = start;
    end--;
}  
else
start++;   

}               
}

quicksort(array,0,start-1);
quicksort(array,start+1,temp);
}




main(){

int x[5] = {3,1,5,2,4};
int i;
quicksort(x,0,4);
for(i=0;i<5;i++)
printf("%d ", x[i]);
getch();
}

Answer 1

What is missing is the point to cancel the algorithm. If you check the control flow of you function, you’ll see that on every path the application can walk though this function the quicksort function is called again. Finding out when you are done is simple. You just need to exit the function without calling quicksort again in case the parameters start and end are equal. That should do the trick.