Why does this quicksort program not give an output? - c++

Question

I am giving an integer array as input to an int array and I am expecting an output of sorted ints.
But there is no output.
I tried to use a recursive quicksort algo that will take an array of ints as input and give out a sorted array as output. I was expecting the sorted array to be shown in the terminal but there was no output.
This is the code
#include <iostream>
#define SIZE 10
using namespace std;
void swap(int* a, int* b)
{
int temp = *a;
*a = *b;
*b = temp;
}
int partition(int* array,int p, int q)
{
int key = array[p];
int i = p;
for(int j = p+1; j <= q; j++ )
{
if(key > array[j])
{
i++;
swap(&array[i],&array[j]);
}
}
swap(&array[i],&array[p]);
return i;
}
void quicksort(int* array,int p,int q)
{
if(p < q)
{
int r = partition(array,p,q);
quicksort(array,p,r-1);
quicksort(array,r,q);
}
}
int main()
{
int unsorted[SIZE] = {6,2,5,-1,11,3,17,-9,1,10};
quicksort(unsorted,0,SIZE-1);
cout << "The sorted array" << endl;
for(int i = 0; i < SIZE; i++)
{
cout << unsorted[i] <<" ";
}
}

Answer 1

The code you pasted enters infinite recursion in the quicksort function. Also, I would recommend using std::vector and pass that by reference instead of using C-style arrays. The std::vector is a more flexible container.
Here is the corrected code https://onlinegdb.com/q4LxWciVC:
#include <iostream>
#define SIZE 10
using namespace std;
void swap(int* a, int* b)
{
int temp = *a;
*a = *b;
*b = temp;
}
int partition(int* array,int p, int q)
{
int i = p-1;
int pivot = array[q]; // make pivot the last element of the array
for (int j{p};j<q;++j)
{
if (array[j] < pivot)
{
i++;
swap(&array[i], &array[j]);
}
}
swap(&array[i+1], &array[q]);
return i+1;
}
void quicksort(int* array,int p,int q)
{
if(p >= q){
return;
}
std::cout<<"\n---------Reached here---------\n";
int r = partition(array,p,q);
quicksort(array, p, r-1);
quicksort(array, r+1, q);
}
int main()
{
int unsorted[SIZE] = {6,2,5,-1,11,3,17,-9,1,10};
quicksort(unsorted,0,SIZE-1);
cout << "The sorted array" << endl;
for(int i = 0; i < SIZE; i++)
{
cout << unsorted[i] <<" ";
}
}
Output:
---------Reached here---------
---------Reached here---------
---------Reached here---------
---------Reached here---------
---------Reached here---------
---------Reached here---------
The sorted array
-9 -1 1 2 3 5 6 10 11 17