Quicksort sorting issues - c

Question

This might not be a conventional of doing quicksort.my first try at it.the numbers are not sorted in the way they should be.I have tried to sort a random list of numbers.However i am unable to identify the logical errors even after a strict checking.
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
int n;
int *expivot;
int *arr;
void quicksort();
void display();
int check();
main()
{
int i;
printf("to continue press 'a' always\n");
while(getch()=='a')
{
printf("Enter the length of list\n");
scanf("%d",&n);
time_t start,end;
double t;
start=clock();
arr=(int *)calloc(n,sizeof(int));
expivot=(int *)calloc(n,sizeof(int));
srand(time(NULL));
for(i=0;i<n;i++)
arr[i]=rand()%RAND_MAX + 1;
printf("\nelements inputted are:");
display();
quicksort();
end=clock();
t=(double)(end-start)/CLOCKS_PER_SEC;
printf("\n\nelements sorted are:");
display();
printf("\ntime take is %.15lf",t);
free(arr);
free(expivot);
}
}
void quicksort()
{
int low,high,temp;
int pivot=rand()%n;//generate random pivot
int store=pivot;
/*location of pivot might change due to swapping,so using store to store pivot location so as to add this to expivot list after running quickort once*/
int flag=1;
if(expivot[pivot]==1) // checks if it's an already used pivot
flag=0;
if(flag==1) //if the pivot is unused
{
low=pivot;
high=pivot;
while(low>0 && expivot[low]==0)
low--;
if(expivot[low]==1)//i
low++;
/*decrements low to a location where a value has been set permanently and then increase by 1,if nothing is set then decrements low to zero*/
/*increments high to a location where a value has been set permanently and then decrease by 1,if nothing is set then increments high to last index*/
while(high<n-1 && expivot[high]==0)
high++;
if(expivot[high]==1)
high--;
while(low<high)
{
if(arr[low]>=arr[pivot] && arr[high]<=arr[pivot])//checks swap possibilty
{
if(low==pivot) //if pivot is to be swapped store new location of pivot
store=high;
else if(high==pivot)
store=low;
temp=arr[low];
arr[low]=arr[high];
arr[high]=temp;
low++;
high--;
}
else
{
if(arr[low]<arr[pivot])
low++;
else if(arr[high]>arr[pivot])
high--;
}
}
expivot[store]=1;
/*final location of pivot,stores info that this location has a permanent value now
and cannot be used as a pivot*/
}
if(check()==1)
quicksort();
}
int check() //checks if there are any unused pivots
{
int i;
for(i=0;i<n;i++)
{
if(expivot[i]==0)
return 1;
}
return 0;
}
void display()
{
int i;
for(i=0;i<n;i++)
printf("%d ",arr[i]);
}

Answer 1

Your method is:
Randomly select a pivot from whole array;
From the pivot, spread a range to both direction, this range will be partitioned by the pivot;
All pivot will be cached in another array (item 5);
The range, mentioned in item 2 above, should apread as large as it can do, but: 1) should not beyond the range of whole array; 2) should not contain another pivot, if it does, stop and shrink one unit;
Partition the range by the pivot it spread from, then cache this pivot;
If all unit in the array has been selected as pivot, sorting is done. If not, repeat as above, over and over again.
There are three problems in your code:
1- "checd()"function should be:
int check() //checks if there are any unused pivots
{
int flag = 0;
int i;
for(i=0;i<n;i++)
{
if(expivot[i]==0)
flag = 1;
}
return flag;
}
You should check all member, see if they ALL satisfy your condition, but not one of them satisfy your condition.
2- While shrink the range, make sure the pivot is between "high" and "low" (equal is well). Keep tracking the index and value of the pivot. The while loop should be:
//"store" is not needed, change it to "pivot", even after this code block.
while(low<high)
{
if(arr[low]>=arr[pivot] && arr[high]<=arr[pivot])//checks swap possibilty
{
if(low==pivot) //if pivot is to be swapped store new location of pivot
pivot=high;
else if(high==pivot)
pivot=low;
temp=arr[low];
arr[low]=arr[high];
arr[high]=temp;
/////////////////////
if (low < pivot)
low++;
if (high > pivot)
high--;
/////////////////////
}
else
{
if(arr[low]<arr[pivot])
low++;
else if(arr[high]>arr[pivot])
high--;
}
}
3- Finally, once you get memory from calloc or malloc, check if it's NULL.
==================================
Additionally, you should make sure all unit in the array can be selected, because the random number in computer is mostly pseudo random number. So, maybe for a certain length, a particular unit cannot be chosen forever.

Answer 2

Quicksort is a Divide and Conquer algorithm. You cannot perform it without using stacks or recursion.
Edit: The function does use recursion (oops!). But this isn't quicksort. If your are changing the method of a standard sorting algorithm, then it is no more that algorithm.