QUICKSORT not working as required.

general
1

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

int partition(int *a,int p, int q)
{
int key=a[q],t,i,j=q-1;
i=p;
    while(i<j){

            while(a[i]<key&&i<q)
                    i++;
            while(a[j]>key&&j>=p)
                    j--;


            if(i<j){
            t=a[i];
            a[i]=a[j];
            a[j]=t;
            }

    }
if(i>j&&j>=p){
     t=a[j];
     a[j]=a[q];
     a[q]=t;
}

return j;
 }

void quicksort(int *a,int ll, int ul){
int pos;
if(ll<ul){
	pos=partition(a,ll,ul);
	quicksort(a,ll,pos-1);
	quicksort(a,pos+1,ul);
}
}


int main(){
int  a[100],i,n;
scanf("%d",&n);
for(i=0;i<n;i++)
    scanf("%d",&a[i]);
    
quicksort(a,0,n-1);
for(i=0;i<n;i++)
    printf("%d  ",a[i]);
    
return 0;
}
2

Your partitioning algorithm is wrong.

Simple example: [45, 23]
You have only two numbers in here. So, in the very first call to partition, p=0 and q=1
Now, key=a[q](=23), i=p(=0), j=q-1(=0)

Now, while (i<j) and if (i > j && ...) are both false. So, it will return j(=0), and the partition is done (according to this implementation). But, the element at a[j] is 45, and there are still elements to tis right (23) which are smaller than this pivot element. So, the partitioning is wrong.

3

You might want to recheck the algorithm, whether you have mistyped some variable somewhere or not.

PS: if there is a repeat until construct in algorithm, like

repeat
    X
until C

then, it is translated into C code as

do {
    X
} while (!C);

Note that,

  1. it is do...while in action and not while
  2. the condition C is negated in the do...while