pls any one tell me why the loop is not ending??
(program for quick sort)

#include<stdio.h>
int partition(int a[],int l,int r);
void quicksort(int a[],int low,int high);
int loc,temp,pivot,a[50],low,high,left,right;
int main()
{ 
  int n,i;
  printf("\nEnter the no: of elements : " );
  scanf("%d",&n);
  printf("\nEnter the numbers  : ");
  for(i=0;i<n;i++)
    {
      printf("   %d ",i);
      scanf("%d",&a[i]);
    }

  //  printf("\n%d %d ok",i,i+900);
  low=0;
  high=n-1;
  quicksort(a,low,high);
  for(i=0;i<n;i++)
    printf("  %d",a[i]);
  return 0;
}


void quicksort(int a[],int low,int high)
{
  left=low;
  right=high;
  while(left<right)
    {
      loc=partition(a,left,right);
      quicksort(a,left,(loc-1));
      quicksort(a,(loc+1),right);
    }
}


int  partition(int a[50],int l,int r)
{
  pivot=a[l];
  low=l-1;
  high=r+1;
  while(low<=high)
    {
      while(pivot<=a[high]&&low<high)
	high=high-1;
      while(pivot>=a[low]&&low<high)
	low=low+1;
      temp=a[high];
      a[high]=a[low];
      a[low]=temp;
    }
  loc=low;
  temp=a[loc];
  a[loc]=a[l];
  a[l]=temp;
  return loc;
}

Please tell me where either left or right are modified in the following loop:

while(left<right)
{
    loc=partition(a,left,right);
    quicksort(a,left,(loc-1));
    quicksort(a,(loc+1),right);
}

If your answer is "nowhere", then you get a gold star. :) That loop shouldn't be a loop at all, it should be a simple if statement and the recursion handles your "looping".

Also I notice that the loc variable used in while loop in the quick sort is not initialized.

Also I notice that the loc variable used in while loop in the quick sort is not initialized.

It's initialized with the return value of the partition function.