help in quicksort

http://ideone.com/QOJxKj please tell me what changes i should do in this quicksort program

You should call the functions on line 20, 21 and 22 as

	int pindex=partition(a,start,end-1);
	pindex=partition(a,start,pindex-1);
	pindex=partition(a,pindex+1,end); 

That is just a instead of a[].

Next, define you array as

static int a[1000001];

before your main() ( that is, define it as a global variable )

Also, change your second scanf() to

    scanf(" %d",&a[i]);

You are also trying to access one value which is not present in the array. Let’s explain that.

You accept t as the number of values to be inserted into the array. Then you send it as an argument to the function qiucksort(). That means you have an array of indices 0 to ( t - 1 ) or ( end - 1 ). But later you are trying to access a[end], which clearly does not have any value you stored in it.

You also have many variables in main(), namely

b,pindex,start,end

which are not even used in your main().

Next, in your function partition() you have

int partition(int a[],int start,int end)
{
	int pivot;
	pivot=a[end];
    int	pindex=a[start];
	for(int i=start;i<end;i++)
	{
		if(a[i]<=pivot)
		{
			int temp;
			temp=pivot;
			pivot=a[i];
			a[i]=temp;
			pindex=pindex+1;
		}
		swap(a[pindex],a[end])
			{
				int temp;
				temp=a[pindex];
				a[pindex]=a[end];
				a[end]=temp;
			}
			return pindex;
	}
 
}

I hope you didn’t mean std::swap in there ( line 41 on your ideone link ). And I don’t even know why you are using that return statement inside your for loop.

These are just some of the mistakes I noticed. I have not yet checked the logic of your code ( simply put, your program ran for me, but it didn’t give the expected output ).

In case you need more help, read this . By reffering to this link, you should understand that your code does not even look like a quicksort algorithm.

Here’s a working quicksort algorithm

#include <iostream>
using namespace std;
void quicksort(int [],int,int);

static    int a[1000001];

int main() {
    int t,i;

    scanf("%d", &t);
    for(i=0;i<t;i++)
       scanf(" %d",&a[i]);

    quicksort( a , 0 , t-1 );

    for(i=0;i<t;i++)
        printf("%d\n",a[i]);
    return 0;
} 
void quicksort(int arr[], int left, int right) {
      int i = left, j = right;
      int tmp;
      int pivot = arr[(left + right) / 2];
 
      /* partition */
      while (i <= j) {
            while (arr[i] < pivot)
                  i++;
            while (arr[j] > pivot)
                  j--;
            if (i <= j) {
                  tmp = arr[i];
                  arr[i] = arr[j];
                  arr[j] = tmp;
                  i++;
                  j--;
            }
      }
  }

This code will help you. If u can not understand, please comment.

#include 
#include 
using namespace std;


int partition(int arr[], int start, int end)
{
    int pivotValue = arr[start];
    int pivotPosition = start;

    for (int i=start+1; i<=end; i++)
    {
        if (pivotValue > arr[i])
        {
            swap(arr[pivotPosition+1], arr[i]);
            swap(arr[pivotPosition] , arr[pivotPosition+1]);
            pivotPosition++;
        }
    }

    return pivotPosition;
}


void quickSort(int arr[], int low, int high)
{
    if (low < high)
    {
        /* pi is partitioning index, arr[p] is now
           at right place */
        int pi = partition(arr, low, high);
        // Separately sort elements before
        // partition and after partition
        quickSort(arr, low, pi - 1);
        quickSort(arr, pi + 1, high);
    }
}



int main()
{
    int arr[10],n;
    cin>>n;

    for(int i=0; i<n; i++)
    {
      cin>>arr[i];
    }

    quickSort(arr, 0, n-1);

    for(int k = 0; k<n; k++)   cout<<arr[k]<<" ";

    return 0;
}

what problems are you getting ?

this program is showing error i m not able to correct it

thanks arjun i will try to again

@rahulsup , check out the code I have posted. Also, my name is Arun and not Arjun ( no j )