Question

Why this code gives me wrong ? In Quick sort , I have picked up the first element as pivot:

I have traced that on paper,nothing is wrong.

    private void QuickSort(ref int [] S ,int l,int h)
    {
        //partioning
        int pivot_index = l;
        int j = 0;
        int temp = 0;
        for(int i=l+1;i<=h;i++)
            if (S[pivot_index] > S[i])
            {
                j++;
                temp = S[i];
                S[i] = S[j];
                S[j] = temp;
            }
        pivot_index = j;
        temp = S[l];
        S[l] = S[j];
        S[j] = temp;
        //end partioning

        if (l < h && pivot_index>l && pivot_index<h)
        {
            QuickSort(ref S, l, pivot_index - 1);
            QuickSort(ref S, pivot_index + 1, h);
        }
    }

here is my main :

        int[] List = get_input(textBox1.Text, ref n);
        //
        QuickSort(ref List, 0, n-1);

Answer 1

Your function is apparently supposed to sort [l, h] range in the array, yet for some reason you are swapping element number i with element number j. The latter (j) is out of [l, h] range (it is always initially 0 and then it becomes 1, 2, 3 and so on). What are you trying to do by this? Why are you swapping your elements to some totally unrelated remote location out of your sorting range?

In other words this does not even remotely look like a QuickSort-style sorting algorithm to me. Your unexplainable manipulations with j is one reason why your implementation cannot really sort anything.