Implementing quicksort algorithm

Question

I found quicksort algorithm from this book

This is the algorithm

QUICKSORT (A, p, r)
if p < r
    q = PARTITION(A, p, r)
    QUICKSORT(A, p, q-1)
    QUICKSORT(A, q+1, r)

PARTITION(A, p, r)
x=A[r]
i=p-1
for j = p to r - 1
  if A <= x
     i = i + 1
     exchange A[i] with A[j]
exchange A[i+1] with A[r]
return i + 1

And I made this c# code:

private void quicksort(int[] input, int low, int high)
{
    int pivot_loc = 0;

    if (low < high)
        pivot_loc = partition(input, low, high);
    quicksort(input, low, pivot_loc - 1);
    quicksort(input, pivot_loc + 1, high);
}

private int partition(int[] input, int low, int high)
{
    int pivot = input[high];
    int i = low - 1;

    for (int j = low; j < high-1; j++)
    {
        if (input[j] <= pivot)
        {
            i++;
            swap(input, i, j);
        }
    }
    swap(input, i + 1, high);
    return i + 1;
}



private void swap(int[] ar, int a, int b)
{
    temp = ar[a];
    ar[a] = ar[b];
    ar[b] = temp;
}

private void print(int[] output, TextBox tbOutput)
{
    tbOutput.Clear();
    for (int a = 0; a < output.Length; a++)
    {
        tbOutput.Text += output[a] + " ";
    }
}

When I call function like this quicksort(arr,0,arr.Length-1); I get this error An unhandled exception of type 'System.StackOverflowException' occurred it pass empty array... when call function like this quicksort(arr,0,arr.Length); I get error Index was outside the bounds of the array. on this line int pivot = input[high]; but array passed successfully.

I also want to print it like this print(input,tbQuick); but where to place it so it would print when quicksort finished?

Answer 1

You did not properly implement the base case termination, which causes quicksort to never stop recursing into itself with sublists of length 0.

Change this:

if (low < high)
    pivot_loc = partition(input, low, high);
quicksort(input, low, pivot_loc - 1);
quicksort(input, pivot_loc + 1, high);

to this:

if (low < high) {
    pivot_loc = partition(input, low, high);
    quicksort(input, low, pivot_loc - 1);
    quicksort(input, pivot_loc + 1, high);
}

Answer 2

In addition to Deestan's answer, you also have this wrong:

for (int j = low; j < high-1; j++)

It should be:

for (int j = low; j < high; j++)