My Quicksort fails when sorting pre-sorted items, how to improve?

Question

This is a standard recursive quickSort implementation, and it succeeds at sorting larger lists for unsorted items but not for "pre-sorted" items. I knew it would take longer, but not fail altogether. Any possible improvements?

void quickSort(int *array, size_t count) {
    int pivot = array[count - 1];
    int max_index = 0;
    for (size_t i = 0; i < count - 1; ++i) 
    {
        if (array[i] < pivot) {
            swap(array[i], array[max_index]);
            ++max_index;
        }
    }
    swap(array[max_index], array[count - 1]);

    if (max_index > 1)
        quickSort(array, max_index);
    if (count - max_index - 1 > 1)
        quickSort(array + max_index + 1, count - max_index - 1);
}

Can you show an example of input that fails, along with the output? — cigien, May 15 '20 at 22:19
Now executing your Quick Sort of 9999 items. Your Quick Sort took 0.005833 Seconds. Now Executing your Quick Sort of 9999 pre-sorted items. (process 16128) exited with code -1073741571. ^^^^That's the output I receive, and it's a tester program for when I put in 9,999 items to sort. It doesn't output the time indicating something went wrong altogether, maybe stack overflow. — sabowsky, May 15 '20 at 22:24
@sabowsky Tester programs can be useful for finding errors, but they might not be the best tool for debugging. A good next step for you would be to create your own test program where you give your function a pre-sorted array of, let's say, three elements and you step through your code with a debugger. — JaMiT, May 15 '20 at 22:27
@JaMiT The issue is that it works fine on 100 items, so I'm thinking it might have to do with the style of recursion and stack overflow — sabowsky, May 15 '20 at 22:30

score 0 · Answer 1 · answered May 15 '20 at 23:15

I think your problem is the choice of a pivot value. You're using the rightmost value (array[count-1]) which can lead to worst-case behavior for an already sorted sequence. You create a partition with size 1 and another partition with size n-1. This causes a chain of n-1 recursive calls, leading to poor performance O(N²) or a stack overflow.

You should consider alternatives like:

A random index
The middle index
The median of three indexes (first, middle, last)
Something more complicated like a median of medians (ninther)

My Quicksort fails when sorting pre-sorted items, how to improve?

1 Answers1