[Tutor] counter not working in Quick Sort script

[Patti Scott]

Mac OS 10.10
Python 3.4.3
I self-study Python and am using it for a Coursera algorithm class.

Hi!  My script sorts correctly on all my test arrays. My accumulator variable to count the number of comparisons returns nonsense.  I'm counting the (length - one) of each sublist that will be sorted in place, not the individual comparisons.  I put in a print statement for the count variable, and the expected values print, but the function seems to be returning the first increment of the count variable instead of the final value.  I reread on the difference between print and return, but I haven't figured this out yet.  I think I'm doing something language-specific wrong would appreciate another set of eyes.


def partition(A, start, stop):
    p = A[start] # make pivot first element in partition
    i = start + 1
    for j in range(i, stop):
        # swap elements if A[j] bigger than pivot
        if A[j] < p:
            A[j], A[i] = A[i], A[j]
            i += 1

    # swap pivot into sorted index
    A[start], A[i-1] = A[i-1], A[start]
    return i



def quick_sort(A, start, stop, count):
    if start < stop:
        # increment count by length of partition less the pivot
        count += (stop - start - 1)
        print(count)
        split = partition(A, start, stop)

        # recursively sort lower partition
        quick_sort(A, start, split-1, count)
        # recursively count upper partition
        quick_sort(A, split, stop, count)

        return count


def main():
    unsorted = [ 1, 2, 3, 4, 5, 6, 7, 8 ]
    count = quick_sort(unsorted, 0, len(unsorted), 0)
    print(unsorted)
    print("There are {} comparisons.".format(count))

main()


This code is giving me this output:

➜  algorithms  python3 quick_sort_first.py
7
13
18
22
25
27
28
28
[1, 2, 3, 4, 5, 6, 7, 8]
There are 7 comparisons.

Thank you,
Patricia