Quicksort partitioning infinite loop

Question

arr = [7, 3, 5, 6, 7, 1, 8, 0, 4, 9, 6, 2]
def partitioning(arr, l, d):
    pivot = 5
    while l <= d:
        while arr[l] < pivot:
            l += 1
        while arr[d] > pivot:
            d -= 1
        arr[l], arr[d] = arr[d], arr[l]
partitioning(arr, 0, len(arr) - 1)
print(arr)

I don't understand why when putting l <= d, when l and d become the same they stop moving and keep swapping to infinity?

Did you "execute" this program step by step, using either your favourite debugger or just pen and paper? — Jasper
– Jasper, Commented Feb 2, 2016 at 19:37

Blckknght · Accepted Answer · 2016-02-02 19:50:02Z

2

Your infinite loop happens when l == d and arr[l] == arr[d] == pivot. In this situation, the inner loops never do anything and the swap also doesn't do anything since the two indexes are the same (so you're swapping the pivot with itself).

You want your top loop to quit in this situation, since the array has been completely partitioned. You should change the <= on the outer loop to <, and it should work.

You'll need to change the pivot-choice logic going forward (since if the pivot is not in the list, your inner loops might run l or d off the end of the list), but I assume that picking the constant 5 is just a preliminary thing.

answered Feb 2, 2016 at 19:50

Blckknght

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