Is this implementation of quick sort considered "in place"?

Question

This is an implementation of quick sort which is easier for me to understand than the others I found. Although, this implementation does not appear to be "in place" as quick sort is apparently supposed to be. I'm thinking it isn't "in place" because you're returning a new array.

Am I correct in thinking that this implementation is not "in place"?

def quick_sort(sequence):
    length = len(sequence)
    if length <= 1:
        return sequence
    else:
        pivot = sequence.pop()

    items_greater = []
    items_lower = []

    for item in sequence:
        if item > pivot:
            items_greater.append(item)
        else:
            items_lower.append(item)

    return quick_sort(items_lower) + [pivot] + quick_sort(items_greater)

This is another implementation I found which I think IS an "in place" version of quick sort. -> https://stackabuse.com/quicksort-in-python/

If somebody could confirm this. I would be very appreciative. Thanks!

No, although it does modify the list you pass in. – juanpa.arrivillaga May 09 '20 at 00:10 — juanpa.arrivillaga, May 09 '20 at 00:10

jotaelesalinas · Answer 1 · 2020-05-09T00:15:26.287

2

No, it is not an "in place" implementation of the quick sort algorithm.

This implementation is easier to grasp, but it is very inefficient. Bear in mind that for the array that you are trying to sort, it creates a new one of the same size, then, for the half of it, it creates another one, then for the half or it another one, and so on... You will be wasting a lot of memory for big arrays.

edited May 09 '20 at 00:15

answered May 09 '20 at 00:10

jotaelesalinas

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It will also kill the runtime. – juanpa.arrivillaga May 09 '20 at 00:11
@juanpa.arrivillaga Do you believe that you can implement a fast quicksort in pure python? If so, I have an almost-new bridge you might be interested in... – Igor Rivin May 09 '20 at 00:16
I believe you can implement an *algorithmically efficient* quick sort. Yes – juanpa.arrivillaga May 09 '20 at 00:20
A hint for in-place sorting - your recursion arguments should be indexes into a list (telling the function what range of positions in the list it needs to operate on), and you should only be modifying the list, never constructing new lists. – Amitai Irron May 09 '20 at 00:33

score 1 · Answer 2 · answered May 09 '20 at 00:07

1

It is definitely not in place - you are generating new arrays (items_lower, etc)

answered May 09 '20 at 00:07

Igor Rivin

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is the implementation in the link "in place" – solobrogrammer May 09 '20 at 00:08
@crescendo01 yes, it is. – juanpa.arrivillaga May 09 '20 at 00:11

Is this implementation of quick sort considered "in place"?

2 Answers2