CODEWITHSUNDEEP
pythonquicksort
David Tran
David Tran

Reputation: 47

Understading a quicksort python implementation

I found this code online.

def quick_sort(items):
""" Implementation of quick sort """
if len(items) > 1:
    pivot_index = len(items) / 2
    smaller_items = []
    larger_items = []
    for i, val in enumerate(items):
        if i != pivot_index:
            if val < items[pivot_index]:
                smaller_items.append(val)
            else:
                larger_items.append(val)

    quick_sort(smaller_items)
    quick_sort(larger_items)
    items[:] = smaller_items + [items[pivot_index]] + larger_items

The one line that gives me trouble is the last one. I believe it's basic concatenation, however, when I change "items[:]" to "items", the algorithm fails. What is special about the [:] at the end of the list?

If anyone one can help, I would really appreciate. Thank you in advance!

Upvotes: 0

Views: 59

Answers (1)

Photon
Photon

Reputation: 3222

This is in-place assignment, without replacing the list object with a new object. This way, the output is placed in the original list and can be read by the caller.

Upvotes: 2

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