CODEWITHSUNDEEP
pythonrecursionquicksort
user1291967
user1291967

Reputation: 23

quicksort and recursion in python

I'm trying to implement quicksort in Python using 2 main functions - partition and quicksort. The partition function is designed so that it returns 2 arrays - bigger and smaller than p. after that quicksort is called on both of them separately. so the quicksort works like this:

def quicksort(array)
    pivot = 0 # pivot choice is irrelevant
    left,right = partition(array,pivot)
    quicksort(left)
    quicksoft(right)
    return left+right

But from my understanding it should be possible to design partition to return just one single index - delimiting bigger and smaller arrays and redesign quicksort as follows:

def quicksort(array)
    pivot = 0 # pivot choice is irrelevant
    i = partition(array,pivot)
    quicksort(array[:i-1])
    quicksoft(array[i:])
    return array

but this implementation returns partially sorted array

original array [5, 4, 2, 1, 6, 7, 3, 8, 9]
sorted array [3, 4, 2, 1, 5, 7, 6, 8, 9]

what am i missing here?

Upvotes: 2

Views: 2360

Answers (3)

Rob Berkes
Rob Berkes

Reputation: 86

I had the same problem, my quicksort was returning partially sorted lists. I found the problem was that I wasn't returning the pivot in it's own array. When I create an array for the pivot, it allows the recursion to work properly.

ie. my partition function returns instead of:

return left, right

it returns

return left, pivotval, right

Upvotes: 0

JiBB
JiBB

Reputation: 251

quicksort(array[:i-1]) doesn't actually call quicksort on the first partition of the array, it calls quicksort on a copy of the first partition of the array. Thus, your code is partitioning the array in place, then creating copies of the halves and trying to sort them (but never doing anything with the resulting arrays), so your recursive calls have no effect.

If you want to do it like this, you'll have to avoid making copies of the list with slicing, and instead pass around the whole list as well as the ranges you want your functions to apply to.

Upvotes: 1

andrew cooke
andrew cooke

Reputation: 46882

without seeing your code it's hard to be sure, but one possible error is the i-1:

>>> [1,2,3,4][:2]
[1, 2]
>>> [1,2,3,4][2:]
[3, 4]

(although you may be simply skipping the pivot?)

also, slices are new lists, not views:

>>> l = [1,2,3,4]
>>> l[2:][0] = 'three'
>>> l
[1, 2, 3, 4]

which is unfortunate (the typical functional program doing quicksort which is not a quicksort at all, dammit, because it's creating a pile of new arrays annoys me too...)

you can work round the second problem by passing the entire list plus lo/hi indices:

def quicksort(data, lo=0, hi=None):
    if hi is None: hi = len(data)
    ....

Upvotes: 4

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