CODEWITHSUNDEEP
pythonrecursionquicksort
tavalendo
tavalendo

Reputation: 877

Quicksort returning wrong order

I am working on an implementation of quicksort in Python with recursion without creating new variables to hold left and right portions of the partitioned array.

I am getting wrong values when running the recursive step. This is what I have done so far:

def swap(a,i,j):
    tmp = a[i]
    a[i] = a[j]
    a[j] = tmp
    
def pivot(a, lo, hi):
    mid = (lo + hi) // 2
    # sort lo, mid, hi:
    if a[mid] < a[lo]:
        swap(a, lo, mid)
    if a[hi] < a[lo]:
        swap(a, lo, hi)
    if a[hi] < a[mid]:
        swap(a, mid, hi)

def partition(a, lo, hi):
    # place the pivot out of the way, in position hi -1 
    mid = (lo + hi)//2
    swap(a, mid, hi - 1)
    
    i = lo
    j = hi - 1
    pivot = a[j]
    
    while True:
        while True:
            i += 1
            if a[i] >= pivot: break
           
        while True:
            j -= 1
            if a[j] <= pivot: break
            
            
        if i >= j: break
        swap(a, i, j)
    swap(a, i, hi - 1)
    return i

Assuming the template code above is "correct". I had to implement the quick sort using the above implementations of pivot and partition. This is what I did:

def quicksort(a):
    _sort(a, 0, len(a)-1)
    
def _sort(a, left, right):
    if(left <  right):
        pivot(a, left, right)
        piv = partition(a, left, right)
        _sort(a, left, piv-1)
        _sort(a, piv+1, right)

When I call the quicksort with an list:

x = [98, 33, 11, 5, 1, 10, 11, 12, 14, 33, 55, 66, 556, 88]
quicksort(x)
print(x)
>>> [1, 5, 10, 11, 11, 12, 14, 33, 33, 55, 66, 88, 556, 98]

You can see that the 98 is misplaced. If I run like this:

x = [9, 7, 5, 11, 12, 2, 14, 3, 10, 6, 55, 66, 888, 33, 556, 10]
quicksort(x)
print(x)
>>> [2, 3, 5, 6, 7, 9, 10, 10, 11, 12, 14, 33, 55, 66, 556, 888]

So, for the above case its correct. But in other smaller cases it fails:

x = [98, 33, 11, 556, 88]
quicksort(x)
print(x)
>>> [33, 11, 88, 556, 98]

Can anyone help me find the error? Thanks :-)

Upvotes: 1

Views: 96

Answers (1)

alec_djinn
alec_djinn

Reputation: 10789

I see two errors, the first one is in your partition() function.

Try substituting swap(a, i, hi - 1) with swap(a, i, hi)

The second one is in _sort(). The last call should be _sort(a, piv, right)

The correct code is:

def partition(a, lo, hi):
    # place the pivot out of the way, in position hi -1 
    mid = (lo + hi)//2
    swap(a, mid, hi - 1)
    
    i = lo
    j = hi - 1
    pivot = a[j]
    
    while True:
        while True:
            i += 1
            if a[i] >= pivot: break
           
        while True:
            j -= 1
            if a[j] <= pivot: break
            
        if i >= j: break
        swap(a, i, j)
    swap(a, i, hi)
    return i

and

def _sort(a, left, right):
    if(left <  right):
        pivot(a, left, right)
        piv = partition(a, left, right)
        _sort(a, left, piv-1)
        _sort(a, piv, right)

Test:

x = [98, 33, 11, 556, 88]
quicksort(x)
print(x)
[11, 33, 88, 98, 556]

Some more tests:

import random
for n in range(10):
    x = [random.randint(1,999) for i in range(random.randint(4,10))]
    quicksort(x)
    print(x)

[104, 226, 721, 769]
[131, 453, 590, 730, 752, 834]
[132, 156, 191, 277, 541, 599, 666, 909, 919]
[114, 210, 280, 919, 968, 978]
[127, 212, 381, 458, 585, 594, 685, 809, 935]
[73, 90, 189, 591, 599, 686, 806, 829, 831, 906]
[89, 115, 208, 601, 774, 813, 842, 981]
[159, 177, 203, 231, 621, 759, 950]
[347, 348, 417, 476, 850, 902]
[8, 50, 51, 483, 499, 696, 842]

Upvotes: 1

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