quick sort python recursion

Question

This is my quick sort code, the partition function works well, but I got a problem while calling the recursion. The pos changes every time it starts the function and then the list limits are change as well. How to fix that?

def partition(lst, start, end):

    pos=0
    if len(lst)<2:
        return 
    for i in range(len(lst[start:end])):
        if lst[i] < lst[end]:
            lst[i],lst[pos]=lst[pos],lst[i]
            pos+=1

        elif i==(len(lst[start:end])-1):
            lst[end],lst[pos]=lst[pos],lst[end]

    return pos

def quick_sort_recursive(lst, start, end):

        pos=partition(lst, start, end)
    if start<=pos<=end :
        quick_sort_recursive(lst, start, pos-1)
        quick_sort_recursive(lst, pos+1, end)
    else:
        return lst

Solution 1

There are numerous problems in your code, here are some fixes just to make it work:

def partition(lst, start, end):
    pos = start                           # condition was obsolete, loop won't
                                          # simply run for empty range

    for i in range(start, end):           # i must be between start and end-1
        if lst[i] < lst[end]:             # in your version it always goes from 0
            lst[i],lst[pos] = lst[pos],lst[i]
            pos += 1

    lst[pos],lst[end] = lst[end],lst[pos] # you forgot to put the pivot
                                          # back in its place
    return pos

def quick_sort_recursive(lst, start, end):
    if start < end:                       # this is enough to end recursion
        pos = partition(lst, start, end)
        quick_sort_recursive(lst, start, pos - 1)
        quick_sort_recursive(lst, pos + 1, end)
                                          # you don't need to return the list
                                          # it's modified in place

Example:

example = [3,45,1,2,34]
quick_sort_recursive(example, 0, len(example) - 1)
print example

Gives:

python test.py

[1, 2, 3, 34, 45]

Solution 2

I think in a pure recursive implementation the partition aux function is not needed:

def quicksort_recursive(a):
    if len(a) == 0:
        return a
    p = len(a) // 2
    l = [i for i in a if i < a[p]]
    m = [i for i in a if i == a[p]]
    r = [i for i in a if i > a[p]]
    return quicksort_recursive(l) + m + quicksort_recursive(r)

Solution 3

Trivial example of Quick Sort algorithm:

*

###  QUICKSORT
A=[44,5,22,0,323,995,94,4,7,15]
def main():
    r=len(A)-1
    p=0
    Result=QuickSort(A,p,r)
    print(Result)
def QuickSort(A,p,r):

    if p<r:
        q=partition(A, p, r)
        QuickSort(A, p, q-1)
        QuickSort(A, q+1, r)
    return A
def partition(A,p,r):
    x=A[r]
    i=p-1
    for j in range(p,r):
        if A[j]<=x:
            i=i+1
            a,b=A.index(A[i]), A.index(A[j])
            A[a],A[b]=A[b],A[a]
    d,c=A.index(A[i+1]),A.index(A[r])
    A[c],A[d]=A[d],A[c]
    return i+1
main()

*

Solution 4

recursive Quicksort algorithm

def quick_sort(array):
    if len(array) < 2:
        return array
    else:
        pivot = array[0]
        less = quick_sort([i for i in array if i < pivot])
        greater = quick_sort([i for i in array if i > pivot])
        return less + [pivot] + greater

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Comments

[shrishinde]

for getting less and greater you are doing list comprehension operation twice on same array. You can improve it to only once. Also if there are duplicate pivot numbers then your code will give wrong answer. e.g. for [6, 6, 5, 8, 2, 1] your code will return only [1, 2, 5, 6, 8]