Quicksort in place Python

Question

I am extremely new to Python and i am trying my hand in Algorithms. I was just trying my hands in the in place sorting of an array using quicksort algo.

Below is my code . When i run this there is an infinite loop as output. Can anyone kindly go through the code and let me know where i am going wrong logically :

def quicksort(arr,start,end):
    if start < end:
        pivot = arr[int(start + (end - start)/2)]
        while end > start:
            while arr[start] < pivot:
                start = start + 1
            while arr[end] > pivot:
                end = end - 1
            if start <= end:
                arr[start],arr[end] = arr[end],arr[start]
                start = start + 1 
                end = end - 1


        quicksort(arr,0,end)
        quicksort(arr,start,len(arr) - 1)
    else:    
        return



arr = list(map(int,input().split(" ")))
quicksort(arr,0,len(arr) - 1)

print ("The final sorted array:",arr)

Thanks for any help in advance.

Answer 1

Your recursion is wrong. After doing Hoare Partition, you should do :

• call quicksort from start (of data that being sorted) to index that run from the end, and
• call quicksort from end (of data that being sorted) to index that run from the start

So you have to create new variables beside the argument to maintain start,end and indices that move toward each other in the partition.

In order to not change a lot of your code, i suggest you to change the argument

def quicksort(arr,left,right):
    start = left
    end = right

and do this in the recursion

quicksort(arr,left,end)
quicksort(arr,start,right)