Hackerrank Minimum swaps 2 – loop goes to infinity from a small change in code

Question

This challenge asks that you find the minimum number of swaps to sort an array of jumbled consecutive digits to ascending order. This is the correct function for the question:

def minimumSwaps(arr):
    n = 0
    i =0
    while i < len(arr):
        index = arr[i]-1
        if arr[i] != arr[index]:
            arr[i], arr[index] = arr[index], arr[i]
            n+=1
            print(arr)
        else:
            i+=1
    
    return n

However if I get rid of the index = arr[i]-1 and replace index with arr[i]-1 everywhere like this:

def minimumSwaps(arr):
    n = 0
    i =0
    while i < len(arr):
        
        if arr[i] != arr[arr[i]-1]:
            arr[i], arr[arr[i]-1] = arr[arr[i]-1], arr[i]
            n+=1
            print(arr)
        else:
            i+=1
    
    return n

The loop goes to infinity and I cant figure out why this is the case.
Any help will be appreciated.

Asked By: Avighna Jha

||

Source

Answer 1

arr[i], arr[arr[i]-1] = arr[arr[i]-1], arr[i] first evaluates the rhs terms arr[arr[i]-1], arr[i] this is unchanged compared to using index, now think of them being stored in temporary variables a and b. Then it assigns to the lhs terms one by one. First a to arr[i] and only after assigns b to arr[arr[i]-1], at this stage arr[i] has alreday changed so this is no longer equivalent to using index.

Answered By: Julien

Hackerrank Minimum swaps 2 – loop goes to infinity from a small change in code

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