Pointer assignment in python is changing the reference pointer list

Question

I was doing a code of rotating the linked list given the rotation amount on leetcode. I came across interesting problem.

Problem: After assigning the head (H) of the linked list to some other pointer (P) and then rotating the pointer P would make the next element of header pointer H to null.

Example:

If you pass any linked list header to function you can easily reproduce this

My purpose is not to traverse the list. I just wants to understand why does the reference of that pointer change and remains only first value of the listed gets printed?

def rotateRight(self, head: Optional[ListNode], k: int) -> Optional[ListNode]:
    dummy = head # Pointer pointing to the head of the linked list
    pre = None
    # Traversing the dummy pointer and keeping backward link to pre pointer
    while dummy:
        nex = dummy.next
        dummy.next = pre
        pre = dummy
        dummy = nex
    while head:
        print(head.val) # This would print only the head of the linked list which is 1 in my example of 1,2,3,4,5 it seems like its next is pointing to null
        head = head.next

Pass value of the reference pointing elements 1,2,3,4,5 and my head reference would only print the first element of the list Could anyone please explain the reason for this?

Since you are using the word "pointer" so many times in throughout the question, I'd point out that all of these are references, not pointers — DeepSpace, May 03 '22 at 18:52
Right. After your first statement. `dummy` and `head` both reference the same object. That does not make a copy. When you change `dummy`, you also change `head`. You don't need to change the object to traverse the list. — Tim Roberts, May 03 '22 at 18:56
Python does not have pointers. Python has *names*. They are just "labels" that you attach to objects. The label can be easily moved from object to object and an object can have many labels. — Bakuriu, May 03 '22 at 19:03

score 0 · Answer 1 · answered May 03 '22 at 18:57

0

You don't need to modify any link node to traverse the list.

def rotateRight(self, head: Optional[ListNode], k: int) -> Optional[ListNode]:
    dummy = head
    pre = None
    while dummy:
        nex = dummy.next
        pre = dummy
        dummy = nex
    while head:
        print(head.val) # This would print only the head of the linked list         
        head = head.next

Or, even simpler:

    while dummy:
        pre, dummy = dummy, dummy.next

answered May 03 '22 at 18:57

Tim Roberts

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Thanks for replying but I do understand that though my purpose is not to reverse the linked list but I want to know the behavior of python referencing. I have updated my question please feel free to comment on that – pujan prajapati May 08 '22 at 23:16

Pointer assignment in python is changing the reference pointer list

If you pass any linked list header to function you can easily reproduce this

My purpose is not to traverse the list. I just wants to understand why does the reference of that pointer change and remains only first value of the listed gets printed?

1 Answers1