CODEWITHSUNDEEP
c#while-looplinked-list
Dave Washington
Dave Washington

Reputation: 9

c# - Why can I alter my constructed link list before a while loop, but not after it?

So I created my own version of a linked list in C# and one of the methods I'm using is removing the last element from the list. After some fooling around trying to get this to work, I realized that for some reason my code does some funny stuff when trying to remove the last element.

Here is my code for the linked list:

class Node
{
  public Node next;
  public int data;
}

class LinkedL 
{
  private Node head;
  //Methods...
}

After creating a linked list consisting of 10, 1223, 53, 93, 2830, and 93, I wrote a function within my LinkedL class called Popback that removes the last element of the linked list, in this case 93.

In the following PopBack code, I purposely added a line to reset the temp.next.next to be null before the while loop and it does what I would want it to do had the list head been 10, 1223, and 53, returning 10, 1223. The while loop doesn't seem to do anything though.

public void PopBack()
{
  if (head == null) { return; }

  Node temp = head;
  temp.next.next = null; //this is the added line

  while (temp.next != null)
  {
    temp = temp.next;
  }

  temp = null;                
}

However, if I were to remove that line, it goes through the while loop but even after setting the updated temp to be null, the list (head) doesn't change. I have a similar function to this where I add an element to the end of the linked list and that works just fine. What could be the cause of this?

Upvotes: 0

Views: 66

Answers (1)

MarcinJuraszek
MarcinJuraszek

Reputation: 125640

Setting temp to null does just that - sets temp variable to point at null. It does not change what the Next property of the previous element points at.

To do what you want and remove the last element you need to modify the second-to-last Nodes Next to point at null:

public void PopBack()
{
    if (head == null) { return; }
    if (head.Next == null) { head = null; return; }

    Node temp = head;
    while (temp.next.next != null)
    {
        temp = temp.next;
    }

    temp.next = null;
}

Update

In one of your comments you mentioned:

Had I used temp = null instead, it would've made head = null like I want it to. Using it after the while loop however, it doesn't seem to affect head at all.

That's not true.

public void PopBack()
{
    if (head == null) { return; }

    Node temp = head;
    temp = null;

    // over here head won't be null!
}

You're making both head and temp point at the same class instance and later on change temp to point at null. head still points at the same class instance it pointed at before.

Now, if instead of doing temp = null you do temp.next = null it is equivalent to doing head.next = null directly. That's because both temp and head point at the same object and you're setting a field on that object to null.

Upvotes: 2

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