using ref to delete Node in the middle of a linked list, given only access to that node

Question

I have the solution. So I do not need help, but I have a question. This code works:

public void Delete(ref Node n)
{
    n = n.next;
}

LinkedList list = new LinkedList();
list.AddTail(4);
list.AddTail(3);
list.AddTail(2);
list.AddTail(1);
list.AddTail(0);

list.Delete(ref list.head.next.next);

But this code does not:

Node n = list.head.next.next;
list.Delete(ref n);

Why?

EDIT:

public class Node
{
    public int number;
    public Node next;

    public Node(int number, Node next)
    {
        this.number = number;
    }
}

@SimonKarlsson My own implementation. I am practicing algorithms. — Orvel, Feb 09 '16 at 12:10
@Orvel then please provide the implementation of your delete — Simon Karlsson, Feb 09 '16 at 12:11
I _think_ it's not working because in the first case you are changing the `next` property of `list.head.next` but in the second case you are just changing the node that your local variable `n` is referencing. — Mark, Feb 09 '16 at 12:23
You might want to read [Parameter passing in C#](http://www.yoda.arachsys.com/csharp/parameters.html) by Jon Skeet. — juharr, Feb 09 '16 at 12:32
All the `ref` keyword does is to not pass a copy. But when you do `Node n = list.head.next.next;` you are making a copy. — juharr, Feb 09 '16 at 12:41
@juharr Well, you are not using the new keyword. So a new object is not being created. — Orvel, Feb 09 '16 at 12:53
@Orvel There isn't a new object, there's a copy of the reference to the same object. It's like you have you address written on a piece of paper. Then I copy that onto another piece of paper. If I then change the address on my piece of paper it doesn't change the address on your piece of paper and it certainly doesn't effect your house. — juharr, Feb 09 '16 at 13:02

score 0 · Accepted Answer · answered Feb 09 '16 at 12:31

When you call

list.Delete(ref list.head.next.next);

You change reference (pointer) to field List.next.

But if you call

list.Delete(ref n);

You change reference (pointer) of local variable.

Just try to inline you code:

list.Delete(ref list.head.next.next);

looks like

list.head.next.next = list.head.next.next.next;

But

Node n = list.head.next.next; 
list.Delete(ref n);

looks like

Node n = list.head.next.next; 
n = n.next;

using ref to delete Node in the middle of a linked list, given only access to that node

1 Answers1