Question

I have this code for pushing an object onto the stack, I am having trouble with understanding what each line does, if someone could explain why the previous node (newNode.next) has to be equal to top, and why then making top equal to the newNode.

void push(AnyClass newbObj)
{
  Node newNode = Node(newObj);
  newNode.next = top;
  top = newNode;
}

Answer 1

This looks like a linked list implementation. In a linked list each item has a pointer (reference) to the next one in the list.

The variable top seems to be a reference to the node at the beginning of the list and therefore represents the top of the stack.

Let’s assume then that the list starts looking like this:

item 1 -> item 2 -> etc...
^----top

The first line is just casting the parameter to the right type, we could instead have defined push() to take a Node instead and remove this line altogether.

Since we want to insert the new node at the front, firstly we need to ensure it points to the rest of the list using:

newNode.next = top;  //Point newNode's 'next' field to the current top of the list

This gives us something that looks like this

newNode -> item 1 -> item 2 -> etc...
           ^----top

But top is still pointing to the old item, so we now update that:

top = newNode; //Re-assign top to point to the new head of the list

Now the list looks like this:

newNode -> item 1 -> item 2 -> etc...
^----top

And we’re done.