C++ linked list adding a new node to the beginning of the list

Question

I'm struggling with how to link the new node I am creating to the rest of my linked list.

template <class T>
void LinkedList<T>::addBeg(T value)
{
      ListNode *nodePtr = head;
      head = nodePtr->next;
      head = nodePtr;
      nodePtr = new ListNode(value);
}

I know what I did wrong here: the new value is not associated the linked list at all.

I think I know what I need to do. I'm pretty sure what I need to do is create the new value, insert into the beginning of the existing list, and then redefine head to be the newly created value.

The problem I'm having, is I don't know how to do this.

So, what I think I need to do (at least logically), is set

*nodePtr = new Listnode(value);

Then set

nodePtr = head;

then set

head = nodePtr-> next;

and then set the

new ListNode(value) = head;

Am I on the right track? I can't shake the nagging feeling that I'm not correctly linking the new ListNode to the existing list and I can't figure out if I am making the wrong steps or if I'm missing a step.

nodePtr = new Listnode(value); nodePtr-> next = head; head=nodePtr; — Steve
– Steve, Commented Oct 23, 2017 at 14:17

eshirima · Accepted Answer · 2017-10-23 14:17:19Z

2

To create a new node to the head of a list, follow these steps

Create a temporary node holding your value
Set the temporary node's next to point to the head
Set the head to temporary

answered Oct 23, 2017 at 14:17

eshirima

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2 Comments

Stephen Rivera Over a year ago

Thanks I thought something was missing. I even toyed with using a temporary node but I couldn't figure out the best way to use it. It was also around hour number 2 of being completely stumped so I'm sure that didn't help either.

eshirima Over a year ago

Hey, it happens to the best of us and glad it helped. Review what to do when someone answered my question

SgtDroelf · Accepted Answer · 2017-10-23 14:17:17Z

1

You first have to create a new Node, then link it to the current head. Then you switch the reference from your previous head to the newly created node.

ListNode *newHead = new ListNode;
newHead->next = head;
head = newHead;

answered Oct 23, 2017 at 14:17

SgtDroelf

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Comments

user2891462 · Accepted Answer · 2017-10-23 14:20:26Z

0

Think carefully about what you have and what you need to do:

You have two nodes that you care about: the current head and your new nodePtr.
You want nodePtr->next to point to your current head, and update head to point to nodePtr so that this is the first node in the list.

The order in which you do things is important: if you assign the new value of head before making nodePtr->next point to the previous value, you will lose your whole list! Therefore:

Create your new node nodePtr.
Make nodePtr->next point to head.
Make head point to nodePtr

answered Oct 23, 2017 at 14:20

user2891462

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Comments

AntonioM · Accepted Answer · 2017-10-23 14:20:28Z

0

You are on the wrong way. The solution is create the new node, link the next node to head, and after refer the head to new node:

template <class T>
void LinkedList<T>::addBeg(T value)
{
    ListNode *nodePtr = new ListNode(value);
    nodePtr->next = head;
    head = nodePtr;
}

answered Oct 23, 2017 at 14:20

AntonioM

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Comments

Austin Garrett · Accepted Answer · 2017-10-23 14:25:34Z

0

If you say nodePtr = head; then you're just overwriting the pointer you just made. You want to change nodePtr->next = head; head = nodePtr;

edited Oct 23, 2017 at 14:25

answered Oct 23, 2017 at 14:19

Austin Garrett

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