Linked List Template Class Insertion

[Dega]

Hi.
I have been scratching my head trying to find out why I am getting a crash every time I try to use the insert() method in a Linked List template class I am working on.

Here is the code I have for the insert method in question.

template <typename T>
void LinkedList<T>::Insert(T value){
//traverse the list and ADD a given value

     //dynamically allocate mem and create new node
     ListNode *NewNode = new ListNode; 
     NewNode->data = value; 
     NewNode->next= NULL; 
 
     //check if our first pointer has a value, if not... point to our new node
     if(first == NULL){
         first=NewNode;
     }
     else{
          //initialize current pointer to start of our list
          ListNode *current= first;
          
          //initialize a previous pointer
          ListNode *prev= NULL;
          
          //traverse our list to the insert location
          while (current->data < value && current->next !=NULL){ 
                prev = current; 
                current = current->next; 
          }//we should now be at our insertion location
          
          //update our pointers
          prev -> next = NewNode; 
          NewNode -> next = current;  
     }
     //increment size
     ListSize++;
}

What is going wrong? I traced the code (drew it out on paper) and I don't see how it is crashing. Inserting the first value (as I am testing) should just be running the following portion, right?:

if(first == NULL){
    first=NewNode;
}

Any idea what I am missing here?

[Dega]

Anyone? I need to get this figured out asap :(

[o1vk4iN6]

In the case there is only 1 node, "prev" remains nullptr and you try to set "prev->next" of a nullptr.

[Dega]

I'm still kinda lost. I am testing the insertion with only 1 value. This should only be doing:

first=NewNode;

right? (based on precondition that first is initialized to NULL, which it is).

[o1vk4iN6]

         ListNode *current= first; // only 1 value in list which is first
         ListNode *prev= NULL;
          
  
          while (current->data < value && current->next !=NULL){ 
                prev = current; // this never gets called as current->next is NULL
                current = current->next; 
          }

          prev -> next = NewNode; // prev is invalid so exception happens here 

[Dega]

Oh. I think I see it now...

So do I just catch it in an if/else statement (right after the while) like so?:

if(current->next == NULL){
    current->next = NewNode;
}

[o1vk4iN6]

Think about what that is doing, do you want NewNode to always be after current ?

[Dega]

lol... I must be thinking too hard at this point or something.
Guess I will have to go back to basics and hit up some tutorial videos or something. Hopefully it magically makes full sense before Monday.

[o1vk4iN6]

Yah it can be done with a pointer to a pointer pretty easily, or if it was a doubly linked list :P.