Cant head insert on doubly linked list

This is a discussion on Cant head insert on doubly linked list within the C++ Programming forums, part of the General Programming Boards category; Okay, this is my first time working with doubly linked lists and I can't get past the first thing... doing ...

  1. #1
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    Cant head insert on doubly linked list

    Okay, this is my first time working with doubly linked lists and I can't get past the first thing... doing a head insert.

    here is my code for the insert:
    Code:
    template <class T>
    void Dlist<T>::front_insert(const T& entry)
    {
         if(head == NULL)
                 head = new Dnode<T>(entry);
                 
         Dnode <T> * temp;
         temp = new Dnode<T>(entry);
         temp->next = head;
         head->prev = temp;
         head = temp;
    }
    the biggest problem im having is linking the new node so it has a previous pointer at NULL and a 'next' pointer at the node that was originally the head. Then I need to set the previous node of what was the head to the new head. consufing?

    Here is my Dlist.h:
    Code:
    #include "Dnode.h"
    
    #include <iostream>
    #include <string>
    
    template <class T>
    class Dlist{
    
      public:
             Dlist();
             Dlist(const Dlist& other);
             ~Dlist();
      /*      
       void list_clear(Dnode<T>*& head_ptr);
       void list_copy(const Dnode<T>* source_ptr, Dnode<T>*& head_ptr, Dnode<T>*& tail_ptr);
       std::size_t list_length(const Dnode<T>* head_ptr);
       void rear_insert(Dnode<T>* tail_ptr, const T& entry);
       void rear_remove(Dnode<T>*& tail_ptr);
       void front_remove(Dnode<T>*& head_ptr);
      */
       void front_insert(const T& entry);
     
      
     private:
              Dnode<T>* head;
              Dnode<T>* tail;
    };
    
    #include "Dlist.template"
    and here is Dnode as well just in case:
    Code:
    template <class T>
    class Dnode{
    
      public:      
      Dnode(T d =  T(), Dnode *n = NULL, Dnode *p = NULL){datafield = d; link_next = n; link_prev = p;}
      
      T& data(){return datafield;}
      Dnode * next(){return link_next;}
      Dnode * prev(){return link_prev;}
    
      void set_data(T d){datafield = d;}
      void set_link(Dnode * n = NULL, Dnode * p = NULL){link_next = n; link_prev = p;}
       
     private: 
                T datafield;
                Dnode * link_next;
                Dnode * link_prev;
    };
    i would really appreciate any instruction on this, I have the idea of what it is supposed to do but I think I am missing some fundamental knowledge in trying to implement this.
    thanks,
    -ac

  2. #2
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    >> the biggest problem im having is linking the new node so it has a previous pointer at NULL and a 'next' pointer at the node that was originally the head. Then I need to set the previous node of what was the head to the new head. consufing?

    The code you post does all that except the part about setting the new node's previous pointer to null.

    Also, don't forget to leave the function if the original head is null after you create the new head.

  3. #3
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    i keep getting this error in my code:

    67 C:\Dev-Cpp\240c1\Dlist.template invalid use of member (did you forget the `&' ?)
    68 C:\Dev-Cpp\240c1\Dlist.template invalid use of member (did you forget the `&' ?)

  4. #4
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    okay still looking for some help on this... i am getting the above two errors on these two lines of code:

    Code:
    temp->next = head;
    head->prev = temp;
    those two lines are from a function which can be seen in its entirety in the first post.

    thanks

  5. #5
    Frequently Quite Prolix dwks's Avatar
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    Code:
    temp->next = head;
    head->prev = temp;
    Well, first of all, next and prev are functions:

    Code:
    public:
    // ...
      Dnode * next(){return link_next;}
      Dnode * prev(){return link_prev;}
    // ... 
     private: 
                Dnode * link_next;
                Dnode * link_prev;
    You want something like

    Code:
    temp->setnext(head);
    head->setprev(temp);
    Where setnext() is public and declared as
    Code:
    void setnext(Dnode *n) {link_next = n;}
    setprev() would be similar.
    dwk

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