Problems with Linked List

[Ehorn]

Hello all;

Having a little trouble setting my head pointer to NULL so that I can check to see if an addition to the linked list is the first one. Here is what I have so far: (this is for a class project btw)

dictionary.h (I am NOT allowed to modify this file)

#ifndef DICTIONARY_H
#define DICTIONARY_H

#include <string>

class Dictionary
{

public:
Dictionary();
void add (std::string word);
bool contains (std::string word) const;
private:
struct LinkedListNode
{
std::string data;
LinkedListNode* next;
};

LinkedListNode* first;

};
#endif

driver_dictionary.cpp (I am NOT allowed to modify this file either)

#include <cassert>
#include <iostream>
#include <string>

#include "Dictionary.h"

using namespace std;

void test1()
{
Dictionary one;
one.add ("ape"); <-- First word to add to dictionary
one.add ("zipper");
one.add ("cat");
one.add ("Bear");
one.add ("Ape");
}

int main(int argc, char** argv)
{
test1();

return 0;
}

And finally, the implementation file that I am trying to write:

#include <iostream>
#include <string>
#include <cstdlib>

#include "dictionary.h"


using namespace std;

Dictionary::Dictionary () <--- Constructor

{
string info;
LinkedListNode *first, *next;
LinkedListNode();
{
next = NULL;
first = NULL; <-- does NOT set first to NULL
}
}
void Dictionary::add (std::string word) <-- Recieves "ape" as the first word
{
string x;
cout << first << endl;
if (first == NULL) <-- first is never NULL so the first node is empty
{
first = new LinkedListNode;
first->data = word;
}
else
{
LinkedListNode *current = first;
LinkedListNode *prev = NULL;

x=current->data; <-- Program crashes here because there is nothing in
the linked list to retrieve data from.
cout << x << endl;

cout << "Current = " << current << endl;
cout << "prev = " << prev << endl ;

while (current != NULL && word < current->data)
{
prev = current;
current = current->next;
}
if (prev == NULL)
{
LinkedListNode *newnode = new LinkedListNode;
newnode->data = word;
newnode->next = first->next;
first = newnode;
}
}
}

I am fairly certain that the "while" loop and beyond has issues also but I am not worried about that right now.

Since I can't modify the .h file how can do I set the head pointer (first) to null before adding the first node?

Thanks :)

[fyrebyrd10]

Your header defines a node in the list as a LinkedListNode :

private:
    struct LinkedListNode
    {
        std::string data;
        LinkedListNode* next;
    };

    LinkedListNode* first;

Each LinkedListNode has 2 objects:
1) next : pointer to next node in list
2) data : the value that node holds

________________________________


Something like :

Dictionary::Dictionary()
{
     first = NULL
}

is all that in necessary when creating a new (empty) Slist.

______________________________________

Also when you add a node, remember to have that nodes pointer point to the next node

void Dictionary::add (std::string word)
{
    if (first == NULL) //EMPTY LIST
    {
        first = new LinkedListNode;   //creates first node in list
        first->data = word;      //sets first node = word
        first->next = NULL;    //first node points to NULL 
    }
    else
    {
        LinkedListNode * temp = first;  //temp node = first node
        first = new LinkedListNode;    //create a new node to go infront
        first->data = word;            //set first node = word
        first->next = temp;    //pointing to the next node(temp)
    }
}

Hope that helps!

[garden86]

There are too many problems with your code. The constructor should only initialize member variables. "first" is the only member variable in class Dictionay. Initialize it to NULL in constructor. For info, first and next, you basicaly declared some local variables in your constructor and never used them.
Dictionary::Dictionary () <--- Constructor

{
string info;
LinkedListNode *first, *next; Redefinition of "first"
LinkedListNode(); The semicolon ends the statement, the following block has nothing to do with node's member variables. If you want to initialize node, you have to define a separate constructor for the node.
{
next = NULL;
first = NULL; <-- does NOT set first to NULL
}

[Ehorn]

Fryebyrd and gd18;

Thank you very much for the help.

Dictionary::Dictionary()
{
first = NULL
}

was the piece I did not understand about the constructor. Once that was implemented I was able to modify the rest of the code to work as intended.

And I agree 100% with you gd18, there were TOO many problems with that code, but once I got the constructor to work correctly I was able to sort though all the problems and get it to work.

Thanks again :)