I was wondering if anyone can give me an idea on how i can modify this function to check for duplicates, if it contains a duplicate, since duplicates are not aloowed, output a message.
Any help would greatly be appreciated.

template<class Type>
void orderedLinkedListType<Type>::insertNode(const Type& newItem)
{
	nodeType<Type> *current; //pointer to traverse the list
	nodeType<Type> *trailCurrent; //pointer just before current
	nodeType<Type> *newNode;  //pointer to create a node

	bool  found;

	newNode = new nodeType<Type>; //create the node
 	assert(newNode != NULL);

	newNode->info = newItem;   //store newitem in the node
	newNode->link = NULL;      //set the link field of the node 
	                           //to NULL

	if(first == NULL)  //Case 1	
	{	
	   first = newNode;
	   count++;
 	}
	else
	{
	   current = first;
	   found = false;

	   while(current != NULL && !found) //search the list
			if(current->info >= newItem)
				found = true;
     		else
			{
				trailCurrent = current;
				current = current->link;
			}
		  
	   if(current == first)  	//Case 2
	   {
			newNode->link = first;
			first = newNode;
 			count++;
	   }
	   else				//Case 3
	   {
			trailCurrent->link = newNode;
			newNode->link = current;
 			count++;
	   }
	}//end else
}//end insertNode

<< moderator edit: added [code][/code] tags >>

It's an ordered list, so all you need to do is break off the search when the node's value is not less than the new item. To test for a duplicate, check and see if the next node has the same value as the new item:

// Empty list case
if ( first == NULL )
  first = newNode;

// First node case
if ( newItem->info < first->info ) {
  newNode->next = first;
  first = newNode;
}

// Find an insertion point
while ( current->next != NULL && current->next->info < newItem )
  current = current->next;

// Check for a duplicate
if ( current->next->info == newItem )
  return DUPLICATE;
else {
  newNode->next = current->next;
  current->next = newItem;
}

since this is an ordered list and it is searching where to place the new item in the list so it is in order, do i need another while loop to check for the duplicates, i know you need to go through the whole list and compare each item by moving another pointer along, i'm just not sure where in the insertNode where to code it. it uses the linkedlist.h file which i'll sent as well. I appreciate your help, thanks one again.

#ifndef H_orderedLinkedListType
#define H_orderedLinkedListType

#include <iostream>
#include <cassert>

#include "linkedList.h"

using namespace std;

template<class Type>
class orderedLinkedListType: public linkedListType<Type>
{
public:
   bool search(const Type& searchItem);
     //Function to determine whether searchItem is in the list.
     //Postcondition: Returns true if searchItem is found in
     //               the list; otherwise, it returns false

   void insertNode(const Type& newItem);
     //Function to insert newItem in the list.
     //Postcondition: first points to the new list and newItem is 
     //               inserted at the proper place in the list.

   void deleteNode(const Type& deleteItem);
     //Function to delete deleteItem from the list.
     //Postcondition: If found, the node containing deleteItem
     //               is deleted from the list; first points 
     //               to the first node of the new list.
     //               If deleteItem is not in the list, an 
     //               appropriate message is printed.
};



template<class Type>
bool orderedLinkedListType<Type>::search(const Type& searchItem)
{
    bool found;
    nodeType<Type> *current; //pointer to traverse the list
	
    found = false;    //initialize found to false
    current = first;  //start the search at the first node

    while(current != NULL && !found)
       if(current->info >= searchItem)
          found = true;
       else
          current = current->link;

      if(found)       
         found = (current->info == searchItem); //test for equality

    return found;
}//end search


template<class Type>
void orderedLinkedListType<Type>::insertNode(const Type& newItem)
{
	nodeType<Type> *current; //pointer to traverse the list
	nodeType<Type> *trailCurrent; //pointer just before current
	nodeType<Type> *newNode;  //pointer to create a node

	bool  found;

	newNode = new nodeType<Type>; //create the node
 	assert(newNode != NULL);

	newNode->info = newItem;   //store newitem in the node
	newNode->link = NULL;      //set the link field of the node 
	                           //to NULL

	if(first == NULL)  //Case 1	
	{	
	   first = newNode;
	   count++;
 	}	
	if(newItem < first->info)
	{
		newNode->link = first;
		first = newNode;
	}
	else
	{
	   current = first;
	   found = false;

	   while(current != NULL && !found && current->link->info < newItem) //search the list
		   current = current->link;
		   
		   if(current->info >= newItem)
				found = true;
     	   else
		   {
			   trailCurrent = current;
			   current = current->link;
		   }
		   if(newNode->link == current->link)
			   cout << "No Duplicates Allowed"<<endl;
		   else
		   {
			   newNode = current->link;
			   current->info = newItem;
		   }
	   if(current == first)  	//Case 2
	   {
			newNode->link = first;
			first = newNode;
 			count++;
	   }
	   else				//Case 3
	   {
			trailCurrent->link = newNode;
			newNode->link = current;
 			count++;
	   }
	}//end else
}//end insertNode

template<class Type>
void orderedLinkedListType<Type>::deleteNode
                                    (const Type& deleteItem)
{
	nodeType<Type> *current; //pointer to traverse the list
	nodeType<Type> *trailCurrent; //pointer just before current
	bool found;

	if(first == NULL) //Case 1
		cerr<<"Cannot delete from an empty list."<<endl;
	else
	{
		current = first;
		found = false;

		while(current != NULL && !found)  //search the list
			if(current->info >= deleteItem)
				found = true;
			else
			{
				trailCurrent = current;
				current = current->link;
			}
		
		if(current == NULL)   //Case 4
			cout<<"The item to be deleted is not in the list."
			    <<endl;
		else
 			if(current->info == deleteItem) //item to be deleted  
											//is in the list
			{
  				if(first == current) 		//Case 2
				{
					first = first->link;

					delete current;
				}
				else     				//Case 3
				{
					trailCurrent->link = current->link;
					delete current;
				}
 				count--;
			}
			else  					//Case 4
				cout<<"The item to be deleted is not in the list."
				    <<endl;
	}
} //end deleteNode

#endif
#ifndef H_LinkedListType
#define H_LinkedListType

#include <iostream>
#include <cassert>
using namespace std;

template <class Type>
struct nodeType
{
	Type info;
	nodeType<Type> *link;
};

template<class Type>
class linkedListType
{
	template<class Type>
	friend ostream& operator<<(ostream&, const linkedListType<Type>&);

public:
    const linkedListType<Type>& operator=
          			      (const linkedListType<Type>&); 
		//Overload the assignment operator.
    void initializeList(); 
 		//Initializes the list to an empty state.
	    //Postcondition: first = NULL, last = NULL,
		//                count = 0
    bool isEmptyList();
 		//Function to determine whether the list is empty. 
		//Postcondition: Returns true if the list is empty;
		//               otherwise, returns false.

	int length();
		//Function to return the number of nodes in the 
		//list.
		//Postcondition: The value of count is returned.
    void destroyList();
 		//Function to delete all the nodes from the list.
  		//Postcondition: first = NULL, last = NULL, 
		//               count = 0
    Type front(); 
 		//Function to return the first element of the list.
 		//Precondition: The list must exist and must not be
		//empty.
  		//Postcondition: If the list is empty, then the 
 		//               program terminates; otherwise, 
	    //               the first element of the list is 
		//               returned.
    Type back(); 
       //Function to return the last element of the
       //list.
		//Precondition: The list must exist and must not
		//be empty.
		//Postcondition: If the list is empty, then the 
		//               program terminates; otherwise,
		//               the last element of the list is 
		//               returned.

   bool search(const Type& searchItem);
		//Function to determine whether searchItem is in 
		//the list.
		//Postcondition: Returns true if searchItem is found
		//               in the list; otherwise, it returns
		//               false.

    void insertFirst(const Type& newItem);
		//Function to insert newItem in the list.
		//Postcondition: first points to the new list 
		//                and newItem is inserted at the
		//                beginning of the list.

    void insertLast(const Type& newItem);
		//Function to return newItem at the end of the
		//list.
	    //Postcondition: first points to the new list, 
		//                newItem is inserted at the end
		//                of the list, and last points to
		//                the last node in the list.

    void deleteNode(const Type& deleteItem);
  		//Function to delete deleteItem from the list.
 		//Postcondition: If found, the node containing 
   		//               deleteItem is deleted from the 
		//                list, first points to the first
		//                node, and last points to the last 
		//                node of the updated list. 

    linkedListType(); 
   		//default constructor
 		//Initializes the list to an empty state.
 		//Postcondition: first = NULL, last = NULL, 
		//               count = 0 

    linkedListType(const linkedListType<Type>& otherList); 
         //copy constructor

    ~linkedListType();   
    	//destructor
   		//Deletes all the nodes from the list.
    	//Postcondition: The list object is destroyed. 

protected:
    int count;		//variable to store the number of 
 					//elements in the list
    nodeType<Type> *first; //pointer to the first node of 
                           //the list
    nodeType<Type> *last;  //pointer to the last node of 
                           //the list 
private:
    void copyList(const linkedListType<Type>& otherList); 
		//Function to make a copy of otherList.
		//Postcondition: A copy of otherList is created 
		//               and assigned to this list.
};

template<class Type>
bool linkedListType<Type>::isEmptyList()
{
	return(first == NULL);
}

template<class Type>
linkedListType<Type>::linkedListType() // default constructor
{
	first = NULL;
	last = NULL;
	count = 0;
}

template<class Type>
void linkedListType<Type>::destroyList()
{
	nodeType<Type> *temp;   //pointer to deallocate the memory 
							//occupied by the node
	while(first != NULL)    //while there are nodes in the list
	{
	   temp = first;        //set temp to the current node
	   first = first->link; //advance first to the next node
	   delete temp;         //deallocate memory occupied by temp
	}

	last = NULL;	//initialize last to NULL; first has already
                   //been set to NULL by the while loop
 	count = 0;
}

	
template<class Type>
void linkedListType<Type>::initializeList()
{
	destroyList(); //if the list has any nodes, delete them
}

template<class Type>
int linkedListType<Type>::length()
{
 	return count;
}  // end length

template<class Type>
Type linkedListType<Type>::front()
{   
    assert(first != NULL);
   	return first->info; //return the info of the first node	
}//end front


template<class Type>
Type linkedListType<Type>::back()
{   
	 assert(last != NULL);
   	 return last->info; //return the info of the first node	
}//end back

template<class Type>
bool linkedListType<Type>::search(const Type& searchItem)
{
    nodeType<Type> *current; //pointer to traverse the list
    bool found;

    current = first; //set current to point to the 
                     //first node in the list
    found = false;   //set found to false

    while(current != NULL && !found)		//search the list
        if(current->info == searchItem)     //the item is found
           found = true;
        else
           current = current->link; //make current point 
                                    //to the next node
     
     return found;
}//end search

template<class Type>
void linkedListType<Type>::insertFirst(const Type& newItem)
{
	nodeType<Type> *newNode; //pointer to create the new node

	newNode = new nodeType<Type>; //create the new node

	assert(newNode != NULL);	//If unable to allocate memory, 
 								//terminate the program

	newNode->info = newItem; 	   //store the new item in the node
	newNode->link = first;        //insert newNode before first
	first = newNode;              //make first point to the 
                                 //actual first node
	count++; 			   //increment count

	if(last == NULL)   //if the list was empty, newNode is also 
                      //the last node in the list
		last = newNode;
}

template<class Type>
void linkedListType<Type>::insertLast(const Type& newItem)
{
	nodeType<Type> *newNode; //pointer to create the new node

	newNode = new nodeType<Type>; //create the new node

	assert(newNode != NULL);	//If unable to allocate memory, 
 							    //terminate the program

	newNode->info = newItem;      //store the new item in the node
	newNode->link = NULL;         //set the link field of newNode
         						//to NULL

	if(first == NULL)	//if the list is empty, newNode is 
     					//both the first and last node
	{
		first = newNode;
		last = newNode;
		count++;		//increment count
	}
	else			//the list is not empty, insert newNode after last
	{
		last->link = newNode; //insert newNode after last
		last = newNode;   //make last point to the actual last node
		count++;		//increment count
	}
}//end insertLast

template<class Type>
void linkedListType<Type>::deleteNode(const Type& deleteItem)
{
	nodeType<Type> *current; //pointer to traverse the list
	nodeType<Type> *trailCurrent; //pointer just before current
	bool found;

	if(first == NULL)    //Case 1; list is empty. 
		cerr<<"Can not delete from an empty list.\n";
	else
	{
		if(first->info == deleteItem) //Case 2 
		{
			current = first;
			first = first->link;
			count--;
			if(first == NULL)    //list has only one node
				last = NULL;
			delete current;
		}
		else  //search the list for the node with the given info
		{
			found = false;
			trailCurrent = first;   //set trailCurrent to point to
                                 //the first node
			current = first->link;  //set current to point to the 
    			   //second node
	
			while(current != NULL && !found)
			{
  				if(current->info != deleteItem) 
				{
					trailCurrent = current;
					current = current->link;
				}
				else
					found = true;
			} // end while

			if(found) //Case 3; if found, delete the node
			{
				trailCurrent->link = current->link;
				count--;

				if(last == current)      //node to be deleted was 
                                     //the last node
					last = trailCurrent;  //update the value of last

				delete current;  //delete the node from the list
			}
			else
				cout<<"Item to be deleted is not in the list."<<endl;
		} //end else
	} //end else
} //end deleteNode


	//Overloading the stream insertion operator
template<class Type>
ostream& operator<<(ostream& osObject, const linkedListType<Type>& list)
{
	nodeType<Type> *current; //pointer to traverse the list

	current = list.first;   //set current so that it points to 
					   //the first node
	while(current != NULL) //while more data to print
	{
	   osObject<<current->info<<" ";
	   current = current->link;
	}

	return osObject;
}

template<class Type>
linkedListType<Type>::~linkedListType() // destructor
{
	destroyList(); 
}//end destructor


template<class Type>
void linkedListType<Type>::copyList
            	      (const linkedListType<Type>& otherList) 
{
   nodeType<Type> *newNode; //pointer to create a node
   nodeType<Type> *current; //pointer to traverse the list

   if(first != NULL)	//if the list is nonempty, make it empty
	  destroyList();

   if(otherList.first == NULL) //otherList is empty
   {
		first = NULL;
		last = NULL;
 		count = 0;
   }
   else
   {
		current = otherList.first;  //current points to the 
									//list to be copied
		count = otherList.count;

			//copy the first node
		first = new nodeType<Type>;  //create the node

 		assert(first != NULL);

		first->info = current->info; //copy the info
		first->link = NULL;  	     //set the link field of 
									 //the node to NULL
		last = first;    		     //make last point to the 
            						 //first node
		current = current->link;     //make current point to  
           							 //the next node

			//copy the remaining list
		while(current != NULL)
		{
			newNode = new nodeType<Type>;  //create a node

			assert(newNode!= NULL);

			newNode->info = current->info;	//copy the info
			newNode->link = NULL;   	    //set the link of 
                                        //newNode to NULL
			last->link = newNode; 		//attach newNode after last
			last = newNode;   			//make last point to
										//the actual last node
			current = current->link;	//make current point to
       									//the next node
		}//end while
	}//end else
}//end copyList


	//copy constructor
template<class Type>
linkedListType<Type>::linkedListType
            	      (const linkedListType<Type>& otherList) 
{
	first = NULL;

	copyList(otherList);
	
}//end copy constructor

	//overload the assignment operator
template<class Type>
const linkedListType<Type>& linkedListType<Type>::operator=
   	 	 		(const linkedListType<Type>& otherList)
{ 
	if(this != &otherList) //avoid self-copy
		copyList(otherList);

	return *this; 
}

#endif

<< moderator edit: fixed [code][/code] tags >>

It's an ordered list, so all you need to do is break off the search when the node's value is not less than the new item. To test for a duplicate, check and see if the next node has the same value as the new item:

// Empty list case
if ( first == NULL )
  first = newNode;

// First node case
if ( newItem->info < first->info ) {
  newNode->next = first;
  first = newNode;
}

// Find an insertion point
while ( current->next != NULL && current->next->info < newItem )
  current = current->next;

// Check for a duplicate
if ( current->next->info == newItem )
  return DUPLICATE;
else {
  newNode->next = current->next;
  current->next = newItem;
}

>do i need another while loop to check for the duplicates
Why would you need to? Since presumably the list is already in order, you stop searching for an insertion point at the same place that there would be a duplicate. The code I gave you is the complete algorithm to do this (insert into an ordered list without duplicates), all you need to do is incorporate it into your code.

If you need to ensure uniqueness then a linked list is not an appropriate data structure. Inserting into a linked list is expected to be a constant time O(1) operation. Checking for duplicates during the insert is going to change that.

You might want to consider a LinkedHashMap. It is basically a LinkedList in which all the Nodes are also kept in a hash. The expected insert time is still constant and duplicate entries can be checked for upon insertion in the hash.

I initially thought you were asking how to check for duplicate node pointers in a linked list , but it appears that you really want to avoid inserting duplicate objects. Please correct me if I am wrong.

Here's one example where a linked list is the appropriate structure for this: hashing, with linked lists used to handle collisions. Or any other situation where the linked list is short.

>If you need to ensure uniqueness then a linked list is not an appropriate data structure.
How do you know? The OP never mentioned how this list is going to be used, so it could very well be an appropriate data structure.

>Inserting into a linked list is expected to be a constant time O(1) operation.
Only if you make certain assumptions, like that the insertion will only be at the head of the list, or at the tail and then only if there is a tail pointer. The average expected time complexity for inserting into a linked list in the general case is O(N/2).

>You might want to consider a LinkedHashMap.
This is not Java. In C++, he would have to implement the functionality of a LinkedHashMap.

>Inserting into a linked list is expected to be a constant time O(1) operation.
Only if you make certain assumptions, like that the insertion will only be at the head of the list, or at the tail and then only if there is a tail pointer. The average expected time complexity for inserting into a linked list in the general case is O(N/2).

And the worst case time complexity is O(N/100000000).

:cool:

And the worst case time complexity is O(N/100000000).

:cool:

There are times when I can't tell if you're joking or not. :confused:

I had to do this and found a much simpler fix, in the insert method starting with the first while(which I included to help) use this:

         while (current != NULL && !found) //search the list
        if (current->info == newItem) 
        {
            cout << "Error" <<endl; 
            return; //Return to the list without including the duplicate value
        }
        else //If there are no duplicates then proceed with the input of the list
            { //Inserted bracket
       if (current->info >= newItem)
           found = true;
       else
       {
           trailCurrent = current;
           current = current->link;
       }
            } //Inserted bracket
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