Thread: Sorting while inserting

When I try to display the list after adding names a pop-up tells me that a unhandled win32 exception occured.

Head is initialized to NULL in the constructor. Is this what you ask?

Thanks for the input. I've changed "if(prev == NULL)" to "if(!prev && !curr)" for checking if the list is empty. I am now getting the same error when I add more than one name. I think I've been staring at this for too long. Here is the updated whole code:

Ack! My spacing got all screwy when i pasted it. I'm fixing it now, sorry.

Code:

#include <iostream>
#include <string.h>

using namespace std;

#include "list.h"

//=========================================================
// Constructor
//=========================================================

list::list()
{
	num_in_list = 0;
	head = NULL;
}

//==========================================================
// Destructor
//==========================================================

list::~list()
{
	node * curr; //temporary pointer for deleting
	
    //deletes vacation photo list
    while(head)
    {
	   curr = head->next;
	   delete [] head->fname;
	   delete [] head->lname;
	   delete head;
	   head = curr;
    }

}


//==========================================================
// Add
//==========================================================
  
int list::add(char firstname[30], char lastname[30])
{
    node * prev = NULL; 
    node * curr = head;  

    //find position of last name
    while(curr != NULL && (strcmp(curr->lname, lastname) < 0))
    {
        prev = curr;
        curr = curr->next;
    }

    if(!curr && !prev)
    {
        head = new node;

        head->fname = new char[strlen(firstname)+1];
        strcpy(head->fname, firstname);

        head->lname = new char[strlen(lastname)+1];
        strcpy(head->lname, lastname);
         
        head->next = curr;
 
        ++num_in_list;

        return 0;
    }

    else
    {
        //if last names are equal, order by firstname
        while(strcmp(curr->lname, lastname) == 0  &&
                 strcmp(curr->fname, firstname) < 0)
        {
            prev = curr;
            curr = curr->next;
        }		
		
        prev->next = new node;
        prev = prev->next;
	    
        head->fname = new char[strlen(firstname)+1];
        strcpy(head->fname, firstname);
    
        head->lname = new char[strlen(lastname)+1];
        strcpy(head->lname, lastname);

        prev->next = curr;

        ++ num_in_list;

        return 0;
        }

        return 0;
}

//==========================================================
// Remove
//==========================================================

int list::remove(char firstname[30], char lastname[30])
{

	node * curr;
	node * prev;

	curr = head;

	while((curr != NULL) && ((strcmp(curr->lname, lastname) + strcmp(curr->fname, firstname) != 0)))
	{
		curr = curr->next;
	}

	if(curr == NULL)
	{
		cout << "NO MATCH FOUND" << endl;
		
		return 0;
	}

	if(curr == head)
	{
		head = curr->next;
		delete [] curr->fname;
		delete [] curr->lname;
		delete curr;
		curr = NULL;

		-- num_in_list;
	}
	else
	{
		prev = head;

		while(prev->next != curr)
		{
			prev = prev->next;
		}

		prev->next = curr->next;

		delete [] curr->fname;
		delete [] curr->lname;
        delete curr;
		curr = NULL;

		-- num_in_list;
	}


	return 0;
}

//==========================================================
// Display
//==========================================================

int list::display()
{
  node * curr;      

  curr = head;

  while(curr)
  {
    cout << curr->fname << " " << curr->lname << endl;
    curr = curr->next;
  }

  return 0;
}

Ok, but follow the code when the list has a name that doesn't match. As I said before, prev will be the item in the list, and curr will be null (since it reached the end without a match).

Follow the flow of the function and notice what happens in that specific instance. Maybe even write things down as you go so you can visualize it. You will find where you are using a null pointer and where you need more logic.

>> (strcmp(curr->lname, lastname) + strcmp(curr->fname, firstname) != 0)
On a separate note, that code is cute, but what if the first compare returns -1 and the second returns positive 1?

Ack! My spacing got all screwy when i pasted it.

Said it before, say it again: different applications and fonts use vastly different tab-to-space ratios. A tab-and-space potpourri lining up in your editor is no guarantee that it'll line up your code elsewhere. This forum itself contains many painful illustrations of what can go wrong. Use spaces or tabs exclusively; don't intermix.