Linked List Unhandled Exception

[alex067]

I'm making a simple linked list class, but when I display the current values, it will display the values but then the compiler will abruptly break and the unhandled exception error produces..

code:

#include <iostream>

using namespace std;
class NodeType
{
private:
	int info;                 	// data
	NodeType * next;// pointer to next node in the list
	friend class list;
};

class list
{
private:
	NodeType *head;
public:
	list(); //default constructor
	void add(int data);
	void addFront(int data);
	void display() const;
	~list();

};
// default constructor
list::list()
{
	head = NULL;
}
void list::add(int data)
{
	NodeType*ptr = new NodeType;
	ptr->info = data;
	if (head == NULL) //if empty list
	{
		//need to add to the front 
		head = ptr;
		ptr->next = NULL;
	}
	else
	{
		NodeType*current = head;
		while (current->next != NULL) // as long as next node is not null
		{
			current = current->next; // keep traversing till you reach the end 
		}
		current->next = ptr;
	}
	ptr = NULL;
}
void list::addFront(int data)
{
	NodeType*ptr = new NodeType;
	ptr->info = data;
	ptr->next = NULL;
	if (head == NULL) //list is empty
	{
		head = ptr;
	}
	else
	{
		head = ptr;
		ptr->next = head->next;
	}
}
void list::display() const
{
	NodeType*current = head;
	while (current)
	{
		cout << current->info << " ";
		current = current->next;
	}

}
list::~list()
{
	NodeType * current = head;
	while (current)
	{
		NodeType *temp = current;
		current = current->next;
		delete  temp;
	}
	current = NULL;
	head = 0;
}

int main()
{
	//going to add 2 values
	list one;
	int addinfo;

	cout << "Enter a value you would like to add: " << endl;
	cin >> addinfo;
	one.add(addinfo);

	cout << "Enter another value you would like to add: " << endl;
	cin >> addinfo;
	one.add(addinfo);

	//display the current info
	one.display();


	system("PAUSE");
	return 0;

}

[cire]

Immediately after line 46, what value does ptr->next hold?

[JayhawkZombie]

		while (current->next != NULL) // as long as next node is not null
		{
			current = current->next; // keep traversing till you reach the end 
		}
		current->next = ptr; //Hmm, what's the value of "next"? 

Like cire said, when will this loop exit? "next" has to not satisfy the condition there. What is curent->next going to be after it exits?

[alex067]

Ah I see, so then would I have to do:

current->next = ptr

ptr->next = NULL?

[cire]

Personally, I would put the ptr->next = NULL; between line 32 and 33, then delete line 37.

But, yes. You could do it that way.

[alex067]

Got it to work! Thank you! One random question though:

i'm implementing a do while loop for a menu, and the user if he/she enters Q, it will quit

But for some reason, when I press Q, my program will say "invalid option" and quit, instead of just quitting

int main()
{
	//going to add 2 values
	list one;
	int addinfo;
	char choice;
	do
	{
		cout << "Welcome to the linked list tutorial!" << endl;
		for (int i = 0; i < 40; i++)
		{
			cout << "*";
		}
		cout << endl;
		cout << "A) To insert a value\nB) To print out the current values\nQ) Quit" << endl;
		cin >> choice;
		choice = toupper(choice);
		if (choice == 'A')
		{
			cout << "Enter a value to add to the list: " << endl;
			cin >> addinfo;
			one.add(addinfo);
		}
		else if (choice == 'B')
		{
			cout << "Printing current values..." << endl;
			one.display();
		}
		else 
			cout << "Invalid!";

	} while(choice != 'Q');
	


	system("PAUSE");
	return 0;

}

[cire]

You only treat 'A' and 'B' as valid choices. Maybe you should consider treating 'Q' like one.

[alex067]

What do you mean? Isn't that what while(choice!='Q') does? isn't this condition saying, as long as the choice doesn't equal Q, redo the menu, if it does equal Q, exit the loop?

[JayhawkZombie]

What happens if the user enters 'Q'? It will print "Invalid" and then exit the loop. It's doesn't impede functionality, it just prints "Invalid" in one case when it shouldn't.