Linked list logic...

Hello, now, i am creating a program to create 3 lists. (List1, list2, list3)

my program will read from a txt file which contains letters.

for example:

Code:

a
b
c
d

my goal is to create 2 identical list, each node contain one letter. After that, i have to put each letter from my list2 to list3, but in reverse order.

therefore, list 1 should contain:
abcd

list 2 should contains
abcd

and list 3 should contains
dcba

Here is my program, i think i have some logical error in my "insertFront" funcion..

Code:

#include <iostream.h>
#include <fstream.h>
#include <stdlib.h>


struct NodeType
{
	char letter;
	NodeType* link;
};

char DelBeginning(NodeType&);
void insertFront(NodeType&, char);
	
int main()
{
	
	NodeType* head1, *current1, *newptr1, *head2, *current2, *newptr2;
	ifstream myFile;
	int count=0;
	char value, letter;
	myFile.open("ulist.txt");
	
	if (myFile.fail())
	{
		cout<<"Cannot open file";
		exit(-1);
	}
	
	head1 = new NodeType;
	myFile>>head1->letter;
	head2 = new NodeType;
	head2->letter=head1->letter;
	myFile>>value;
	current1=head1;
	current2=head2;
	
	while (!myFile.eof())
	{
		newptr1=new NodeType;
		newptr1->letter=value;
		newptr2=new NodeType;
		newptr2->letter=newptr1->letter;
		current1->link=newptr1;
		current1=newptr1;
		current2->link=newptr2;
		current2=newptr2;
		myFile>>value;
		count++;
	}
	count++;
	current1->link=NULL;
	current2->link=NULL;
	

	for (int i=0; i<count; i++)
	{
		letter=DelBeginning(head2);
		insertFront(head2,letter);
		
	}
	myFile.close();
	return 0;
}

char DelBeginning(NodeType& head)		//Remove an item from the front and returns it
{
	char value;
	value=head->letter;
	NodeType* current=head->link;
	delete head;
	head=current;
	return (value);

}
void insertFront(NodeType& head, char letter)		//Inserts the item at the front of list3
{
	NodeType* newptr, * head3;
	newptr=new NodeType;
	newptr->item=value;
	
	
	if (head==NULL)
	{
		newptr->link=NULL;
		head3=newptr;
	}
	else
	{
		newptr->link=head3;
		head3=newptr;
	}
}

Code:

NodeType* newptr, * head3;

You're going to want to make head3 global, otherwise it will not be preserved between function calls.

Or make it static

Originally posted by rmullen3
Or make it static

If it was made global he would be able to access it from the main function.

sorry, picked through the code too quickly

ok thanks,
now i make 'head3' to global...

Code:

#include <iostream.h>
#include <fstream.h>
#include <stdlib.h>


struct NodeType
{
	char letter;
	NodeType* link;
};

char DelBeginning(NodeType&);
void insertFront(NodeType&, char);
NodeType* head3;
	
int main()
{
	
	NodeType* head1, *current1, *newptr1, *head2, *current2, *newptr2;
	ifstream myFile;
	int count=0;
	char value, letter;
	myFile.open("ulist.txt");
	
	if (myFile.fail())
	{
		cout<<"Cannot open file";
		exit(-1);
	}
	
	head1 = new NodeType;
	myFile>>head1->letter;
	head2 = new NodeType;
	head2->letter=head1->letter;
	myFile>>value;
	current1=head1;
	current2=head2;
	
	while (!myFile.eof())
	{
		newptr1=new NodeType;
		newptr1->letter=value;
		newptr2=new NodeType;
		newptr2->letter=newptr1->letter;
		current1->link=newptr1;
		current1=newptr1;
		current2->link=newptr2;
		current2=newptr2;
		myFile>>value;
		count++;
	}
	count++;
	current1->link=NULL;
	current2->link=NULL;
	

	for (int i=0; i<count; i++)
	{
		letter=DelBeginning(head2);
		insertFront(head2,letter);
		
	}
	myFile.close();
	return 0;
}

char DelBeginning(NodeType& head)		//Remove an item from the front(List 2) and returns it to the main function
{
	char value;
	value=head->letter;
	NodeType* current=head->link;
	delete head;
	head=current;
	return (value);

}
void insertFront(NodeType& head, char letter)		//Inserts the item at the front of list3
{
	NodeType* newptr;
	newptr=new NodeType;
	newptr->item=value;
	
	
	if (head==NULL)         //make the node point to nothing
	{
		newptr->link=NULL;
		head3=newptr;
	}
	else               //This part make the node in the front
	{
		newptr->link=head3;
		head3=newptr;
	}
}

but i think i have logical error in this part:

Code:

if (head==NULL)
	{
		newptr->link=NULL;
		head3=newptr;
	}
	else
	{
		newptr->link=head3;
		head3=newptr;
	}

I think i will got a NULL in the beginning instead at the end.

sorry guys,

please go to my new topic instead of this messy one with syntax error....

my new topic is "Linked list part 2 (Syntax error free version)"

Instead of making head3 global, just declare it in main() and pass it by reference to insertFront().