Thread: Dbl Link Problem

Hi all,

I am having a problem with a double link list.
The list is loaded from a file, I got the list to move forward but cant seem to move backward.

Account.h:

Code:

#include <iostream>
#include <cstring>

using namespace std;

class Account
{
	int age;
	public:
		Account(){age=0;};
                Account(int a){age=a;};
		int retAge(){return age;};
		Account *next;
		Account *prev;
};

Code:

main.cpp:

#include <cstdlib>
#include <iostream>
#include <cstring>
#include "Account.h"
#include <fstream>
using namespace std;

void loadFile();
void saveFile(int);

Account *currP; //current account pointer
Account *rootP;//root wont change

int main(int argc, char *argv[])
{
	saveFile(12);
	saveFile(13);
	saveFile(14);
	loadFile();
	
    system("PAUSE");
    return EXIT_SUCCESS;
}

void saveFile(int yr)
{
	fstream sfile("Account.txt",ios::binary|ios::app|ios::out);
	if(!sfile.is_open()) sfile.open("Account.txt",ios::binary|ios::out);
	
	Account nAcnt(yr);
	sfile.write(reinterpret_cast<char*>(&nAcnt),sizeof(Account));
	sfile.close();
}

void loadFile()
{
	fstream ofile("Account.txt",ios::binary|ios::in);
	if(!ofile.is_open()) return ; //
	
	rootP=new Account;
	Account *tempAcnt;

        int sizeAcnt=sizeof(Account);
	ofile.read(reinterpret_cast<char*>(rootP),sizeAcnt);
	currP=rootP;
	rootP->prev=0;  //points to nothing

        cout<<"Root Next Pointer: " <<rootP->retAge()<<endl; //check output

         int count=1;
	while(!ofile.eof())
	{
			tempAcnt=new Account;
			currP->next=new Account;
			currP->prev=new Account;
			cout<<"loop: "<<count<<endl;

                        ofile.read(reinterpret_cast<char*>(tempAcnt),sizeAcnt);
		
			cout<<"currP-retAge() : "<<currP->retAge()<<endl;
			currP->prev = currP;
			currP->next = tempAcnt;
			
			currP=tempAcnt;		
			cout<<"new currP-retAge() : "<<currP->retAge()<<endl;
			count++;
	}
	
	ofile.close();

        currP=rootP;  //reset point to root
	while(currP->retAge()!=0)
	{
		cout<<"current Previous Pointer: "<<currP->retAge()<<endl;
		currP=currP->next;
	}

	currP=currP->prev;   //problem is at either this line or the next
	cout<<currP->retAge();   //line cause returning age cause an error

return ; 
}

rootP is a pointer to an Account object, so it's a variable that can store the address of an Account object. However, it looks to me like you are trying to read an Account object into rootP, which means you are trying to read an Account object into the memory set aside for an address with this line:

Code:

ofile.read(reinterpret_cast<char*>(rootP),sizeAcnt);

Originally Posted by 7stud

rootP is a pointer to an Account object, so it's a variable that can store the address of an Account object. However, it looks to me like you are trying to read an Account object into the memory set aside for an address with this line:

Code:

ofile.read(reinterpret_cast<char*>(rootP),sizeAcnt);

rootP is a pointer to an Account object but was instantiate at:

Code:

rootP=new Account

its in the loadFile() function

I have no problem moving the current Pointer (currP) starting from root to the last node, but i have a problem moving it back.

even if i do this:

Code:

currP=rootP;
currP=currP->next;
currP=currP->prev;
cout<<currP-retAge();

if the root's retAge() return 55 and the next node returns 44.
The code returns 44 instead of 55 which it is suppose to.

It looks to me like you are creating new objects and assigning them to pointers, and then immediately reassigning other objects to the pointers, which is creating memory leaks and makes no sense. In addition, what do you think this line does:

currP->prev = currP;

Do you really think that sets the previous pointer to the previous object in the list? Or, does it set the prev pointer to the current object in the list just like a plain English reading of the statement would suggest?

With linked lists you need to be able to draw pictures. Get a pencil, an eraser, and a sheet of paper, and go through your code line by line. Everytime you create a new Account object draw a box and inside the box write the words "prev" and "next". Underneath the box write the pointer names which were assigned the address of the object, and draw an arrow pointing from the name to the box. When you assign an object to the prev or next pointers inside the box, draw an arrow that starts at "prev" or "next" and points to the object. Anytime you assign an object to a pointer, erase any old line and draw a new line from that pointer to the object.

You don't need a return; statement at the end of a void function.

I see a new, but no delete . . . .

I got the list to move forward but cant seem to move backward

Maybe your backwards links are incorrect?

In loadFile(), should next be set to zero like prev?

Code:

rootP->prev=0;

Just suggestions.