Thread: Linked List with files problem - need help!

Hey everyone.

I just joined this forum, so I might be doing something wrong, but I made a program with linked lists, but there are errors that occur:

1. Whenever I try to save a new movie, it overwrite the previous data stored in the file. [FIXED]
2. When the program exits, all information in movie.ini disappears (I have no idea why) [FIXED]
3. I need to getlines to get the movie name. [FIXED]

Code:

#include <iostream>
#include <string>
#include <conio.h>
#include <windows.h>
#include <math.h>
#include <time.h>
#include <iomanip>
#include <limits>
#include <fstream>

using namespace std;

bool checkFile(char filename[255])
{
     ifstream check (filename);
     if (check)
        return 0;
     else
        return 1;
}

typedef class movie
{
        public:
        char *title;
        char *cat;
        char *genre;
        movie *next;
};

int mainMenu()
{
    int option;
    
    cout << "+-----------------------------+" << endl;
    cout << "|          MAIN MENU          |" << endl;
    cout << "+-----------------------------+" << endl <<
            "| 1. Enter a new movie        |" << endl << 
            "| 2. Show media               |" << endl <<
            "| 3. Search media             |" << endl << 
            "| 4. Help                     |" << endl << 
            "| 5. About                    |" << endl <<
            "| 6. Quit                     |" << endl <<
            "+-----------------------------+" << endl <<
            "|     :ENTER AN OPTION:       |" << endl <<
            "+-----------------------------+" << endl;
    cin >> option;
    cin.ignore(numeric_limits<streamsize>::max(),'\n');
    return option;
}

void decide(int opt, movie *head)
{
     string title;
     string cat;
     string genre;
     int a = 0;
     ifstream check ("movie.ini");
     if (!check)
     {
                a = 1;
     }
     check.close();
     
     if (opt == 1)
     {
             ofstream fOut ("movie.ini", ios::app);
             cout << "Enter movie title: ";
             getline(cin,title,'\n');
             cout << "Enter movie category: ";
             getline(cin,cat,'\n');
             cout << "Enter movie genre: ";
             getline(cin,genre,'\n');
             if (a != 1)
                fOut << endl;
                
             fOut << title << endl << cat << endl << genre;
             fOut.close();
     }
     if (opt == 2)
     {
          int cnt = 1;
          movie *list;
          list = head;
          system("cls");
          cout << setw(3) << left << "#" << setw(40) << left << "Movie" <<  setw(17) << left << "Category" <<  setw(17) << left << "Genre\n" << endl;
          while (list != NULL)
          {
                cout << setw(3) << left << cnt << setw(40) << left << list->title <<  setw(17) << left << list->cat <<  setw(17) << left << list->genre << endl;
                list = list->next;
                cnt++;
          }
     }
     if (opt == 4)
        cout << endl << endl << " -- Error opening help file!" << endl << endl;
     if (opt == 5)
        MessageBox(NULL,"(C) 1991-2006 Matt Nesterenko","About Media Manager",MB_OK);
     if (opt == 6)
        exit(0);
}

void menu(string menu, movie *head)
{
     int opt;
     if (menu == "main")
        opt = mainMenu();
     decide(opt, head);
}

int main (int argc, char *argv[])
{
    movie *head;
    movie *mNode;
    movie *tail;

    while (1)
    {
         ifstream input( "movie.ini");
         
         head = tail = NULL;
         
         if (input)
         {
             while (! input.eof())
             {
                  mNode = new movie;
                  mNode->title = new char[255];
                  mNode->cat = new char[255];
                  mNode->genre = new char[255];
                  mNode->next = NULL;
                  input.getline(mNode->title, 255, '\n');
                  input.getline(mNode->cat, 255, '\n');
                  input.getline(mNode->genre, 255, '\n');
                  if (head ==NULL)
                     head = mNode;
                  else
                      tail->next = mNode;
                  tail = mNode;
             }
         }
         menu("main", head);
         cout << endl << endl;
         system("pause");
         system("cls");
    }
    
    system("pause");
    return 0;
}

Code:

mNode = new movie;
mNode->title = (char *) malloc(255);

Why are you mixing new and malloc? You really shouldn't even be using malloc in C++.

The syntax is new. ...and the big reason you're having a problem is because you're using character pointers rather than std::strings, like you should be in C++.

Code:

#include <string>

struct yourStruct {
   std::string this_is_a_string_object;  // No need to allocate memory, it's a solid object 
                                         // that can store a string as long as you want it.
   yourStruct *next;
};

You should get yourself a C++ book, you're looking at this too much from a C perspective and it's getting you a little screwed up. C++ is not C. There are ways to do things in C and much better ways to do it in C++.

No, there is no need to make your string a pointer. The only reason you have character pointers currently is because you want them to hold more than one character. A string object can do that already.

And yes, with new, the proper syntax would be

Code:

char *foo = new char[255];

// ...and don't forget

delete[] foo;

Quite impressive for code that doesn't interact with any ofstreams whatsoever. No, your actual problem lies here:

Code:

ofstream fOut ("movie.ini");

A seemingly harmless line of code, right? Well consider this code:

Code:

#include <fstream>

int main() {
   std::ofstream myFile("myTextFile.txt");
   
   myFile << "Look at all this text! ";
   myFile.close();
   myFile.open("myTextFile.txt");
   myFile << "Some more text!";
   
   return 0;
}

So what does the file say? Does it say "Look at all this text! Some more text!"? You would think so, right? You did, after all, write that to the file. Unfortunately, ofstream's default is to truncate the file, erasing the contents every time you open it. (every time you call the function), so this text output will actually generate a file that simply says "Some more text!". What you're looking for is:

Code:

ofstream fOut ("movie.ini", ios::app);

No, it's not a bug. It can all be traced back to this:

Code:

cin >> option;

The cin >> call leaves a trailing newline character '\n' in the input stream. This then gets eaten by the next getline function call which of course sees the newline and therefore thinks the user pressed the "enter" key and continues on without actually getting any data into your title variable the first time the function is called. The second call doesn't find a newline character and so it waits for user input and successfully retrieves the title information entered.

The solution to this is to throw away that trailing newline character after any cin >> that comes before a getline call.

Code:

cin >> option;
cin.ignore(numeric_limits<streamsize>::max(),'\n');

You'll need to include <limits> for that numeric_limits.