Question

Why does this file always contain an extra Fraction structure? If a user enters option 3 to view all fractions when the program just starts (the file should be empty), 0/0 is printed out. If a user selects option one and enters a fraction 1/1, and then selects option 3, two fractions are printed out 1/1 1/1.

Note: to run this you will need to create an empty .dat file called "fractions.dat"

I really appreciate your help!

#include <iostream>
#include <fstream>

using namespace std;

struct Fraction
{
    int num, den;
};

int search(fstream& file, Fraction* f);
int menu();
void proccessChoice(int i);
Fraction* readFrac();

Fraction* fracs[100];
int index;
string fileName = "fractions.dat";
fstream file(fileName,  ios::in | ios::out | ios::binary | ios::trunc);

//structure called Fraction with num and den as its only integer members
int main()
{
    if (!file)
    {
        cout << "Error opening file. Program aborting.\n";
        system("pause");
        exit(1);
    }
    int choice;
    do
    {
        choice = menu();
        proccessChoice(choice);
    } while(choice != 4);


    system("pause");
    return 0;
}



int menu()
{
    int c;
    cout << "1.Enter new fraction" << endl;
    cout << "2.Display/update a fraction" << endl;
    cout << "3.Display all fractions" << endl;
    cout << "4.Quit" << endl;
    cin >> c;
    return c;
}

void proccessChoice(int i)
{
    switch(i)
    {
    case 1:
        {
            cout << "Enter a fraction to add: ";
            fracs[index] = readFrac();
            file.seekp(0, ios::end);
            file.write(reinterpret_cast<char*>(fracs[index]), sizeof(Fraction));
            /*cout << fracs[index]->num << "/" << fracs[index]->den ;*/
            index++;
        }
            break;
    case 2:
        {
            cout << "Enter fraction to view/update: ";
            Fraction* fToFind = readFrac();
            int i = search(file, fToFind);
            if (i == -1)
            {
                cout << "Fraction not found." << endl;
            }
            else
            {
                char r;
                cout << fToFind->num << "/" << fToFind->den << " found at index " << i << ". Update? [y/n]";
                cin >> r;
                if(r == 'y' || r == 'Y')
                {
                    cout << "Enter a new fraction: ";
                    Fraction* fToWrite = readFrac();
                    file.seekp(sizeof(Fraction) * (i));
                    file.write(reinterpret_cast<char*>(fToWrite), sizeof(Fraction));
                }
            }
            cout << i << endl;
            cout << "Case 2" << endl;
        }
            break;
    case 3:
        {
            Fraction* cur = new Fraction();
            //int pos = -1;
            if (!file)
            {
                cout << "Error opening file. Program aborting.\n";
                system("pause");
                exit(1);
            }
            file.seekg(0, ios::beg);
            while(!file.eof())
            {
                file.read(reinterpret_cast<char *>(cur), sizeof(Fraction));
                cout << cur->num << "/" << cur->den << endl;
            }
                    cout << "Case 3" << endl;
        }
            break;
    }
}

int search(fstream& file, Fraction* f)
{
    Fraction* cur = new Fraction();
    int pos = -1;
    if (!file)
    {
        cout << "Error opening file. Program aborting.\n";
        system("pause");
        exit(1);
    }
    file.seekg(0, ios::beg);
    while(!file.eof())
    {
        file.read(reinterpret_cast<char *>(cur), sizeof(Fraction));
        if((cur->num == f->num) && (cur->den == f->den))
        {
            cout << "\nFrac found!" << cur->num << "/" << cur->den<<endl;
            pos = file.tellg()/sizeof(Fraction) - 1;
            return pos;
        }
    }
    return pos;
}

Fraction* readFrac()
{
    Fraction* f = new Fraction();
    char slash;
    cin >> f->num >> slash >> f->den;
    return f;
}

Attempted solution 1: works for the first attempt of the user to view all fractions but on the second attempt the file is not found.

case 3:
        {
            Fraction* cur = new Fraction();
            //int pos = -1;
            if (!file)
            {
                cout << "Error opening file. Program aborting.\n";
                system("pause");
                exit(1);
            }
            file.seekg(0, ios::beg);
            while(!file.eof())
            {
                if(file.read(reinterpret_cast<char *>(cur), sizeof(Fraction)).gcount()== sizeof(Fraction))
                {
                    cout << cur->num << "/" << cur->den << endl;
                }
            }
                    cout << "Case 3" << endl;
        }
            break;
    }

Answer 1

Because you didn’t yet reach the end of the file!

while(!file.eof())

You opened the empty file, however, note that the read cursor is currently at position 0. As long as you didn’t attempt to read data file.eof() will be false, as the end of file hasn’t been reached yet by the cursor.

You’ll need to check whether you were actually able to extract the Fraction:

if(
   file.read(reinterpret_cast<char *>(cur), sizeof(Fraction)).gcount() 
   == sizeof(Fraction)
){
     // Fraction has been extracted
}
else
     // either eof() or some other error.

Note that this gets much simpler if you provide a custom operator for this:

std::istream& operator>>(std:istream& in, Fraction& f){
    return in.read(reinterpret_cast<char *>(&f), sizeof(Fraction));
}

// ....
while(file >> *cur){
   // ... your code.
}