while(!file.eof) function going infinite

[ankit137]

#include<iostream.h>
#include<conio.h>
#include<fstream.h>
#include<stdio.h>
class a
{
int rollno;
char name[10];
public:
void mod()
{
cout<<"Enter roll no ";cin>>rollno;
cout<<"Enter name ";gets(name);
}
int call()
{
return a;
}
void out()
{
cout<<"Rollno : "<<rollno;
cout<<"\nName : "<<name;
}
};
void main()
{
clrscr(); a x;
fstream file("n.doc",ios::in|ios::out);int pos,n;
cout<<"Enter roll number to be modded ";cin>>n;
while(!file.eof())
{pos=file.tellg();
if(x.call()==n)
{
cout<<"Enter modifications ";x.mod();
file.seekg(pos);
file.write((char*)&x,sizeof(x));
}
}
file.seekg(0);
cout<<"The file now contains ";
while(!file.eof())
{
file.read((char*)&x,sizeof(x));
x.out();
}
file.close();
}


This was a program which I made to modify data in a file called "n.doc".But the eof function is going infinite, and I can't modify anything.Please help!!

[Chervil]

Looks like Turbo C++ ?
The code is non-standard in lots of ways, It's hard to give good advice in such a case, other than to use an up-to-date compiler.

As for the code, this is an error, it doesn't compile.

int call()
{
     return a;
}

There are two eof loops, I would get rid of both of them. The first has no way to read from the file, it simply does nothing useful.

The second loop, I'd replace with this:

while (file.read((char*)&x, sizeof(x)))
{
    x.out();
}

[ankit137]

I didn't understand by 'eof does nothing useful'.Please clarify.Yes its Turbo c++.

[Chervil]

The first loop looks like this:

while (!file.eof())
{
    pos=file.tellg();
    if (x.call()==n)
    {
        cout<<"Enter modifications ";x.mod();
        file.seekg(pos);
        file.write((char*)&x,sizeof(x));
    }
}

The first time around, the file has just been opened, so it is positioned at the start.
At line 3, pos is set to zero.
At line 4, variable x has not been initialised, definitely nothing has been read from the file (no file.read() statement). So the condition (x.call()==n) will be false. So the loop continues from the start. At line 2, pos is set to zero, the if statement is false ... round and round it goes.