filestream processing in c++(tellg() function)

Question

i wrote the following code....

#include< iostream>
#include< fstream>
using namespace std;  
int main()  
{   
ifstream in("text1.dat",ios::in);    
enum choice{zero=1, credit, debit, exit};  
choice your;  
int balance;  
char name[50];  
int option;  
while(cin>>option)  
{
if(option==exit)  
 break;

switch(option)  
 {case zero:
     while(!in.eof())
     {in>>balance>>name;
      if(balance==0)
      cout<<balance<<" "<<name<<endl;
      cout<<in.tellg()<<endl;
     }   
     in.clear(); 
     in.seekg(0);
     break;}

// likewise there are cases for debit and credit

system("pause");
return 0;   
}    

In text1.dat the entry was:

10 avinash  
-57 derek  
0 fatima  
-98 gorn  
20 aditya

and the output was:

1 //i input this  
16  
27  
0 fatima  
36  
45  
55  
-1  //(a)  
3 //i input this  
10 avinash  
16  
27  
36  
45  
20 aditya  
55  
20 aditya //(b) 
-1  

my questions are:

1. the output marked 'a' is -1...what does -1 mean as an output of tellg()?
2. the output marked 'b' is repeated...why so?

Answer 1

You are observing the same behavior as many other novice C++ programmers. Read for example this question.

What happens is that in.eof() is set to true after you've tried to read something from in and the operation failed because there was no more data. When a read operation fails due to end-of-file, it sets both, eofbit and failbit. When a stream is in fail state, the tellg function is documented to return -1.

To solve the problem, test for eof after you perform a read operation and before you do anything else. Even better, check that the operation just 'failed', since you don't want to distinguish between an end-of-file and an incorrect input (e.g. if a string is fed instead of an number for the balance, your code enters an infinite loop):

for(;;)
{
  in>>balance>>name;
  if(!in)
    break;
  if(balance==0)
    cout<<balance<<" "<<name<<endl;
  cout<<in.tellg()<<endl;
}

The !in condition checks that either failbit or badbit are set. You can simplify this by rewriting as:

while(in>>balance>>name)
{
  if(balance==0)
    cout<<balance<<" "<<name<<endl;
  cout<<in.tellg()<<endl;
}