c++中的文件流处理（tellg（）函数）

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 (eg 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;
}