istream not working in DEV C++

Question

Hi. I'm using ifstream and ofstream operations in DEV c++ but they don't seem to work correctly. I've been trying to write a little prime generator code but it doesn't work :\ When I display fstream::tellg() at any point, it displays -1. Please check the code. Thanx.

#include<iostream>
#include<math.h>
#include<fstream>
using namespace std;
int prime (unsigned long long n)
{   
    ifstream f1; 
    ofstream f2;             
    unsigned long long i,m,root;
    int flag=0;
    for(i=2;i<=n;i++)
    {   
        f1.open("prime2.txt",ios::binary);    
        if(!f1.is_open())
        {   
            cout<<"NOT OPEN";    
        }              
        cout<<f1.tellg()<<" ";      //Displaying getpointer pos    
        flag=0;
        root=(unsigned long long)sqrt(i);
        while(f1.read((char*)&m,sizeof(m)))
        {   
            if((i%m)==0)
            {   
                flag=1;
                break;
            }      
            if(m>root)
            {   
                break;
            }                                       
        }             
        f1.close();
        if(!flag)
        {   
            f2.open("prime2.txt",ios::app|ios::binary);
            f2.write((char*)&i,sizeof(i));
            cout<<i<<" ";             //Displaying num being written
            f2.close();
        }   
    }    
    return 1;
}   

int main(int argc, char* argv[])
{   
    prime(50);
    system("pause");
}   

(I'm sorry, i just couldn't make the whole thing get in one codeblock. I guess theres something wrong with [code] tag?)

Output:

NOT OPEN-1 2 -1 3 -1 4 -1 5 -1 6 -1 7 -1 8 -1 9 -1 10 -1 11 -1 12 -1 13 -1 14 -1 15 -1 16 -1 17 -1 18 -1 19 -1 20 -1 21 -1 22 -1 23 -1 24 -1 25 -1 26 -1 27 -1 2 8 -1 29 -1 30 -1 31 -1 32 -1 33 -1 34 -1 35 -1 36 -1 37 -1 38 -1 39 -1 40 -1 41 -1 42 -1 43 -1 44 -1 45 -1 46 -1 47 -1 48 -1 49 -1 50 Press any key to continue . . .

PS: I'm relatively new to programming, and only just started on DEV cpp a couple of days ago. Before that i used Turbo C...and a (similar) code worked well on it.

Answer 1

The return value -1 of tellg() indicates failure. Check that the file path is correct. Your output clearly indicates that the file cannot be opened. If a file cannot be opened, you shouldn't perform read/write operation on the file, or in this case stream.

EDIT

f1.open("prime2.txt",ios::binary);

Here the location of prime2.txt is relative. Since you changed compiler there is a good chance that the file cannot be found. Try with some absolute value first (like C:\myprograms\test\prime2.txt) and then figure out where to put prime2.txt for your program to get it by only specifying prim2.txt in open().

Answer 2

The first thing in your ouput is 'NOT OPEN'. If file open fails, don't expect anything else to work.

Check file is present in the dir where your program executes. Try with absolute path in the filename instead of relative.

It might be better to build this logic up step by step, testing as you go, instead of writing the whole thing and then trying to debug a pile of output from failing code.