Question

i wrote a code in C++ where it opens a .txt file and reads its contents, think of it as a (MAC address database), each mac address is delimited by a (.), my problem is after i search the file for total number of lines , iam unable to return the pointer to the initial position of the file in here i use seekg() and tellg() to manipulate the pointer to the file.

here is the code:

#include <iostream>
#include <fstream>
#include <conio.h>


using namespace std;

int main ()
{
 int i = 0;
string str1;

ifstream file;
file.open ("C:\\Users\\...\\Desktop\\MAC.txt");  


 //this section calculates the no. of lines

while (!file.eof() )
{
  getline (file,str1); 
 for (int z =0 ; z<=15; z++)
 if (str1[z] == '.')
 i++;   
}


file.seekg(0,ios::beg);
getline(file,str2);

cout << "the number of lines are " << i << endl; 
cout << str2 << endl;

file.close();


      getchar();
      return 0;
      }

and here is the contents of the MAC.txt file:

0090-d0f5-723a.

0090-d0f2-87hf.

b048-7aae-t5t5.

000e-f4e1-xxx2.

1c1d-678c-9db3.

0090-d0db-f923.

d85d-4cd3-a238.

1c1d-678c-235d.

here the the output of the code is supposed to be the first MAC address but it returns the last one .

Answer 1

file.seekg(0,ios::end);

I believe you wanted file.seekg(0,ios::beg); here.

Zero offset from the end (ios::end) is the end of the file. The read fails and you’re left with the last value you read in the buffer.

Also, once you’ve reached eof, you should manually reset it with file.clear(); before you seek:

file.clear();
file.seekg(0,ios::beg);
getline(file,str2);

The error would have been easier to catch if you checked for errors when you perform file operations. See Kerrek SB’s answer for examples.