Read file line by line using ifstream in C++

The contents of file.txt are:

5 3
6 4
7 1
10 5

11 6
12 3
12 4

Where 5 3 is a coordinate pair.
How do I process this data line by line in C++?

I am able to get the first line, but how do I get the next line of the file?

ifstream myfile;
myfile.open ("text.txt");

Answer

First, make an ifstream:

#include 
std::ifstream infile("thefile.txt");

The two standard methods are:

1. Assume that every line consists of two numbers and read token by token:

   
   
   

   int a, b;
   while (infile >> a >> b)
   {
       // process pair (a,b)
   }
   
   




2. Line-based parsing, using string streams:

   
   
   

   #include 
   #include 
   
   std::string line;
   while (std::getline(infile, line))
   {
       std::istringstream iss(line);
   
       int a, b;
       if (!(iss >> a >> b)) { break; } // error
   
       // process pair (a,b)
   }
   

You shouldn't mix (1) and (2), since the token-based parsing doesn't gobble up newlines, so you may end up with spurious empty lines if you use getline() after token-based extraction got you to the end of a line already.