Read file line by line using ifstream in C++

Question

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 ("file.txt"); 

Answer 1

First, make an ifstream:

#include <fstream> 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 <sstream> #include <string>  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.