Kevin Duke Kevin Duke - 3 months ago 21
C++ Question

Read file line by line

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?

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

Answer

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.