Iterating over a text file using Fortran like format in C++

Question

I am making an application that deals with txt file data.

The idea is that txt files may come in different formats, and it should be read into C++.

One example might be 3I2, 3X, I3, which should be done as: "first we have 3 integers of length 2, then we have 3 empty spots, then we have 1 integer of length 3.

Is the best to iterate over the file, yielding lines, followed by iterating over the lines as strings? What would be an effective approach for iterating smartly leaving out the 3 spots to be ignored?

E.g.

101112---100
102113---101
103114---102

to:

10, 11, 12, 100
10, 21, 13, 101
10, 31, 14, 102

Answer 1

The link given by Kyle Kanos is a good one; *scanf/*printf format strings map pretty well onto fortran format strings. It's actually easier to do this using C-style IO, but using C++ style streams is doable as well:

#include <cstdio>
#include <iostream>
#include <fstream>
#include <string>

int main() {
    std::ifstream fortranfile;
    fortranfile.open("input.txt");

    if (fortranfile.is_open()) {

        std::string line;
        getline(fortranfile, line);

        while (fortranfile.good()) {
            char dummy[4];
            int i1, i2, i3, i4;

            sscanf(line.c_str(), "%2d%2d%2d%3s%3d", &i1, &i2, &i3, dummy, &i4);

            std::cout << "Line: '" << line << "' -> " << i1 << " " << i2 << " "
                      << i3 << " " << i4 << std::endl;

            getline(fortranfile, line);
        }
    }

    fortranfile.close();

    return 0;
}

Running gives

$ g++ -o readinput readinput.cc
$ ./readinput
Line: '101112---100' -> 10 11 12 100
Line: '102113---101' -> 10 21 13 101
Line: '103114---102' -> 10 31 14 102

Here the format string we're using is %2d%2d%2d%3s%3d - 3 copies of %2d (decimal integer of width 2) followed by %3s (string of width 3, which we read into a variable we never use) followed by %3d (decimal integer of width 3).