Question

I have a text file with numbers as follows:
num1 TAB num2 TAB…. num22 newline
.
.
.

I would like to read num1 check to see if it is equal to 3 and if yes copy the entire row to a new file. What is the fastest way to do this? The file is quite big 80Mb+. Also, num 1 is repetitive, i.e it goes from 0 to 3 in steps of 0.001. So I just have to read every so many steps. I am not sure how to tell the computer to a-priori skip x-lines?

Thanks.

Answer 1

Given you’ve said that runtime performance is not a primary concern, then the following is clear and concise:

#include <string>
#include <fstream>

void foo(std::string const& in_fn, std::string const& out_fn)
{
    std::ifstream is(in_fn);
    std::ofstream os(out_fn);

    std::string line;
    while (std::getline(is, line))
        if (line.size() && std::stoi(line) == 3)
            os << line << '\n';
}

(C++11 support assumed; error handling omitted for brevity.)