Question

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In C++ is there a way to go to a specific line in a text file?

Whats the smarter way to read specific set of lines (line number A to line number B) from a text file in C++ using standard C++ library (without opting to Boost)?

Answer 1

If the line length isn’t fixed and you don’t have an index you can’t do better than counting the \ns.

Consider this example file:

Hello\nThis is a multi\n-line\nfile, where is this?\n\nAgain?

Line 1 starts at byte 0, Line 2 at 6, Line 3 at 22, Line 4 at 28, Line 5 at 49 and Line 6 at 50 – there’s no pattern.

If we knew that information in advance, e.g. at the beginning of the file we had that information in some table we could compute a byte offset into the file for the lines we cared about and use a seek to jump straight there.

If the line width is fixed at 20 bytes:

Hello               \nThis is a multi     \n-line               \nfile, where is this?\n                    \nAgain?

Then we can compute the start of a line as a simple multiplication – an offset into the file.

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If you’re looking for a “generic” way of doing this I’d suggest something like:

#include <sstream>
#include <fstream>
#include <iostream>
#include <algorithm>
#include <string>

template <typename Iter, typename Count, typename T>
Iter find_nth(Iter it, const Iter end, Count c, const T match) {
  while(c > 0 && it != end) {
    if (match == *it++)
      --c;
  }
  return it;
}

int main() {
  std::ifstream in("test.txt");
  std::ostringstream str;
  str << in.rdbuf();

  const std::string& s = str.str();
  const int A=2, B=4;
  const std::string::const_iterator begin=find_nth(s.begin(),s.end(), A, '\n');
  const std::string::const_iterator end  =find_nth(begin,s.end(), B-A, '\n');

  const std::string range(begin,end);
  std::cout << range << std::endl;
}

This is appropriate for small-ish files (it reads the entire file into a std::string). For larger files you might want do do the same, but with mmap instead, using the mapped region as iterators. Or you could do this with a RandomAccess iterator that uses seek() within the file. (std::istream_iterator does not do this, it’s only a ForwardIterator so wouldn’t be appropriate).