Ignoring certain lines in a txt file when parsing

Question

I want to read files from a txt file and compare some lines with regex. The first line of the txt file should start with the string #FIRST. And if the string should start with a '#' the line should be ignored and it should continue. So counter should have the value 1 which it does and it should go to the second if statement if(counter==1). However it doesn't go to the second if statement.

txt file:

#FIRST
#
#haha

I expect the output to be good\ngood after the code is run once.

The output is:

   good.

And it should be

          good.
          good.

.........

#include <iostream> 
#include <string> 
#include <vector> 
#include <regex> 
#include <fstream> 
#include <sstream>

  int main() {

    std::ifstream input("test.txt");
    std::regex e("#FIRST");
    std::regex b("haha");
    int counter;
    for (counter = 0; !input.eof(); counter++) {
      std::cout << counter << "\n";

      std::string line;
      if (counter == 0) {
        getline(input, line);
        if (std::regex_match(line, e)) {
          std::cout << "good." << std::endl;
          counter++;

        } else
          std::cout << "bad." << std::endl;
        break;
      }

      getline(input, line);
      if (line[0] == '#')
        continue;

      if (counter == 1) {
        getline(input, line);
        if (std::regex_match(line, b)) {
          std::cout << "good." << std::endl;
        } else
          std::cout << "bad." << std::endl;
        break;

      }
    }

    return 0;
  }

Count how may `getline` calls are happening. counter = 0 => 2, counter = 1 => 2 but you only have 3 lines... Simple debug (i.e. printing out the line) would help you find this. — John3136, Mar 26 '19 at 22:15
@john I have printed out the lines and I still can't find the problem — senpai, Mar 26 '19 at 22:43
Some restructuring of the code would be advisable considering https://stackoverflow.com/questions/4533063/how-does-ifstreams-eof-work — J.R., Mar 26 '19 at 22:49
Expanding on @J.R. 's comment, [Why is iostream::eof inside a loop condition considered wrong?](https://stackoverflow.com/questions/5605125/why-is-iostreameof-inside-a-loop-condition-considered-wrong) — user4581301, Mar 26 '19 at 23:01

score 0 · Answer 1 · answered Mar 26 '19 at 22:17

The issue is with the break statement in the first if clause. After getting the first line of the input the program encounters the break statement and breaks out of the loop immediately. No further statements are executed within the for loop which I believe is the behavior you are seeing. You will have to restructure the program to be something like:

for loop {
  getline()
  if (counter == <>) {
    // no break
  } else if (line[0] == '#') {
    continue;
  } else {
    // whatever else you want to get done
  }
}

I used your approach and I still face the same problem – senpai Mar 26 '19 at 22:43 — senpai, Mar 26 '19 at 22:43

Ignoring certain lines in a txt file when parsing

1 Answers1