You have typo in your code and it did not work, It should have been '\0' instead of '\n'.

So the following fix works.

string to_postfix(const string& infix) {
  int pos = find(infix.begin(), infix.end(), '\0') - infix.begin();
  return to_postfix_NEW(infix.substr(0, pos)) + infix.substr(pos, infix.size() - pos);
}

It says that the frequency-values will never be greater than 100, which means that it can be 100 as well

Point taken. I've changed the description.

And also, it must specify that columns are two-space separated from each othe

It is already clear from the example

Would have been better to work with ...

Thanks for your advice. I will try harder next time.