foo.cc: In function ‘int main()’:
foo.cc:12:30: warning: comparison of integer expressions of different signedness: ‘int’ and ‘std::__cxx11::basic_string<char>::size_type’ {aka ‘long unsigned int’} [-Wsign-compare]
   12 |                 for(int i=0;i<s.size();i++)
      |                             ~^~~~~~~~~
foo.cc:15:25: warning: use of an operand of type ‘bool’ in ‘operator++’ is deprecated [-Wdeprecated]
   15 |                         a++;
      |                         ^
foo.cc:17:25: warning: use of an operand of type ‘bool’ in ‘operator++’ is deprecated [-Wdeprecated]
   17 |                         b++;
      |                         ^
foo.cc:19:25: warning: use of an operand of type ‘bool’ in ‘operator++’ is deprecated [-Wdeprecated]
   19 |                         a++;
      |                         ^
foo.cc:20:33: error: ‘F’ was not declared in this scope
   20 |                         if(s[i]>F)
      |                                 ^
foo.cc:21:25: warning: use of an operand of type ‘bool’ in ‘operator++’ is deprecated [-Wdeprecated]
   21 |                         d++;
      |                         ^