foo.cc: In function ‘int main()’:
foo.cc:9:30: warning: comparison of integer expressions of different signedness: ‘int’ and ‘std::__cxx11::basic_string<char>::size_type’ {aka ‘long unsigned int’} [-Wsign-compare]
    9 |                 for(int j=0;j<s.size();j++)
      |                             ~^~~~~~~~~
foo.cc:24:32: warning: suggest explicit braces to avoid ambiguous ‘else’ [-Wdangling-else]
   24 |                         else if(s[j]=='3')
      |                                ^