foo.cc: In function ‘std::string f(std::string, int, int)’:
foo.cc:14:22: warning: comparison of integer expressions of different signedness: ‘int’ and ‘std::__cxx11::basic_string<char>::size_type’ {aka ‘long unsigned int’} [-Wsign-compare]
   14 |         for(int i=0;i<a.length();++i){
      |                     ~^~~~~~~~~~~
foo.cc:19:22: warning: comparison of integer expressions of different signedness: ‘int’ and ‘std::__cxx11::basic_string<char>::size_type’ {aka ‘long unsigned int’} [-Wsign-compare]
   19 |         for(int i=0;i<a.length();++i){
      |                     ~^~~~~~~~~~~