/in/foo.cc: In function 'int output(std::vector<Student>)':
/in/foo.cc:17:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<a.size();i++)
              ~^~~~~~~~~
/in/foo.cc:21:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<20&&i<a.size();i++)
                    ~^~~~~~~~~
/in/foo.cc:27:1: warning: no return statement in function returning non-void [-Wreturn-type]
 }
 ^