/in/foo.cc: In function 'int main()':
/in/foo.cc:10:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<s.length();i++)
              ~^~~~~~~~~~~
/in/foo.cc:13:9: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   if(idx<s.length())
      ~~~^~~~~~~~~~~