/in/foo.cc: In function 'int main()':
/in/foo.cc:20:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     while(i ++< a.length())
           ~~~~^~~~~~~~~~~~
/in/foo.cc:42:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for(int i=0;i<=c.length();i++)
                 ~^~~~~~~~~~~~