/in/foo.cc: In function 'int main()':
/in/foo.cc:10:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (i = 0; i < a.length() && i < b.length(); i++) cout << a[i] << b[i];
              ~~^~~~~~~~~~~~
/in/foo.cc:10:34: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (i = 0; i < a.length() && i < b.length(); i++) cout << a[i] << b[i];
                                ~~^~~~~~~~~~~~
/in/foo.cc:11:8: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  if (i == a.length())
      ~~^~~~~~~~~~~~~
/in/foo.cc:13:12: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for (; i < b.length(); i++) cout << b[i];
          ~~^~~~~~~~~~~~
/in/foo.cc:17:12: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for (; i < a.length(); i++) cout << a[i];
          ~~^~~~~~~~~~~~
/in/foo.cc:6:9: warning: unused variable 'j' [-Wunused-variable]
  int i, j;
         ^