/in/foo.cc: In function 'int main()':
/in/foo.cc:12:10: warning: array subscript has type 'char' [-Wchar-subscripts]
    c[a[i]]++;
          ^
/in/foo.cc:15:10: warning: array subscript has type 'char' [-Wchar-subscripts]
    d[b[i]]++;
          ^