/in/foo.cc: In function 'int main()':
/in/foo.cc:10:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for(int i=0;i<a.length();i++)
                ^
/in/foo.cc:12:17: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
    for(int j=0;j<a.length();j++)
                 ^