/in/foo.cc: In function 'int main()':
/in/foo.cc:5:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<a.length();i++){
              ~^~~~~~~~~~~
/in/foo.cc:7:17: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
    for(int j=0;j<a.length();j++){
                ~^~~~~~~~~~~
/in/foo.cc:15:17: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
    for(int j=0;j<a.length();j++){
                ~^~~~~~~~~~~
/in/foo.cc:23:17: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
    for(int j=0;j<a.length();j++){
                ~^~~~~~~~~~~