/in/foo.cc: In function 'int main()':
/in/foo.cc:23:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<s.length();i++)
              ~^~~~~~~~~~~
/in/foo.cc:27:7: error: 'a' was not declared in this scope
    if(a[j]==s[i])
       ^