Main.cc: In function 'int main()': Main.cc:12:16: warning: comparison with string literal results in unspecified behaviour [-Waddress] else if(a>="0"&&a<"9"){ ^ Main.cc:12:16: error: ISO C++ forbids comparison between pointer and integer [-fpermissive] Main.cc:12:23: warning: comparison with string literal results in unspecified behaviour [-Waddress] else if(a>="0"&&a<"9"){ ^ Main.cc:12:23: error: ISO C++ forbids comparison between pointer and integer [-fpermissive]