Compilare: main.cpp: In function 'std::__cxx11::string eval()': main.cpp:14:11: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] if (i >= s.size()) ~~^~~~~~~~~~~ main.cpp:17:44: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] while ('a' <= s[i] && s[i] <= 'z' && i < s.size()) ~~^~~~~~~~~~ main.cpp:20:44: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] while ('0' <= s[i] && s[i] <= '9' && i < s.size()) ~~^~~~~~~~~~ main.cpp: In function 'int main()': main.cpp:51:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i = 0; i < s.size(); i++) ~~^~~~~~~~~~