Compilare: main.cpp: In function 'std::__cxx11::string eval()': main.cpp:16:8: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] if(i >= s.size()) ~~^~~~~~~~~~~ main.cpp:19:41: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] while('a' <= s[i] && s[i] <= 'z' && i < s.size()) ~~^~~~~~~~~~ main.cpp:22:41: 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:50:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i = 0; i < s.size(); i++) ~~^~~~~~~~~~