Compilare: user.cpp: In function ‘int main()’: user.cpp:49:24: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] while(i<s.size()) ^ user.cpp:55:49: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] while(isdigit(s[i]) && i<s.size()) ^ user.cpp:62:25: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] if(i<s.size() && s[i]=='x') ^ user.cpp:68:25: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] if(i<s.size() && s[i]=='-') ^ user.cpp:73:25: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] if(i<s.size() && s[i]=='+') ^