Compilare: user.cpp: In function ‘int main()’: user.cpp:27:43: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i = 0; i < expression.length(); ++i) { ^ user.cpp:31:42: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] while (i < expression.length() && '0' <= expression[i] && expression[i] <= '9') { ^ user.cpp:19:10: warning: unused variable ‘c’ [-Wunused-variable] char c; ^