Compilare: user.cpp: In function ‘int main()’: user.cpp:16:35: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i = 0, x; i < a.size(); i++) ^ user.cpp:16:21: warning: unused variable ‘x’ [-Wunused-variable] for (int i = 0, x; i < a.size(); i++) ^ user.cpp:21:32: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i = 0; i < a.size(); i++) ^