Compilare: user.cpp: In function ‘int main()’: user.cpp:52:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(i = 0; i < N; ++i) ^ user.cpp:72:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(i = 1; i < N; ++i) ^ user.cpp:84:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(i = 0; i < M; ++i) ^