Compilare: user.cpp: In function ‘big_number operator+(big_number, big_number)’: user.cpp:55:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(i=0; i<a.size(); ++i) ^ user.cpp: In function ‘big_number operator-(big_number, big_number)’: user.cpp:75:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(i=0; i<b.size(); ++i) ^ user.cpp:81:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(; i<a.size(); ++i) ans.push_back(a[i]); ^ user.cpp: In function ‘void refresh_val()’: user.cpp:99:28: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(; j<Nr[i].size(); ++j) ^