Compilare: user.cpp: In function ‘long long int sum(int, int)’: user.cpp:16:36: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i=0;i<S[x][y].size();++i) ^ user.cpp:18:43: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int j=0;j<S[x][y][i].size();++j) ^ user.cpp: In function ‘std::vector<std::basic_string<char> > split(std::string, char)’: user.cpp:45:26: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i=1;i<s.size();++i) ^ user.cpp: In function ‘int main()’: user.cpp:73:34: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i=0;i<s.size();++i) ^