Compilare: user.cpp: In function ‘void fold(std::__cxx11::string, int)’: user.cpp:12:10: warning: statement has no effect [-Wunused-value] for(i; i<s.length() && p!=0; ++i){ ^ user.cpp:12:13: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(i; i<s.length() && p!=0; ++i){ ^ user.cpp: In function ‘int main()’: user.cpp:61:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(i=0; i<s.length() && s[i]!='('; ++i) ^