Compilare: user.cpp: In function ‘int main()’: user.cpp:13:30: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(i=0;i<s.size();i++)if(((s[i]>='a')&&(s[i]<='z'))||((s[i]>='A')&&(s[i]<='Z'))){ok=1;break;} ^ user.cpp:15:30: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(i=0;i<s.size();i++) ^