Compilare: user.cpp: In function ‘int main()’: user.cpp:27:34: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int j=0;j<s.size();++j) ^ user.cpp:31:93: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] while(((s[j]>='a' && s[j]<='z') ||(s[j]>='0' && s[j]<='9')) && j<s.size()) ^