Compilare: user.cpp: In function ‘int main()’: user.cpp:12:22: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(i=0;i<a.size();++i) ^ user.cpp:21:98: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] if(!((a[i+1]>='a' && a[i+1]<='z')||(a[i+1]>='A' && a[i+1]<='Z')) && i==a.size()-1) ^