I must say, I'm not entirely sure what you want to know. I would honestly recommend a good book on C++. Post test loops are not super popular in C++ (they are of the form "do.. while" where "while" loops / pre test loops are much more common). A little more information is available here: "Play It Again Sam"
EDIT: you need to get data from the user, test it, and then do stuff based on it. Your best bet is something along the lines of
static const int SENTINEL = ??;
int num;
cout << "please input a number" << endl;
cin >> num;
while( num != SENTINEL ) {
// DO STUFF HERE
// Now get the next number
cout << "please input a number" << endl;
cin >> num;
}