What function tells you if a vector is empty? (Hint: isempty)
What do you want? Not empty. So what is the logical negation operator in MATLAB? (Hint: ~)
Given that, what would ~isempty(Q) do? How would it help you?
Finally, read the help for while, since it looks like you don't know the basic syntax for a loop in MATLAB.
Essentially, you need to learn MATLAB. Start with the getting started tutorials.