[R] Q: appending to non-existent vector?

[D. R. Evans]

This is a real newbie question. What makes it worse is that I know
I've seen the answer somewhere, but I can no longer find it.

If I have a loop that is supposed to generate a vector piecemeal,
adding an element each time through the loop, what do I do to stop it
failing the first time around the loop, when the vector doesn't yet
exist (so I can't use the append() function)?