Whack-a-mole Game in AS3

Hey:

I am building a whack-a-mole game in AS3. I have the basic structure down but am trying to figure out how to have one instance of a random ‘enemy’ appear in one slot at a time. So there would not be a duplicate ‘enemy’ anywhere else simultaneously as well as only one instance can occupy a slot at a given time.

Anyone have any suggestion on what direction? Array? for loop? if? tutorial?

Thanks