Yes. They can do that. Bad luck.
The OP was supposed to be:
I know I shouldn’t take anything from hollywood, I’ve seen a lot of references saying that if you ask a cop if he’s a cop, he has to truthfully answer you. Is there any truth to this?? If so, doesn’t that kinda defeat the purpose of an undercover cop?
There is no truth to this.
I think that it is a rumor started by undercover cops to make their job easier.
i figured as much. thanks 
In general, no, a policeman working undercover does not have to identify himself if asked. There is probably some locality somewhere that has a law like that on the books so it’s possible that it’s technically true somewhere, but there aren’t any state or federal laws along those lines so you can’t just say ‘are you a cop’ three times before [illegal act] to ensure you’re not about to get busted.
Now, a LEO does have to identify himself as such if he’s using LEO powers for something; generally you wouldn’t be guilty of resisting arrest if the cop didn’t show a badge or otherwise identify himself (depening on state laws on citizens arrest etc.), and in some places you could only get ‘assault’ charges for a physical attack, not ‘assault on an officer’ (which is generally worse).