IF THEN ELSE

[CAD-NY]

I need some help here...(greatly appreciated)
Is there a way to create a IF THEN ELSE condition flow in GH? (Hoe
about Nested IFs?)
Something like this: IF a circle radius is X mm THEN Pipe ELSE Extrude
etc.
I love GH but most of the time is way above my limited specialized
skills.
This forum is a mind opener and I am learning a ton.
Thank you.