R error in unique.default(x) unique() applies only to vectors

Question

I created a dataset named state from the built-in matrix state.x77 with two continuous variables (Population and Income) and two factor variables

Answer 1

This is a very common mistake, you need to use the named argument FUN:

ave(state$inc, state$region, FUN = mean)

otherwise mean will be interpreted as another grouping variable (part of the ... argument to ave.)