Ghost factor levels in R [duplicate]

Question

Possible Duplicate:
dropping factor levels in a subsetted data frame in R

I have subsetted away observations with a certain factor level. When checking whether this has been done with summary() the levels were still listed, but with zero observations. Shouldn't they disappear during the subsetting?

Answer 1

To make the extra levels disappear, use drop=TRUE when subsetting:

newfactor <- oldfactor[indices, drop=TRUE]

Incidentally, one reason this is not the default is that factors with different levels cannot be compared. So if you want to compare your factors with the original vector, or perhaps a different subset of the vector, you'd need to keep the extra levels.

Answer 2

Subsetting doesn't drop empty levels. Why this is the case is that it is a feature. Think of it as your factor levels determine the possible/potential categories of a thing. If you only take a subset of these things, the possible categories of thing don't change, your subset just doesn't contain any of them.

If you want to drop these empty levels, see ?droplevels.