Extracting levels from data.table

Question

For extracting levels from a data.table, is the standard way to lapply over the data.table as a list or do it inside the brackets somehow?

For example, using the npk builtin data, I know that the first 4 columns are factors and I want to extract the levels.

dat <- as.data.table(npk)

This is what I want, a list of the levels

levs <- lapply(dat[,1:4,with=FALSE], levels)

But, am I missing the data.table way that would be something like this? (this isnt right though because it repeats the levels to match the longest one).

levs2 <- dat[, lapply(.SD, levels), .SDcols=names(dat)[1:4]]

ps. sorry if this seems dumb, I am just trying to pick up the proper data.table idioms.

Answer 1

Your first example seems reasonable to me, and I don't think that you can do it within the brackets of the data.table, as the return type should be a list.

Another option is Filter(Negate(is.null),lapply(DT,levels)), which has the added benefit of not needing to know which columns are factors beforehand