[R] drop unused levels in subset.data.frame

[baptiste auguie]

Dear list,

subset has a 'drop' argument that I had often mistaken for the one in
[.factor which removes unused levels.
Clearly it doesn't work that way, as shown below,

d <- data.frame(x = factor(letters[1:15]), y = factor(LETTERS[1:3]))
s <- subset(d, y=="A", drop=TRUE)
str(s)
'data.frame':	5 obs. of  2 variables:
 $ x: Factor w/ 15 levels "a","b","c","d",..: 1 4 7 10 13
 $ y: Factor w/ 3 levels "A","B","C": 1 1 1 1 1

The subset still retains all the unused factor levels. I wonder how
people usually get rid of all unused levels in a data.frame after
subsetting? I came up with this but I may have missed a better
built-in solution,

dropit <- function (d, columns = names(d), ...)
{
    d[columns] = lapply(d[columns], "[", drop=TRUE, ...)
    d
}

str(dropit(s))
'data.frame':	5 obs. of  2 variables:
 $ x: Factor w/ 5 levels "a","d","g","j",..: 1 2 3 4 5
 $ y: Factor w/ 1 level "A": 1 1 1 1 1


Best regards,

baptiste