I apologize if this is not appropriate for this mailing list.
In R, there is already functionality to drop unused factor levels. However, I
am proposing the code below that I wrote. In some occasions, it was faster than
applying function 'factor'. In any case, there is no restriction for anyone to
use the code below.
droplevels2 <- function(x) {
if (is.null(levels(x)))
stop("no 'levels' attribute")
nlev <- length(levels(x))
y <- unclass(x)
tb <- tabulate(y, nlev)
used <- as.logical(tb)
tb[used] <-
seq(length=sum(used))
y[] <- tb[y]
levels(y) <- levels(x)[used]
attr(y, "class") <-
attr(x, "class")
y
}
Alternatively, one may use 'levels<-.factor' by assigning NA to unused levels,
like below.
levels(x)[ tabulate(
unclass(x),
length(levels(x))) == 0 ] <-
NA
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