[Rd] Another code to drop factor levels

Suharto Anggono Suharto Anggono suharto_anggono at yahoo.com
Wed Nov 7 17:24:44 CET 2012

This is a variant that does not use 'tabulate'. As before, there is no restriction for anyone to use this code.

droplevels2i <- function(x)
if (is.null(levels(x)))
 stop("no 'levels' attribute")
nlev <- length(levels(x))
y <- unclass(x)
used <- logical(nlev)
used[] <- FALSE
used[y] <- TRUE
tb <- used
tb[used] <-
y[] <- tb[y]
levels(y) <- levels(x)[used]
attr(y, "class") <-
 attr(x, "class")
 Pada Sen, 5 Nov 2012 23:49 ICT Suharto Anggono Suharto Anggono menulis:

 >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")
 >Alternatively, one may use 'levels<-.factor' by assigning NA to unused levels, like below.
 >levels(x)[ tabulate(
 >length(levels(x))) == 0 ] <-

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