R devel - Nov 2012 - Another code to drop factor levels

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

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] <-
 seq(length=sum(used))
y[] <- tb[y]
levels(y) <- levels(x)[used]
attr(y, "class") <-
 attr(x, "class")
y
}
------------------------------
 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")
 >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