After searching, I see that
https://stat.ethz.ch/pipermail/r-help/2011-April/276274.html has mentioned this
issue, perhaps more clearly.
Thanks for pointing out "Arguments" section about 'exclude'. That documents the
code
exclude <- as.vector(exclude, typeof(x))
A note: if x is a factor, factor(x, exclude=y) doesn't always do nothing other
than dropping unused levels.
> x <- 2:3
> x
[1] 2 3
> xf <- factor(x, levels=x)
> xf
[1] 2 3
Levels: 2 3
> factor(xf, exclude=2)
[1] <NA> 3
Levels: 3
> x <- c(2:3, "a")
> x
[1] "2" "3" "a"
> xf <- factor(x, levels=x)
> xf
[1] 2 3 a
Levels: 2 3 a
> factor(xf, exclude=2)
[1] <NA> 3 a
Levels: 3 a
> sessionInfo()
R version 2.15.2 (2012-10-26)
Platform: i386-w64-mingw32/i386 (32-bit)
locale:
[1] LC_COLLATE=English_United States.1252
[2] LC_CTYPE=English_United States.1252
[3] LC_MONETARY=English_United States.1252
[4] LC_NUMERIC=C
[5] LC_TIME=English_United States.1252
attached base packages:
[1] stats graphics grDevices utils datasets methods base
--- On Thu, 6/12/12, Lorenz, David <lor...@usgs.gov> wrote:
From: Lorenz, David <lor...@usgs.gov>
Subject: Re: [Rd] factor(x, exclude=y) if x is a factor
To: "Suharto Anggono Suharto Anggono" <suharto_angg...@yahoo.com>
Cc: R-devel@r-project.org
Date: Thursday, 6 December, 2012, 10:12 PM
Suharto, I think that the key is to read the definition of exclude in the
Arguments section:a vector of values to be excluded when forming the set of
levels. This should
be of the same type as x, and will be coerced if necessary. Because the levels
already exist for x as a factor, they are not formed or revised, except to drop
unused levels in the case where exclude=NULL (or the default value). To drop
level a from x use:
factor(as.character(x), exclude="a") or, on creation:
x <- factor(c("a", "b"), exclude="a")
Dave
On Wed, Dec 5, 2012 at 11:39 PM, Suharto Anggono Suharto Anggono
<suharto_angg...@yahoo.com> wrote:
factor(x, exclude=factor("a", levels=c("a","b")))
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