levels update

On Fri, 5 Dec 2008, jim holtman wrote:

try this:

df <- data.frame(factor(c("a","a","c","b","b")), c(4,5,6,7,8), 
c(9,1,2,3,4))
names(df) <- c("X1","X2","X3")

my.sub <- subset(df, X1 == "a" | X1 == "b")
levels(my.sub$X1)

[1] "a" "b" "c"

my.sub$X1 <- factor(my.sub$X1)

I find

my.sub$X1 <- my.sub$X1[drop=TRUE]

a lot more self-explanatory.  See ?"[.factor".  However, if you find 
yourself wanting to do this, ask why you have a factor (rather than a 
character vector) in the first place.

levels(my.sub$X1)

[1] "a" "b"

On Fri, Dec 5, 2008 at 7:50 AM, Antje <niederlein-rstat at yahoo.de> wrote:

Hello,

I hope this question is not too stupid. I would like to know how to 
update
levels after subsetting data from a data.frame.

df <- data.frame(factor(c("a","a","c","b","b")), c(4,5,6,7,8), 
c(9,1,2,3,4))
names(df) <- c("X1","X2","X3")

my.sub <- subset(df, X1 == "a" | X1 == "b")
levels(my.sub$X1)

# still gives me "a","b","c", though the subset does not contain entries
with "c" anymore

I guess, the solution is rather simple, but I cannot find it.

Antje

-- 
Jim Holtman
Cincinnati, OH
+1 513 646 9390

What is the problem that you are trying to solve?