levels update

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

Two questions in one afternon; aren't I good to you!

levels(my.sub$X1[,drop=TRUE])
[1] "a" "b"
levels(factor(my.sub$X1))
[1] "a" "b"

Regards,
Richie.

Mathematical Sciences Unit
HSL


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