two apparent anomalies

My explanation for No2: 

When coercing a character vector to factor, the current levels are stored.
By choosing a subvector of the factor you don't change the levels of the
factor. So levels(a[1:3]) is still [1] "a" "b" "c" in the last line ... 

If you want to reduce levels you need to tell R.

levels(a[1:3, drop=TRUE])

[1] "a" "b"

________________
Moritz Grenke
http://www.360mix.de

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Von: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] Im
Auftrag von analyst41 at hotmail.com
Gesendet: Samstag, 22. Januar 2011 15:17
An: r-help at r-project.org
Betreff: [R] two apparent anomalies

(1)

a = c("a","b")
mode(a)

[1] "character"

b = c(1,2)
mode(b)

[1] "numeric"

c = data.frame(a,b)
mode(c$a)

[1] "numeric"

(2)

a = c("a","a","b","b","c")
levels(as.factor(a))

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

levels(as.factor(a[1:3]))

[1] "a" "b"

a = as.factor(a)
levels(a)

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

levels(a[1:3])

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

Any explanation would be helpful.  Thanks.

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