two apparent anomalies

(1)

a <- c("a", "b")
str(a)

chr [1:2] "a" "b"

b <- c(1,2)
str(b)

num [1:2] 1 2

c <- data.frame(a, b)
str(c)

'data.frame':	2 obs. of  2 variables:
 $ a: Factor w/ 2 levels "a","b": 1 2
 $ b: num  1 2

mode(c$a)

[1] "numeric"

c2 <- data.frame(a, b, stringsAsFactors=FALSE)
str(c2)

'data.frame':	2 obs. of  2 variables:
 $ a: chr  "a" "b"
 $ b: num  1 2

mode(c2$a)

[1] "character"


(2)

a <- c("a", "a", "b", "b", "c")
levels(as.factor(a))

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

b <- a[1:3]
b

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

levels(as.factor(b))

[1] "a" "b"

a <- as.factor(a)
a

[1] a a b b c
Levels: a b c

a[1:3]

[1] a a b
Levels: a b c


On Sat, Jan 22, 2011 at 9:16 AM, analyst41 at hotmail.com

(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.

Sarah Goslee
http://www.functionaldiversity.org

On Sat, 22 Jan 2011 06:16:43 -0800 (PST)

(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"

R> str(c)
'data.frame':	2 obs. of  2 variables:
 $ a: Factor w/ 2 levels "a","b": 1 2
 $ b: num  1 2

Character vectors are turned into factors by default by data.frame().

OTOH:

R> c = data.frame(a,b, stringsAsFactors=FALSE)
R> mode(c$a)  
[1] "character"

(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"

Subsetting factors does not get rid of no-longer used levels by default.

OTOH:

R> levels(a[1:3, drop=TRUE])
[1] "a" "b"

or

R> levels(factor(a[1:3]))
[1] "a" "b"


HTH.

Cheers,

	Berwin

========================== Full address ============================
Berwin A Turlach                      Tel.: +61 (8) 6488 3338 (secr)
School of Maths and Stats (M019)            +61 (8) 6488 3383 (self)
The University of Western Australia   FAX : +61 (8) 6488 1028
35 Stirling Highway                   
Crawley WA 6009                e-mail: berwin at maths.uwa.edu.au
Australia                        http://www.maths.uwa.edu.au/~berwin

On Sat, 22 Jan 2011 06:16:43 -0800 (PST)

"analys... at hotmail.com" <analys... at hotmail.com> wrote:

(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"

R> str(c)
'data.frame': ? 2 obs. of ?2 variables:
?$ a: Factor w/ 2 levels "a","b": 1 2
?$ b: num ?1 2

Character vectors are turned into factors by default by data.frame().

OTOH:

R> c = data.frame(a,b, stringsAsFactors=FALSE)
R> mode(c$a) ?
[1] "character"

(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"

Subsetting factors does not get rid of no-longer used levels by default.

OTOH:

R> levels(a[1:3, drop=TRUE])
[1] "a" "b"

or

R> levels(factor(a[1:3]))
[1] "a" "b"

HTH.

Cheers,

? ? ? ? Berwin

Thanks for both responses.

is there a difference between the "as.factor" and "factor" commands
and also between "as.data.frame" and "data.frame"?

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

-----Urspr?ngliche Nachricht-----
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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