R help - Sep 2008 - Repeated factor levels - inconsistency of factor and levels<- functions?

Hello,

I have a vector x containing letters ("a", "b" etc.). Now I
want to
convert it to factor and group some letters into one common level. If I do
it by factor function, giving the same label names for all values I want
to group, it doesn't work:
> x<-letters[1:5]
> x
[1] "a" "b" "c" "d"
"e"
> f<-factor(x,levels=letters[1:5],
        
labels=c("vowel","consonant","consonant","consonant","vowel"))
> levels(f)
[1] "vowel"     "consonant" "consonant"
"consonant" "vowel"

But, after it, if I update level names by a single assignment, levels with
the same names will group, even when I don't change all of them:
> levels(f)[1]<-"vowel" #changing only one vector item will make
ALL
levels to group
> levels(f)
[1] "vowel"     "consonant"

I'm rather confused! I think this behavior is double inconsistent. First,
the labeling in factor function should work similarly as in levels<- ,
i.e. they should group levels with the same names either BOTH or NONE.
Second, if I change only one vector item, it should not change anything
else, especially it should not make any "invisible" grouping.

Or am I wrong? Or is it a bug?

Jan Hucin

Honza Hucin wrote:
> Hello,
>
> I have a vector x containing letters ("a", "b" etc.).
Now I want to
> convert it to factor and group some letters into one common level. If I do
> it by factor function, giving the same label names for all values I want
> to group, it doesn't work:
>
>   
>> x<-letters[1:5]
>> x
>>     
> [1] "a" "b" "c" "d" "e"
>   
>> f<-factor(x,levels=letters[1:5],
>>     
>         
labels=c("vowel","consonant","consonant","consonant","vowel"))
>   
>> levels(f)
>>     
> [1] "vowel"     "consonant" "consonant"
"consonant" "vowel"
>
> But, after it, if I update level names by a single assignment, levels with
> the same names will group, even when I don't change all of them:
>
>   
>> levels(f)[1]<-"vowel" #changing only one vector item will
make ALL
>>     
> levels to group
>   
>> levels(f)
>>     
> [1] "vowel"     "consonant"
>
> I'm rather confused! I think this behavior is double inconsistent.
First,
> the labeling in factor function should work similarly as in levels<- ,
> i.e. they should group levels with the same names either BOTH or NONE.
> Second, if I change only one vector item, it should not change anything
> else, especially it should not make any "invisible" grouping.
>
> Or am I wrong? Or is it a bug?
>
>   
I asked Brian Ripley the same thing half a year ago and his answer was:
"Back compatibility ...."

I'm at a loss trying to figure out what kind of code would depend on
current behaviour, but the workaround is rather obvious, so the
motivation for fixing (changing!) it is not too great.


-- 
   O__  ---- Peter Dalgaard             ?ster Farimagsgade 5, Entr.B
  c/ /'_ --- Dept. of Biostatistics     PO Box 2099, 1014 Cph. K
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