R help - Jun 2004 - subset(..., drop=TRUE) doesn't seem to work.

Hello!

If I read ?subset, the workings of the argument drop (to me) seem to imply 
equivalence of A and B (R 1.9.0):

#A
dd <- data.frame(rt=rnorm(10), c=factor(gl(2,5)))
dd <- subset(dd, c==1)
dd$c <- dd$c[, drop=TRUE]
table(dd$c)

1
5
	

#B
dd <- data.frame(rt=rnorm(10), c=factor(gl(2,5)))
dd <- subset(dd, c==1, drop=TRUE)
table(dd$c)

1 2
5 0

So to lose the second level of dd$c, in method B I still need to 'dd$c <-
dd$c[, drop=TRUE]', while the manual seems to imply that with the drop 
argument to subset() this would not be necessary.


Could you comment?

kind regards,
Paul


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Paul Lemmens <P.Lemmens at nici.kun.nl> writes:
> Hello!
> 
> If I read ?subset, the workings of the argument drop (to me) seem to
> imply equivalence of A and B (R 1.9.0):
> 
> #A
> dd <- data.frame(rt=rnorm(10), c=factor(gl(2,5)))
> dd <- subset(dd, c==1)
> dd$c <- dd$c[, drop=TRUE]
> table(dd$c)
> 
> 1
> 5
> 	
> 
> #B
> dd <- data.frame(rt=rnorm(10), c=factor(gl(2,5)))
> dd <- subset(dd, c==1, drop=TRUE)
> table(dd$c)
> 
> 1 2
> 5 0
> 
> So to lose the second level of dd$c, in method B I still need to 'dd$c
> <-
> dd$c[, drop=TRUE]', while the manual seems to imply that with the drop
> argument to subset() this would not be necessary.
> 
> 
> Could you comment?
Looks like a documentation bug. The actual code ends up  doing

    x[r, vars, drop = drop]

and "[.data.frame" will not drop factor levels. I wonder if it ever
did... 

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