is.na(v)<-b (was: Re: [R] Beginner's query - segmentation fault)
Uwe Ligges
ligges at statistik.uni-dortmund.de
Wed Oct 8 10:28:13 CEST 2003
Richard A. O'Keefe wrote:
> I am puzzled by the advice to use is.na(x) <- TRUE instead of x <- NA.
>
> ?NA says
> Function `is.na<-' may provide a safer way to set missingness. It
> behaves differently for factors, for example.
>
> However, "MAY provide" is a bit scary, and it doesn't say WHAT the
> difference in behaviour is.
>
> I must say that "is.na(x) <- ..." is rather repugnant, because it doesn't
> work. What do I mean? Well, as the designers of SETL who many years ago
> coined the term "sinister function call" to talk about f(...)<-...,
> pointed out, if you do
> f(x) <- y
> then afterwards you expect
> f(x) == y
> to be true. So let's try it:
>
> > x <- c(1,NA,3)
> > is.na(x) <- c(FALSE,FALSE,TRUE)
> > x
> [1] 1 NA NA
> > is.na(x)
> [1] FALSE TRUE TRUE
> vvvvv
> So I _assigned_ c(FALSE,FALSE,TRUE) to is.na(x),
> but I _got_ c(FALSE,TRUE, TRUE)> instead.
> ^^^^^
> That is not how a well behaved sinister function call should work,
> and it's enough to scare someone off is.na()<- forever.
>
> The obvious way to set elements of a variable to missing is ... <- NA.
> Wouldn't it be better if that just plain worked?
>
> Can someone give an example of is.na()<- and <-NA working differently
> with a factor? I just tried it:
>
> > x <- factor(c(3,1,4,1,5,9))
> > y <- x
> > is.na(x) <- x==1
> > y[y==1] <- NA
> > x
> [1] 3 <NA> 4 <NA> 5 9
> Levels: 1 3 4 5 9
> > y
> [1] 3 <NA> 4 <NA> 5 9
> Levels: 1 3 4 5 9
>
> Both approaches seem to have given the same answer. What did I miss?
As mentioned in another mail to R-help. I'm pretty sure there was (is?)
a problem with character (and/or factor) and assignment of NAs, but I
cannot (re)produce an example. I think something for the "x <- NA" case
has been fixed during the last year.
What prevents me to think I'm completely confused is that the is.na()<-
usage is proposed in: ?NA, S Programming, the R Language Definition
manual, R's News file, but I cannot find it in the green book right now.
Uwe Ligges
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