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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