On Tue, 07 Oct 2003 14:37:26 +0200, Uwe Ligges <[EMAIL PROTECTED]> wrote :
2. Use "is.na(x) <- TRUE" instead of "x <- NA": is.na(temp[temp[ ,"t1"] == -999.00, "t1"]) <- TRUE
I hadn't heard this advice before. The online help ?is.na gives this cryptic advice:
Function 'is.na<-' may provide a safer way to set missingness. It behaves differently for factors, for example.
I assume it means "safer than assigning NA", and "differently than assigning NA", but how exactly is it safer, and how is it different?
Hmmmm. I was sure I had constructed an problematic example for a course I hold last year in Dortmund, but I cannot find it again. Maybe something for NA assignmets (into character vectors, I remember?) has been fixed in the meantime.
Anyway, I have had the experience that there are cases, where "is.na(x) <- TRUE" is the more secure way of doing it.
Additionally, it's mentioned in ?NA and in "S Programming", for example.
Uwe
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