Concerning x[i] <- NA vs is.na(x[i]) <- TRUE
Brian Ripley wrote:
I don't think it can ever `go wrong', but it can do things other
than the user intends.
If the user writes x[i] <- NA, the user has clearly indicated his intention
that the i element(s) of x should become NA. There isn't any clearer way to
say that. The only way it could ever do something "other than the user
intends" is if the mode of x changes or the selected elements are set to
something other than NA.
The ?NA help page *hints* that this might be the case, but does not give
an example.
The question remains, *WHAT* can x[i]<-NA do that might be other than
what the user intends? An example (especially one added to the ?NA help)
would be very useful.
The intention of is.na<- is clearer,
I find this extremely puzzling. x[i] <- NA is an extremely clear and
obvious way of saying "I want the i element(s) of x to become NA".
is.na(x) <- ... is not only an indirect way of doing this, it is a way
which is confusing and error-prone.
Bear in mind that one way of implementing something is is.na() would be
to associate a bit with each element of a vector; is.na() would test and
"is.na<-"() would set that bit. It would be possible to have a language
exactly like R -except- that
x <- 1
is.na(x) <- TRUE
x
=> NA
is.na(x) <- FALSE
x
=> 1
would work. The very existence of an "is.na<-" which accepts a logical
vector containing FALSE as well as TRUE strongly suggests this. But it
doesn't work like that. As I've pointed out,
is.logical(m) && length(m) == length(x) && done{is.na(x) <- m}
=> identical(is.na(x), m)
is the kind of behaviour that has been associated with well-behaved
sinister function calls for several decades, and yet this is not a fact
about R.
and so perhaps user error is less likely?
I see no reason to believe this; the bad behaviour of "is.na<-" surely
makes user error *more* likely rather than *less* likely.
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