Simon Fear <[EMAIL PROTECTED]> suggested that
> a<-"a"
> a<-NA
> mode(a)
[1] "logical"
> a<-"a"
> is.na(a) <- T
> mode(a)
[1] "character"
might be a relevant difference between assigning NA and using is.na.
But the analogy is flawed: is.na(x) <- operates on the _elements_ of
x, while x <- affects the variable x. When you assign NA to
_elements_ of a vector, the mode does not change:
> a <- "a"
> is.na(a) <- TRUE
> mode(a)
[1] "character"
> b <- "b"
> b[TRUE] <- NA
> mode(b)
[1] "character"
> c <- "c"
> c[1] <- NA
> mode(c)
[1] "character"
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