is.na(v)<-b (was: Re: [R] Beginner's query - segmentation fault)

[Richard A. O'Keefe]

Simon Fear <Simon.Fear at synequanon.com> 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"