Re: [R] how to test for the empty set

From: Duncan Murdoch <murdoch_at_stats.uwo.ca>
Date: Mon, 24 Nov 2008 12:54:56 -0500

On 24/11/2008 12:41 PM, G. Jay Kerns wrote:
> Dear R-help,
>
> I first thought that the empty set (for a vector) would be NULL.
>
> x <- c()
> x
>
> However, the documentation seems to make clear that there _many_ empty
> sets depending on the vector's mode, namely, numeric(0), character(0),
> logical(0), etc. This is borne out by
>
> y <- letters[1:3]
> z <- letters[4:6]
> intersect(y,z)
>
> which, of course, is non-NULL:
>
> is.null(character(0)) # FALSE
>
> So, how can we test if a vector is, say, character(0)? The following
> doesn't (seem to) work:
>
> x <- character(0)
> x == character(0) # logical(0)
>
> More snooping led to the following:
>
> wiki.r-project.org/rwiki/doku.php?id=tips:surprises:emptysetfuncs
>
> and at the bottom of the page it says "logical(0) is an empty set,
> thus is TRUE". However, I get
>
> isTRUE(logical(0)) # FALSE
>
> but, on the other hand,
>
> all.equal(x, character(0)) # TRUE
>
> This would seem to be the solution, but am I missing something? and in
> particular, is there an elegant way to check in the case that the mode
> of the vector is not already known?
>
> Thanks in advance for any insight you may have.

Why not just check the length?

length(x) == 0

will be true for NULL, and all of the empty vectors as well.

Duncan Murdoch



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