# [R] how to test for the empty set

Stavros Macrakis macrakis at alum.mit.edu
Mon Nov 24 22:16:50 CET 2008

```Doesn't length(x)=0 do the trick?

In general, the cardinality of a set represented as a vector or list
of elements is length(unique(x)).  Unique uses `identical` as its
equivalence relation, so length(  unique( list( 1L, 1.0, 1+0i,
as.logical(NA), as.character(NA), as.numeric(NA), NULL, logical(),
character() ))) => 9.

-s

On Mon, Nov 24, 2008 at 12:41 PM, G. Jay Kerns <gkerns at ysu.edu> 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.
>
> Best,
> Jay
>
>
>
>
> ***************************************************
> G. Jay Kerns, Ph.D.
> Associate Professor
> Department of Mathematics & Statistics
> Youngstown State University
> Youngstown, OH 44555-0002 USA
> Office: 1035 Cushwa Hall
> Phone: (330) 941-3310 Office (voice mail)
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> E-mail: gkerns at ysu.edu
> http://www.cc.ysu.edu/~gjkerns/
>
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>

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