[R] how to test for the empty set

[Niels Richard Hansen]

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)

You could use

length(x) == 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)
> -3302 Department
> -3170 FAX
> E-mail: gkerns at ysu.edu
> http://www.cc.ysu.edu/~gjkerns/
> 
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.

-- 
Niels Richard Hansen
Associate Professor
Department of Mathematical Sciences
University of Copenhagen
Universitetsparken 5
2100 Copenhagen Ø
Denmark
+45 353 20783