[R] how to test for the empty set

[Daniel Malter]

Hi, any empty set has (or should have) length 0. 

y <- letters[1:3]
z <- letters[4:6]
length(intersect(y,z))

Cheers,
Daniel

-------------------------
cuncta stricte discussurus
-------------------------

-----Ursprüngliche Nachricht-----
Von: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] Im
Auftrag von G. Jay Kerns
Gesendet: Monday, November 24, 2008 12:41 PM
An: r-help at r-project.org
Betreff: [R] how to test for the empty set

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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