[R] how to test for the empty set

[G. Jay Kerns]

Dear Everybody,

Thanks, length() is the answer.

Best,
Jay

On Mon, Nov 24, 2008 at 1:15 PM, Ted Harding
<Ted.Harding at manchester.ac.uk> wrote:
> On 24-Nov-08 17:41:25, 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
>
> In the above (and below) cases, would not
>
>  (length(x)==0)
>
> do?
> Ted.
>
>> 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
>
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> Date: 24-Nov-08                                       Time: 18:15:31
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G. Jay Kerns, Ph.D.
Associate Professor
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