[R] Exclusion of elements in a vector

[Douglas Bates]

On Thu, Sep 25, 2008 at 8:15 AM, Chuck Cleland <ccleland at optonline.net> wrote:
> On 9/25/2008 7:43 AM, Stefan Fritsch wrote:
>> Dear R Users,
>>
>> I want to exclude elements in a vector by:
>>
>> vector[-exclude]
>>
>> is it intended to cause an error if no elements are excluded?
>>
>>> vector <- 1:10
>>> exclude <- NULL
>>> vector[-exclude]
>> Error in -exclude
>>
>> or am I just definig exclude wrong, if no elements should be excluded?

Yes.  I'm not sure what you want to do can be done in a clean way.
The problem is with the semantics of the indexing operator for numeric
indices.  A "natural" way of expressing the empty numeric vector is

numeric(0)

When you use a numeric vector as a set of indices the first check is
to determine if all the values are negative, in which case the indices
are used to exclude rather than include.  However, you can't tell
whether the elements of the empty numeric vector are positive or
negative because there aren't any elements.  Thus

vector[numeric(0)]

is the same as

vector[-numeric(0)]

> (1:10)[numeric(0)]
integer(0)
> (1:10)[-numeric(0)]
integer(0)

The error message you were getting results from the class of NULL,
which is different from the class of numeric(0).  Arithmetic
operations are defined, to some extent, for numeric(0) but not for
NULL.

>  One approach would be to set "exclude" to a number greater than the
> length of the vector:
>
> x <- 1:10
>
> exclude <- length(x) + 1
>
> x[-exclude]
>  [1]  1  2  3  4  5  6  7  8  9 10
>
> library(fortunes)
>
> fortune("dog")
>
> Firstly, don't call your matrix 'matrix'. Would you call your dog 'dog'?
> Anyway, it might clash with the function 'matrix'.
>   -- Barry Rowlingson
>      R-help (October 2004)
>
>> with kind regards,
>>
>> Stefan Fritsch
>>
>> ______________________________________________
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>> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>
> --
> Chuck Cleland, Ph.D.
> NDRI, Inc. (www.ndri.org)
> 71 West 23rd Street, 8th floor
> New York, NY 10010
> tel: (212) 845-4495 (Tu, Th)
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> fax: (917) 438-0894
>
> ______________________________________________
> 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.
>