[R] negative indexing with null index sets

[Gabor Grothendieck]

Here are a few possibilities.  The first three
use seq_along and the last two use if and length:

x[ !seq_along(x) %in% a[s] ]

x[ !match(seq_along(x), a[s], 0) ]

x[ setdiff(seq_along(x), a[s]) ]

if (length(a[s])) x[-a[s]] else x

x[ if (length(a[s])) -a[s] else TRUE ]


On Thu, Jun 5, 2008 at 3:16 PM, roger koenker <rkoenker at uiuc.edu> wrote:
> Negative indexing is often handy, but I'm in need of  an appropriate idiom
> for handling  cases in which the index set can be null:
>
> x <- rnorm(5)
> a <- 1:5
> s <- rep(FALSE,5)
> y <- x[-a[s]]
>
> # I'd like y == x  but instead one has x[-a[s]] == x[a[s]] == numeric(0),
> which is rather
> # unfortunate -- so far the best I have come up with is:
>
> as <- ifelse(length(a[s]),-a[s],TRUE)
> y <- x[as]
>
> which, doesn't read well.  Apologies if this has been aired previously...
>
> url:    www.econ.uiuc.edu/~roger            Roger Koenker
> email    rkoenker at uiuc.edu            Department of Economics
> vox:     217-333-4558                University of Illinois
> fax:       217-244-6678                Champaign, IL 61820
>
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