[Rd] extract function "[" and empty index
ggrothendieck at gmail.com
Sun Mar 9 13:18:17 CET 2008
On Sun, Mar 9, 2008 at 5:05 AM, Laurent Gautier <lgautier at gmail.com> wrote:
> Dear list,
> I am having a question regarding the extract function "[".
> The man page says that one usage with k-dimensional arrays is to
> specify k indices to "[", with an empty index indicating that all
> entries in that dimension are selected.
> The question is the following: is there an R object qualifying as an
> "empty index" ? I understand that the lazy evaluation of parameters
> allows one to
> have genuinely missing parameters, but I would like to have an object
> instead. I understand that one can always have an if/else workaround,
> but I thought should ask, just in case.
> I tried with NULL but with little success, as it appears to give the
> same results
> as an empty vector.
> > m = matrix(1, 2, 2)
> > m[1, NULL]
> > m[1, integer(0)]
> Since I was at it, I noted that the result obtained with "numeric(0)"
> definitely makes sense but could as well be seen as challenging the
> concept of an empty index presented in the man page (One could somehow
> expect the presence of an object meaning "everything" rather than
> "missingness" meaning it).
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