[Rd] extract function "[" and empty index

Laurent Gautier lgautier at gmail.com
Sun Mar 9 10:05:56 CET 2008

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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