Message-ID: <E403A03C-E45E-4665-8E58-74B83F7E0BA1@comcast.net>
Date: 2011-12-22T15:35:24Z
From: David Winsemius
Subject: Indexing multi-dimensional table
In-Reply-To: <125d01ccc05a$acd96620$068c3260$@net>
On Dec 21, 2011, at 10:34 PM, David A Vavra wrote:
> I want to take slices of a multi-dimensional table (or array) without
> knowing the number of dimensions in advance.
>
> As a test I tried using (in this example a 3d table):
>
> do.call(`[`, list(tbl, x,NULL,NULL)]
Surely that was meant to be:
do.call(`[`, list(tbl, x,NULL,NULL) )
(This does imply you knew the number of dimensions was 3.)
>
> where I built the list on the fly. It works great as long as I only
> want the
> first dimension however when I try a different dimension, say with
> list(tbl,NULL,x,NULL), I get "<0 x 0> matrix" as the result. I
> thought this
> was because I wasn't calling the right function but there is no
> `[.table` or
> `[.matrix` or even `[.array`.
It is interesting to look at what that returns.
tbl <- array(1:27, c(3,3,3))
x=1
str( do.call(`[`, list(tbl, x,NULL,NULL) ) )
int[1, 0 , 0 ]
It looks as though the Nulls became 0's. So if you wanted to use
do.call(`[` then this succeeds:
> do.call(`[`, list(tbl, x, 1:dim(tbl)[2], 1:dim(tbl)[3]) )
[,1] [,2] [,3]
[1,] 1 10 19
[2,] 4 13 22
[3,] 7 16 25
As does this using the "empty comma" approach:
eval(parse(text= paste("tbl[ " ,x, " , , ]")) )
[,1] [,2] [,3]
[1,] 1 10 19
[2,] 4 13 22
[3,] 7 16 25
(Which after looking at the `r.utils::extract` code suggested by
Bengtsson, was what he did .... after considerably better sanity
checking than above.)
>
> Am I going about this the wrong way?
>
> DAV
>
> [[alternative HTML version deleted]]
--
David Winsemius, MD
West Hartford, CT