# [R] extract element, row, etc from general array structure

Jason Turner jasont at indigoindustrial.co.nz
Sun Oct 28 12:03:10 CET 2001

```This one has me a bit stumped, and I know there may well be a really
simple way of doing it in R.  Sorry for the long-winded explanation.

Short version:
given an array of arbitrary number of dimensions D, how can we
retrieve the Nth element of dimension D without knowing  in
advance what D will be?  For example, given that array
A has dimensions c(3,4,5), and N=2, can we write a function
that will "know" to return A[,,2], and not mistake that
for A[2] or A[,2]?  Bonus question - can such a function
be written without relying purely on an arbitrarily high
number of "if" statements?

Long version:
I've been using Greg Warnes' wapply function (thanks Greg!),
and modifying it to suit my own ends.  wapply() takes a vector,
and repeatedly applies a function FUNC (decided at run-time) to
windows of the data.  The modification I've made is to generalize
the return structures - if a scalar is returned by FUNC, wapply
returns a vector.  If FUNC gives a vector, wappaly returns a
matrix.  etc...

A complementary function I'm trying to build is one that extracts
the results of FUNC for window N.  As I see it, the logic would
be something like this:

if:
foo<-wapply(some.time.series, some.vector.returning.function)

foo is a matrix.  The "extract" function would grab the column
of foo for window N.  e.g. N=5, extract would return foo[,5]
(maybe even with classes set appropriately, if I get particularly
clever).

Since we don't know in advance how many dimensions foo will have,
and the last dimension is the one we're interested in, how do
we express in R:
if it's a vector, get foo[5],
if it's a matrix, get foo[,5],
if it's a 3d array, get foo[,,5],
...
without being so ugly or having an arbitrary upper limit based on how
many "if" statements I was prepared to type?

If there's no elegant way, I'll work around it by keeping records
of data size, and pushing structures back to vectors when I
need to automatically pull segments of data out.  If there's an
easier way, I'd be most appreciative.

Cheers

Jason
--
Indigo Industrial Controls Ltd.
64-21-343-545
jasont at indigoindustrial.co.nz
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```