[R] Reversing one dimension of an array, in a generalized case

Bert Gunter bgunter.4567 at gmail.com
Thu Jun 1 19:45:35 CEST 2017

How about this:

f <- function(a,wh){ ## a is the array; wh is the index to be reversed
   l<- lapply(dim(a),seq_len)
   l[[wh]]<- rev(l[[wh]])

## test
z <- array(1:120,dim=2:5)

##  I omit the printouts




Bert Gunter

"The trouble with having an open mind is that people keep coming along
and sticking things into it."
-- Opus (aka Berkeley Breathed in his "Bloom County" comic strip )

On Thu, Jun 1, 2017 at 9:51 AM, Roy Mendelssohn - NOAA Federal
<roy.mendelssohn at noaa.gov> wrote:
> Hi All:
> I have been looking for an elegant way to do the following,  but haven't found it,  I have never had a good understanding of any of the "apply" functions.
> A simplified idea is I have an array, say:
> junk(5, 10, 3)
> where  (5, 10, 3) give the dimension sizes, and I want to reverse the second dimension, so I could do:
> junk1 <- junk[, rev(seq_len(10), ]
> but what I am after is a general function that will do that where the array could be two, three or four dimensions,  and I pass to the function which dimension I want to reverse, that is the function can not assume the number of dimensions of the array nor which dimension to reverse.
> For example,  if i try:
> junk1 <- apply(junk, 2, rev)
> junk1 comes out as two-dimensional,  not three-dimensional.
> It is probably something obvious but I am not getting it.
> Thanks for any help.
> -Roy
> **********************
> "The contents of this message do not reflect any position of the U.S. Government or NOAA."
> **********************
> Roy Mendelssohn
> Supervisory Operations Research Analyst
> Environmental Research Division
> Southwest Fisheries Science Center
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