# [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]])
do.call(`[`,c(list(a),l))
}

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

##  I omit the printouts

f(z,2)

f(z,3)

Cheers,
Bert

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
>
>
> **********************
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> **********************
> Roy Mendelssohn
> Supervisory Operations Research Analyst
> NOAA/NMFS
> Environmental Research Division
> Southwest Fisheries Science Center
> 110 McAllister Way
> Santa Cruz, CA 95060
> Phone: (831)-420-3666
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> e-mail: Roy.Mendelssohn at noaa.gov www: http://www.pfeg.noaa.gov/
>
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>
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