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

[David Winsemius]

> On 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.

It was clear whether you wanted every slice along a particular dimension reversed or just one particular slice in a particular dimension. I thought probably the former:

Try:

(arr <- array(1:(2*3*4), dim=c(2,3,4) ) )
 
array( apply(arr, 3, rev), dim (arr) )

-- 
David.
> 
> 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
> NOAA/NMFS
> Environmental Research Division
> Southwest Fisheries Science Center
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> 
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David Winsemius
Alameda, CA, USA