Reversing one dimension of an array, in a generalized case

z <- array(1:24,dim=2:4)
all.equal(f(z,3),f2(z,3))

dim(f(z,3))

dim(f2(z,3))

Here is an alternative approach using apply(). Note that with apply() you are reversing rows or columns not indices of rows or columns so apply(junk, 2, rev) reverses the values in each column not the column indices. We actually need to use rev() on everything but the index we are interested in reversing:

f2 <- function(a, wh) {
    dims <- seq_len(length(dim(a)))
    dims <- setdiff(dims, wh)
    apply(apply(a, dims, rev), dims, t)
}

# Your example
j1 <- junk[ , rev(1:10), ]
j2 <- f2(junk, 2)
all.equal(j1, j2)
# [1] TRUE

# Bert's example
z1 <- f(z, 2)
z2 <- f2(z, 2)
all.equal(z1, z2)
# [1] TRUE

-------------------------------------
David L Carlson
Department of Anthropology
Texas A&M University
College Station, TX 77840-4352






-----Original Message-----
From: R-help [mailto:r-help-bounces at r-project.org] On Behalf Of Bert Gunter
Sent: Thursday, June 1, 2017 12:46 PM
To: Roy Mendelssohn - NOAA Federal <roy.mendelssohn at noaa.gov>
Cc: R-help <r-help at r-project.org>
Subject: Re: [R] Reversing one dimension of an array, in a generalized case

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
***Note new street address***
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Santa Cruz, CA 95060
Phone: (831)-420-3666
Fax: (831) 420-3980
e-mail: Roy.Mendelssohn at noaa.gov www: http://www.pfeg.noaa.gov/

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