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

Bert Gunter bgunter.4567 at gmail.com
Thu Jun 1 21:00:27 CEST 2017

```??

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

[1] "Attributes: < Component “dim”: Mean relative difference: 0.4444444 >"
[2] "Mean relative difference: 0.6109091"

In fact,

> dim(f(z,3))
[1] 2 3 4

> dim(f2(z,3))
[1] 3 4 2

Have I made some sort of stupid error here? Or have I misunderstood
what was wanted?

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 11:34 AM, David L Carlson <dcarlson at tamu.edu> wrote:
> 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
>
>
> 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
>>
>>
>> **********************
>> "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
>> ***Note new street address***
>> 110 McAllister Way
>> 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/
>>
>> "Old age and treachery will overcome youth and skill."
>> "From those who have been given much, much will be expected"
>> "the arc of the moral universe is long, but it bends toward justice" -MLK Jr.
>>
>> ______________________________________________
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>> https://stat.ethz.ch/mailman/listinfo/r-help
>> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>
> ______________________________________________
> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.

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