R help - Jun 2017 - Reversing one dimension of an array, in a generalized case

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.

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
>
>
> **********************
> "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.
>
> ______________________________________________
> 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.

Thank you very much.  I have a little test example I have been working with, 
and it does seem to work.    I will have to go through and parse this to
understand what you are doing

What I had been doing is building up a string with the arguments and calling it,
it works but very kludgy and very fragile.

Thanks again.

-Roy

> On Jun 1, 2017, at 10:45 AM, Bert Gunter <bgunter.4567 at gmail.com>
wrote:
> 
> 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
>> 
>> 
>> **********************
>> "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.
>> 
>> ______________________________________________
>> 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.
**********************
"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.

Not sure it is "obvious", but this function implements what you
describe:

revdim <- function( a, d ) {
    dims <- attr( a, "dim" )
    idxs <- lapply( seq_along( dims )
                  , function( dd ) {
                      if ( d == dd ) seq.int( dims[ dd ], 1, -1 )
                      else seq.int( dims[ dd ] )
                    }
                  )
    do.call( `[`, c( list( a ), idxs ) )
}

revdim( junk1, 2 )

On Thu, 1 Jun 2017, Roy Mendelssohn - NOAA Federal 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
>
>
---------------------------------------------------------------------------
Jeff Newmiller                        The     .....       .....  Go Live...
DCN:<jdnewmil at dcn.davis.ca.us>        Basics: ##.#.       ##.#.  Live
Go...
                                       Live:   OO#.. Dead: OO#..  Playing
Research Engineer (Solar/Batteries            O.O#.       #.O#.  with
/Software/Embedded Controllers)               .OO#.       .OO#.  rocks...1k

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
>
>
> **********************
> "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.
>
> ______________________________________________
> 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.
______________________________________________
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.

> 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
> ***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.
> 
> ______________________________________________
> 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.
David Winsemius
Alameda, CA, USA