Reversing one dimension of an array, in a generalized case

On 1 Jun 2017, at 22:42, Roy Mendelssohn - NOAA Federal <roy.mendelssohn at noaa.gov> wrote:

Thanks to all for responses/.  There was a question of exactly what was wanted.  It is the generalization of the obvious example I gave,  

junk1 <- junk[, rev(seq_len(10), ]

so that

junk[1,1,1 ] = junk1[1,10,1]
junk[1,2,1]  = junk1[1,9,1]

etc.

The genesis of this is the program is downloading data from a variety of sources on (time, altitude, lat, lon) coordinates,  but all the coordinates are not always there, and sometime the latitude coordinates go from north to south and sometimes from south to north.  I want to always return the data going from south to north, so if I find that the data is north to south,  I have to first reverse the array with the coordinate values (easy enough),  and then reverse the one dimension in the data array that corresponds to latitude. The downloaded information tells me which dimension is latitude plus how many coordinates are in the data.

As I the said,  I haven't done extensive testing on what Bert sent,  but on a toy 3-dimensional example I have it appeared to do what I want.

Thanks again,

-Roy

On Jun 1, 2017, at 12:22 PM, David L Carlson <dcarlson at tamu.edu> wrote:

My error. Clearly I did not do enough testing.

z <- array(1:24,dim=2:4)

all.equal(f(z,1),f2(z,1))

[1] TRUE

all.equal(f(z,2),f2(z,2))

[1] TRUE

all.equal(f(z,3),f2(z,3))

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

# Your earlier example

z <- array(1:120, dim=2:5)
all.equal(f(z,1),f2(z,1))

[1] TRUE

all.equal(f(z,2),f2(z,2))

[1] TRUE

all.equal(f(z,3),f2(z,3))

[1] "Attributes: < Component ?dim?: Mean relative difference: 0.4444444 >"
[2] "Mean relative difference: 0.1262209"                                 

all.equal(f(z,4),f2(z,4))

[1] "Attributes: < Component ?dim?: Mean relative difference: 0.5714286 >"
[2] "Mean relative difference: 0.5855162"  

David C

-----Original Message-----
From: Bert Gunter [mailto:bgunter.4567 at gmail.com] 
Sent: Thursday, June 1, 2017 2:00 PM
To: David L Carlson <dcarlson at tamu.edu>
Cc: Roy Mendelssohn - NOAA Federal <roy.mendelssohn at noaa.gov>; R-help <r-help at r-project.org>
Subject: Re: [R] 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))

[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

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.

**********************
"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 <mailto:Roy.Mendelssohn at noaa.gov> www: http://www.pfeg.noaa.gov/ <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.