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

[Roy Mendelssohn - NOAA Federal]

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