From @v|gro@@ @end|ng |rom ver|zon@net  Sat Feb 13 02:24:13 2021
From: @v|gro@@ @end|ng |rom ver|zon@net (Avi Gross)
Date: Fri, 12 Feb 2021 20:24:13 -0500
Subject: [Rd] Unexpected behavior of '[' in an apply instruction
In-Reply-To: <8de49c5d-8e9b-31fd-4c2b-212db94a2ccf@insa-toulouse.fr>
References: <a8eac065-c1ea-a165-2d92-c4802b4d0373@sapo.pt>
 <CAHHjBM5PhQfxw3xSZXxuQAuWqTi83eN=EiuCQC7rtC35bS3R9A@mail.gmail.com>
 <f13cf55f-1210-cfb6-bf74-ea73fcaaa707@sapo.pt>
 <8de49c5d-8e9b-31fd-4c2b-212db94a2ccf@insa-toulouse.fr>
Message-ID: <0b6501d701a6$f0517c70$d0f47550$@verizon.net>

Just to be different, the premise was that you do not know how many dimensions the array had. But that is easily available using dim() including how many items are in each dimension. So, in principle, you can use a normal indexing method perhaps in a loop to get what you want. Not sexy but doable. You can treat the array x as a vector just like lower level R does and access the contents using the formula it uses.

-----Original Message-----
From: R-devel <r-devel-bounces at r-project.org> On Behalf Of Sokol Serguei
Sent: Friday, February 12, 2021 5:50 PM
To: r-devel at r-project.org
Subject: Re: [Rd] Unexpected behavior of '[' in an apply instruction

Le 12/02/2021 ? 22:23, Rui Barradas a ?crit :
> Hello,
>
> Yes, although there is an accepted solution, I believe you should post 
> this solution there. It's a base R solution, what the question asks for.
>
> And thanks, I would have never reminded myself of slice.index.

There is another approach -- produce a call to `[`() putting there "required number of commas in their proper places" programmatically. 
Even if it does not lead to a very readable expression, I think it merits to be mentioned.

   x <- array(1:60, dim = c(10, 2, 3))
   ld=length(dim(x))
   i=1 # i.e. the first row but can be a slice 1:5, whatever
   do.call(`[`, c(alist(x, i), alist(,)[rep(1,ld-1)], alist(drop=FALSE)))

Best,
Serguei.

>
> Rui Barradas
>
> ?s 20:45 de 12/02/21, robin hankin escreveu:
>> Rui
>>
>>  > x <- array(runif(60), dim = c(10, 2, 3))
>>  > array(x[slice.index(x,1) %in% 1:5],c(5,dim(x)[-1]))
>>
>> (I don't see this on stackoverflow; should I post this there too?) 
>> Most of the magic package is devoted to handling arrays of arbitrary 
>> dimensions and this functionality might be good to include if anyone 
>> would find it useful.
>>
>> HTH
>>
>> Robin
>>
>>
>> <mailto:hankin.robin at gmail.com>
>>
>>
>> On Sat, Feb 13, 2021 at 12:26 AM Rui Barradas <ruipbarradas at sapo.pt 
>> <mailto:ruipbarradas at sapo.pt>> wrote:
>>
>>     Hello,
>>
>>     This came up in this StackOverflow post [1].
>>
>>     If x is an array with n dimensions, how to subset by just one 
>> dimension?
>>     If n is known, it's simple, add the required number of commas in 
>> their
>>     proper places.
>>     But what if the user doesn't know the value of n?
>>
>>     The example below has n = 3, and subsets by the 1st dim. The 
>> apply loop
>>     solves the problem as expected but note that the index i has
>>     length(i) > 1.
>>
>>
>>     x <- array(1:60, dim = c(10, 2, 3))
>>
>>     d <- 1L
>>     i <- 1:5
>>     apply(x, MARGIN = -d, '[', i)
>>     x[i, , ]
>>
>>
>>     If length(i) == 1, argument drop = FALSE doesn't work as I 
>> expected it
>>     to work, only the other way does:
>>
>>
>>     i <- 1L
>>     apply(x, MARGIN = -d, '[', i, drop = FALSE)
>>     x[i, , drop = FALSE]
>>
>>
>>     What am I missing?
>>
>>     [1]
>> https://stackoverflow.com/questions/66168564/is-there-a-native-r-synt
>> ax-to-extract-rows-of-an-array
>>
>>     Thanks in advance,
>>
>>     Rui Barradas
>>
>>     ______________________________________________
>>     R-devel at r-project.org <mailto:R-devel at r-project.org> mailing list
>>     https://stat.ethz.ch/mailman/listinfo/r-devel
>>
>
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[Rd] Unexpected behavior of '[' in an apply instruction
Avi Gross