From @oko| @end|ng |rom |n@@-tou|ou@e@|r  Fri Feb 12 23:49:50 2021
From: @oko| @end|ng |rom |n@@-tou|ou@e@|r (Sokol Serguei)
Date: Fri, 12 Feb 2021 23:49:50 +0100
Subject: [Rd] Unexpected behavior of '[' in an apply instruction
In-Reply-To: <f13cf55f-1210-cfb6-bf74-ea73fcaaa707@sapo.pt>
References: <a8eac065-c1ea-a165-2d92-c4802b4d0373@sapo.pt>
 <CAHHjBM5PhQfxw3xSZXxuQAuWqTi83eN=EiuCQC7rtC35bS3R9A@mail.gmail.com>
 <f13cf55f-1210-cfb6-bf74-ea73fcaaa707@sapo.pt>
Message-ID: <8de49c5d-8e9b-31fd-4c2b-212db94a2ccf@insa-toulouse.fr>

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-syntax-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
>>
>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel



[Rd] Unexpected behavior of '[' in an apply instruction
Sokol Serguei