# [Rd] Unexpected behavior of '[' in an apply instruction

Sokol Serguei @oko| @end|ng |rom |n@@-tou|ou@e@|r
Sat Feb 13 00:08:48 CET 2021

```Le 12/02/2021 à 23:49, Sokol Serguei a écrit :
> Le 12/02/2021 à 22:23, Rui Barradas a écrit :
>> 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
>>
>> 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)))

Or slightly shorter:

do.call(`[`, alist(x, i, ,drop=FALSE)[c(1,2,rep(3,ld-1),4)])

>
> Best,
> Serguei.
>
>>
>>
>> Às 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 using gmail.com>
>>>
>>>
>>> On Sat, Feb 13, 2021 at 12:26 AM Rui Barradas <ruipbarradas using sapo.pt
>>> <mailto:ruipbarradas using 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,
>>>
>>>
>>>     ______________________________________________
>>>     R-devel using r-project.org <mailto:R-devel using r-project.org> mailing list
>>>     https://stat.ethz.ch/mailman/listinfo/r-devel
>>>
>>
>> ______________________________________________
>> R-devel using r-project.org mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-devel
>
>

```