[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
>> 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)))
Or slightly shorter:
do.call(`[`, alist(x, i, ,drop=FALSE)[c(1,2,rep(3,ld-1),4)])
>
> Best,
> Serguei.
>
>>
>> Rui Barradas
>>
>> 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,
>>>
>>> Rui Barradas
>>>
>>> ______________________________________________
>>> R-devel using r-project.org <mailto:R-devel using r-project.org> mailing list
>>> https://stat.ethz.ch/mailman/listinfo/r-devel
>>>
>>
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>
>
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