[Rd] Unexpected behavior of '[' in an apply instruction
Sokol Serguei
@oko| @end|ng |rom |n@@-tou|ou@e@|r
Fri Feb 12 23:49:50 CET 2021
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)))
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
>>
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>
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