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

[Rui Barradas]

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.

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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