Unexpected behavior of '[' in an apply instruction

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

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