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

[Rui Barradas]

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