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

Rui Barradas ru|pb@rr@d@@ @end|ng |rom @@po@pt
Fri Feb 12 12:25:48 CET 2021


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?


Thanks in advance,

Rui Barradas

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