Question

I was quite surprised when I found out that for x <- array(0, c(5,3,1)), e.g. x[2,,] returns a vector instead of a two-dimensional array (or matrix).

Why is it that this array is obviously interpreted as 5 vectors of length 3 instead of 5 3-by-1 arrays? attr(array(0, c(5,3,1)), "dim") yields [1] 5 3 1 as expected, so it seems that the last dimension didn’t get lost.

How can I make sure that I get a two-dimensional array? I understand that arrays are nothing but vectors with additional attributes, but I don’t understand this apparent “inconsistent” behaviour.

Please enlighten me 🙂 I’m using a three-dimensional array in the context of another function in order to store several matrices. In general, these matrices have n-by-m shape where, in particular, m can be 1 (although it is usually higher).

Answer 1

It’s a classic, and has been in the R FAQ for over a decade too: use drop=FALSE to prevent the collapsing of a 1-row / col matrix to a vector.

R> M <- matrix(1:4,2,2)
R> M
     [,1] [,2]
[1,]    1    3
[2,]    2    4
R> M[,1]
[1] 1 2
R> M[,1,drop=FALSE]
     [,1]
[1,]    1
[2,]    2
R>