[R] Unexpected behavior of apply() over a 3d array

[Gabor Grothendieck]

Read ?apply carefullly.    If FUN returns as a scalar as it does here
then the result dimensions are dim(X)[MARGIN]. For example,

apply(X, 1, max) has three components which are
max(X[1,,]), max(X[2,,]) and max(X[3,,])

and apply(X, 3, max) has three components which are
max(X[,,1]), max(X[,,2]) and max(X[,,3])

Also try apply(X, 1:2, max), etc.

On 9/20/06, Toby Muhlhofer <toby.m at mail.utexas.edu> wrote:
> Dear listeRs,
>
> I'm finding that apply() behaves strangely when used on a 3-d array. For
> example:
>
>  > at <- array(1:27,dim=c(3,3,3))
>  > at
> , , 1
>
>      [,1] [,2] [,3]
> [1,]    1    4    7
> [2,]    2    5    8
> [3,]    3    6    9
>
> , , 2
>
>      [,1] [,2] [,3]
> [1,]   10   13   16
> [2,]   11   14   17
> [3,]   12   15   18
>
> , , 3
>
>      [,1] [,2] [,3]
> [1,]   19   22   25
> [2,]   20   23   26
> [3,]   21   24   27
>
>  > apply(at, 1, max)
> [1] 25 26 27
>
> If, for the MARGIN argument in apply() 1 is rows, I would have expected
> as output a 3x3 matrix something like
>
> 7 16 25
> 8 17 16
> 9 18 27
>
> Either that, or maybe the transpose of that, but a single vector seems
> rather random. Especially when you go
>
>  > apply(at, 3, max)
> [1]  9 18 27
>
> What is that the max of? Each submatrix? The diagonal? I'm confused. Can
> anyone clarify this?
>
> Besides this, is there a function that will work on a 3d array, the way
> I'm implying, or do I need to write an explicit loop that takes 2d
> slices of my 3d array?
>
> Thanks!
>
>        Toby
>
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