[R] Unexpected behavior of "apply" when FUN=sample

[Rui Barradas]

Hello,

The problem is that apply returns the results vector by vector and in R 
vectors are column vectors. This is not exclusive of apply with sample 
as the function to be called, but of apply in general. Try, for instance

apply(test, 1, identity)  # transposes the array

The rows are returned as column vectors. And you should expect this 
behavior from apply with MARGIN = 1.
And this is in fact documented, in the Value section of ?apply:

Value

If each call to FUN returns a vector of length n, then apply returns an 
array of dimension c(n, dim(X)[MARGIN]) if n > 1.

The length of the returned vector is the number of rows and the number 
of columns is the dim corresponding to MARGIN...

Hope this helps,

Rui Barradas

Em 14-05-2013 09:52, Luca Nanetti escreveu:
> Dear experts,
>
> I wanted to signal a peculiar, unexpected behaviour of 'apply'. It is not a
> bug, it is per spec, but it is so counterintuitive that I thought it could
> be interesting.
>
> I have an array, let's say "test", dim=c(7,5).
>
>> test <- array(1:35, dim=c(7, 5))
>> test
>
>       [,1] [,2] [,3] [,4] [,5]
> [1,]    1    8   15   22   29
> [2,]    2    9   16   23   30
> [3,]    3   10   17   24   31
> [4,]    4   11   18   25   32
> [5,]    5   12   19   26   33
> [6,]    6   13   20   27   34
> [7,]    7   14   21   28   35
>
> I want a new array where the content of the rows (columns) are permuted,
> differently per row (per column)
>
> Let's start with the columns, i.e. the second MARGIN of the array:
>> test.m2 <- apply(test, 2, sample)
>> test.m2
>
>       [,1] [,2] [,3] [,4] [,5]
> [1,]    1   10   18   23   32
> [2,]    7    9   16   25   30
> [3,]    6   14   17   22   33
> [4,]    4   11   15   24   34
> [5,]    2   12   21   28   31
> [6,]    5    8   20   26   29
> [7,]    3   13   19   27   35
>
> perfect. That was exactly what I wanted: the content of each column is
> shuffled, and differently for each column.
> However, if I use the same with the rows (MARGIIN = 1), the output is
> transposed!
>
>> test.m1 <- apply(test, 1, sample)
>> test.m1
>
>       [,1] [,2] [,3] [,4] [,5] [,6] [,7]
> [1,]    1    2    3    4    5   13   21
> [2,]   22   30   17   18   19   20   35
> [3,]   15   23   24   32   26   27   14
> [4,]   29   16   31   25   33   34   28
> [5,]    8    9   10   11   12    6    7
>
> In other words, I wanted to permute the content of the rows of "test", and
> I expected to see in the output, well, the shuffled rows as rows, not as
> column!
>
> I would respectfully suggest to make this behavior more explicit in the
> documentation.
>
> Kind regards,
> Luca Nanetti
>