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

[Gabor Grothendieck]

On Tue, May 14, 2013 at 4:52 AM, Luca Nanetti <luca.nanetti at gmail.com> wrote:
> 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.

aaply in the plyr package works in the way you expected.

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