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
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