On Tue, Mar 1, 2011 at 4:55 PM, Darcy Webber <darcy.web...@gmail.com> wrote:
> Dear R users,
>
> I am having some difficulty arranging some matrices and wondered if
> anyone could help out. As an example, consider the following matrix:
>
> a <- matrix(1:32, nrow = 4, ncol = 8)
> a
> [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8]
> [1,] 1 5 9 13 17 21 25 29
> [2,] 2 6 10 14 18 22 26 30
> [3,] 3 7 11 15 19 23 27 31
> [4,] 4 8 12 16 20 24 28 32
>
> I would like it to look like the following matrix:
>
> [,1] [,2] [,3] [,4]
> [1,] 1 5 9 13
> [2,] 2 6 10 14
> [3,] 3 7 11 15
> [4,] 4 8 12 16
> [5,] 17 21 25 29
> [6,] 18 22 26 30
> [7,] 19 23 27 31
> [8,] 20 24 28 32
>
> I can achieve this using the following:
>
> a1 <- a[, 1:4]
> a2 <- a[, 5:8]
> b <- rbind(a1, a2)
>
> However, my initial matrix often has a varibale number of columns (in
> multiples of 4, and I still want to split the columns into blocks of 4
> and stack these). I have considered working out how many blocks the
> matrix must be split into using: no.blocks <- ncol(a)/4. My problem is
> then implementing this information to actually split the matrix up and
> then stack it. Any guidance on this would be much appreciated.
>
> Regards
> Darcy Webber
>
Try converting to a 3d array, swapping the last two dimensions and
reconstituting it as a matrix:
> matrix(aperm(array(a, c(4, 4, 2)), c(1, 3, 2)), nc = 4)
[,1] [,2] [,3] [,4]
[1,] 1 5 9 13
[2,] 2 6 10 14
[3,] 3 7 11 15
[4,] 4 8 12 16
[5,] 17 21 25 29
[6,] 18 22 26 30
[7,] 19 23 27 31
[8,] 20 24 28 32
--
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email: ggrothendieck at gmail.com
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