[R] splitting and stacking matrices

[Darcy Webber]

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