interleave rows of matrix stored in a list in R

Question

I want to create interleaved matrix from a list of matrices.

Example input:

> l <- list(a=matrix(1:4,2),b=matrix(5:8,2))
> l
$a
     [,1] [,         


        

Answer 1

Here is a one-liner:

do.call(rbind, l)[order(sequence(sapply(l, nrow))), ]
#      [,1] [,2]
# [1,]    1    3
# [2,]    5    7
# [3,]    2    4
# [4,]    6    8

To help understand, the matrices are first stacked on top of each other with do.call(rbind, l), then the rows are extracted in the right order:

sequence(sapply(l, nrow))
# a1 a2 b1 b2 
#  1  2  1  2 

order(sequence(sapply(l, nrow)))
# [1] 1 3 2 4

It will work with any number of matrices and it will do "the right thing" (subjective) even if they don't have the same number of rows.