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rmatrixdiagonal

Build a block diagonal matrix from a known matrix

I want to build a block diagonal matrix (A) from a known matrix (B) by putting B in diagonal positions of A.

Let's say my B:

> matrix(c(1,3,4,5),nrow=2)
      [,1] [,2]
[1,]    1    4
[2,]    3    5

I am looking for a function like this: function(B,3) (3 is just a random number) which returns matrix A like this:

[1,] 1 4 . . . .
[2,] 3 5 . . . .
[3,] . . 1 4 . .
[4,] . . 3 5 . .
[5,] . . . . 1 4
[6,] . . . . 3 5

Really appreciate any help

Solution

  • We can use bdiag

    library(Matrix)
bdiag(replicate(3, B, simplify = FALSE))
#6 x 6 sparse Matrix of class "dgCMatrix"

#[1,] 1 4 . . . .
#[2,] 3 5 . . . .
#[3,] . . 1 4 . .
#[4,] . . 3 5 . .
#[5,] . . . . 1 4
#[6,] . . . . 3 5

    Can we wrapped in a function

    fdiag <- function(mat, n) {
      bdiag(replicate(n, mat, simplify = FALSE))
}

fdiag(B, 3)