Passing multiple arguments as one in Julia

Question

I am trying to make a diagonal block matrix in Julia. I have an nxn array that I want to make P copies of as a block matrix down the diagonal and the rest of the matrix is sparse.

If arr is my array, I know I can use:

blockdiag(sparse(arr),sparse(arr))

to create a block with P=2 copies of the array down the diagonal.

However, for large P, how can I do this in a general way with variable P?

I tried making an array that is (nxnxP), however BlockDiag() does not accept a 3D array.

Answer 1

fill can repeat elements without actually making a copy. Hence you can just do:

blockdiag(fill(sparse(arr), 2)...)

Here is a full Julia session:

julia> using SparseArrays

julia> arr=Matrix(sprand(Float64,3,3,0.25))
3×3 Matrix{Float64}:
 0.0       0.016897  0.0
 0.219705  0.0       0.0
 0.0       0.0       0.893547


julia> blockdiag(fill(sparse(arr), 2)...)
6×6 SparseMatrixCSC{Float64, Int64} with 6 stored entries:
  ⋅        0.016897   ⋅         ⋅         ⋅         ⋅
 0.219705   ⋅         ⋅         ⋅         ⋅         ⋅
  ⋅         ⋅        0.893547   ⋅         ⋅         ⋅
  ⋅         ⋅         ⋅         ⋅        0.016897   ⋅
  ⋅         ⋅         ⋅        0.219705   ⋅         ⋅
  ⋅         ⋅         ⋅         ⋅         ⋅        0.893547