Consider a 2x2 matrix of 2x2 blocks, e.g.,
using LinearAlgebra
using StaticArrays
M = zeros(SMatrix{2, 2, Float64, 4}, 2, 2)
M[1,1] = SA[1 0; 0 2]
M[2,2] = SA[0 3; 4 0]
What is the best way to convert M into a 4x4 matrix of scalars?
# how to convert M into this?
M2 = [1 0 0 0;
0 2 0 0;
0 0 0 3;
0 0 4 0]
In my real problem the matrix sizes will be larger, but the general question remains: How to flatten a block matrix into one larger matrix.
I recommend using BlockArrays.jl:
julia> using BlockArrays
julia> mortar(M)
2×2-blocked 4×4 BlockMatrix{Float64, Matrix{SMatrix{2, 2, Float64, 4}}, Tuple{BlockedUnitRange{Vector{Int64}}, BlockedUnitRange{Vector{Int64}}}}:
1.0 0.0 │ 0.0 0.0
0.0 2.0 │ 0.0 0.0
──────────┼──────────
0.0 0.0 │ 0.0 3.0
0.0 0.0 │ 4.0 0.0
You can of course do Matrix(mortar(M)) to get back to a "normal" matrix. However, if you have this kind of data structure you should like staying with the BlockArray.