Linear algebra -- block diagonal matrix?

In MATLAB, there is a constructor diagm for block diagonal matrices.

I seem to recall that this is missing in Julia. If I am wrong, what is the most elegant way to create one?

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for sparse matrices, there is SparseArrays.blockdiag

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There’s also BlockBandedMatrices · Julia Packages


There’s also BlockDiagonals.jl I have not used that package myself, but it looks useful.

However, packages like BlockBandedMatrices and BlockDiagonals do not create Matlab-style matrices, but instead define new types to hold diagonal matrices. That can make some operations (such as matrix-matrix or matrix-vector multiplication) run faster, but also means that some things that are possible in Matlab (such as adding entries outside of the blocks) won’t work.

SparseArrays.blockdiag(...) gives basically the same thing as sparse(blkdiag(..)) would give in Matlab.

For a plain-vanilla Julia matrix, just use any of the above, and then pass the result to the Matrix constructor. This is the most Matlab-like option, but therefore it will also be as slow and as memory-consuming as Matlab.

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