[SciPy-user] Sparse indexing workarounds

[Bill Baxter]

Does anyone have an easy (and efficient way) to update a submatrix of
a big sparse matrix?

  K = scipy.sparse.lil_matrix(bigN,bigN)
  ...
  conn = [1,4,11,12]
  K[ix_(conn,conn)] += elemK

where elemK is a 4x4 dense matrix.

This kind of thing is very commonly needed in FEM codes for assembling
the global stiffness matrix.  But sparse doesn't seem to support
either += or indexing with the open grid of indices returned by ix_.

Thanks,
--bb