Re: Condensing indices in a matrix

On Tue, Dec 22, 2020 at 12:49 AM Kai Torben Ohlhus <[email protected]>
wrote:

>
> It has been a longer project, but I hope it was worth the outcome.
> Looking at your matrices M with the "spy" command, you can clearly see a
> sparse band structure and create them more efficiently with my function
> "createMfast" below.
>
> Your function needs for dimension N = 50 (5100 x 5100 dense matrix)
> about 4 seconds and about 200 MB on my machine.  From N=100 the
> computational cost become too huge.
>
> The function "createMfast" uses sparse banded and boolean matrices (as
> your values only deal with {0,1} and for N=500 (501000 x 501000 sparse
> banded matrix) only 0.1 seconds creation time are required and it uses
> only 17 MB of main memory.  Thus you can grow larger if you want.
>
> The following experiment shows a comparison up to dimension 50 and a
> small timing benchmark..
>
> HTH,
> Kai
>

Thank you very much! spdiags is the key I'd been missing. spy will also be
very useful as I learn to use spdiags and troubleshoot.
... and to say that the runtime comparison was impressive is an
understatement.