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From: arun <aragorn168b@gmail.com>
Newsgroups: comp.soft-sys.matlab
Subject: Re: size(sparse matrix) > size(full matrix)
Date: Thu, 17 Sep 2009 04:17:21 -0700 (PDT)
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On Sep 17, 1:10 pm, "Bruno Luong" <b.lu...@fogale.findmycountry>
wrote:
> Rune Allnor <all...@tele.ntnu.no> wrote in message <a56978d9-99f2-4ff6-8073-a77ab61bb...@o21g2000vbl.googlegroups.com>...
>
> > Sparse matrices are not used to gain speed, but to save
> > space.
>
> Wrong, many linear algebra algorithms are designed to gain speed with sparse matrix since Matrix-vector product is much faster than for full matrix. That lead to the class of iterative method (gradient conjugate, GMRES, Lanczos, etc...) that is suitable when working with sparse matrix. The full matrix is not even competitive, even for speed.
>
> Bruno

Bruno,
Yes, its true, however I see that when operations involving sparse-
matrices result in a more-or-less full matrix (as in my case at some
intermediate point), it becomes slow. But as Rune pointed out, my data
set is quite huge 1200 * 312000. So I had to split them and I thought
I could also save more space by storing sparse as my calculations are
faster already and shouldn't take much *even if* sparse makes it
slow.

thank you,
best, arun.