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Subject: Re: [ublas] Assignement operation proposal.
From: David Bellot (david.bellot_at_[hidden])
Date: 2010-03-23 03:47:53

you cannot know how much time you will save us. Especially during testing !

On Mon, Mar 22, 2010 at 23:18, Gunter Winkler <guwi17_at_[hidden]> wrote:

> Hello Nasos,
> this is a feature that was missing long time. So I definitely vote for
> it. What do others think?
> Am Sunday 21 March 2010 schrieb Nasos Iliopoulos:
> > Insipired by the fucntionality of boost.assignment, i decided to
> > implement something similar in ublas. Unfortunately due to the
> > special way ublas is handling its containers I did not find it that
> > convenient to use boost.assignment.
> I opened a new ticket to track progress.
> > - Where do you think this should be added? As a separate header file,
> > or included in vector_ and matrix_ expression headers?
> it should be a separate header, maybe it is then included from
> matrix/vector.hpp
> I don't know if "assigner.hpp" is a good name, but I could not find a
> better one, yet.
> > - I found it incovenient to implement this functionallity using
> > iterators
> For dense, triangular and banded matrices it is efficient to access the
> matrix elements by index. However for most sparse types you will get a
> very bad performance because calling the mutable operator(i,j) results
> in the following steps:
> * look up element (i,j) - this takes O(nnz) operations in worst case
> * if element (i,j) exists return a reference to the value
> * otherwise insert the element (i,j) into the sparse data structure and
> return a reference to this element - this is another O(nnz) operation
> in worst case
> (if BOOST_UBLAS_NO_ELEMENT_PROXIES is not defined then a proxy is
> returned instead of the real element and the insert is performed during
> the destruction of the proxy - obviously this makes things only worse)
> For the several sparse types we would need more intelligent assignments.
> Personally I don't see any use here (except for toy examples).
> Do you think it is possible to use triples (i,j,value) to fill a sparse
> matrix, e.g. like
> cm << sparse_element(0,0,1.0), sparse_element(1,3,2.0) ;
> mfg
> Gunter
> (But I am still not convinced that this makes any benefit)
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David Bellot, PhD