Re: [boost] ublas: matrix clear() and operator=

From: Noel Yap (yap_noel_at_[hidden])
Date: 2002-05-01 15:17:27

How does the Boost matrix work compare with MTL?

Thanks,
Noel
--- Joerg Walter <jhr.walter_at_[hidden]> wrote:
> ----- Original Message -----
> From: "Peter Schmitteckert (boost)"
> <boost_at_[hidden]>
> To: <boost_at_[hidden]>
> Sent: Tuesday, April 30, 2002 8:36 PM
> Subject: Re: [boost] ublas: matrix clear() and
> operator=
> 
> 
> > Salut,
> >
> > On Tuesday 30 April 2002 18:09, Toon Knapen wrote:
> > > Currently, the matrices can be clear()-ed such
> that all elements are set
> to
> > > 0. I would find it even more convenient to be
> able to use
> > > operator=(some_value_type) to assign a specific
> value to all the entries
> in
> > > the matrix. But I figure there's some good
> reason one has to use clear()
> > > instead of operator=() ?
> >
> > As fas as I understand, this is only true for raw
> memory, if you use
> > std::vector, then std::vector().clear() will be
> called, erasing the
> elements.
> > If I'm correct than one has to be carefull to use
> ublas::matrix<
> > std::vector<..>, ... > to interface with FORTRAN
> libraries, since one
> > must not call clear() before calling FORTRAN
> routines.  (This question
> > was raised some time ago on this list).
> > Jörg, am I'm correct ?
> 
> AFAIK, Toon needs to first initialize a matrix to
> zero and then to add parts
> consecutively, something like
> 
> matrix<> m;
> m.clear ();
> for (...)
>     m += ...;
> 
> He wants to express the intent of m.clear() more
> clearly ;-). This is one of
> the drawbacks of the decision not to initialize
> numerical vectors and
> matrices for performance reasons.
> 
> May be, it would be a good thing to add initializing
> constructors.
> 
> > Concerning your operator, one might instead define
> a matrix with all
> elements
> > set to 1, similar to matrix_identity, otherwise
> > matrix A = 2
> > might be confusin. I'd expect this expression to
> set the diagonal elements
> > only, while you might expect that all elements are
> set to 2:
> > Matrix A = 2 * matrix_identity;
> > Matrix B = 2 * matrix_all_elements_one;
> 
> I've already played with something like a
> scalar_matrix<>, which is constant
> in all elements. But such a (dense) matrix doesn't
> harmonize with sparse
> matrices.
> 
> Regards
> 
> Joerg
> 
> 
> _______________________________________________
> Unsubscribe & other changes:
http://lists.boost.org/mailman/listinfo.cgi/boost

__________________________________________________
Do You Yahoo!?
Yahoo! Health - your guide to health and wellness
http://health.yahoo.com