
Ublas : 
From: Nico Galoppo (nico_at_[hidden])
Date: 20061013 10:02:46
Gunter Winkler wrote:
>> bone>get_core().save_solution(ublas::subrange(  b(), c, c+6));
>
>
> Why do you use (b)() and not just (b)? The latter is the vector_expression?
> If you have difficulties to define the correct overload of subrange(), then
> you can directly try to use project().
>
Doh! Sorry, I don't know why I didn't try that first. I assumed it wouldn't
work, probably because I had issues with that in the past, where I was actually
acessing b's data, where you obviously have to evaluate the expression. Your
suggestion worked perfectly though, thanks.
BTW, if I understand the concept of expression templates, I assume that this
will not construct a negated temporary of b. Is that correct?
> vector_expression<V> is the base type of any ublas vectorlike type. (See
> BartonNackmanTrick or CRTP,
> http://en.wikipedia.org/wiki/Curiously_Recurring_Template_Pattern )
> Thus creating an intance of vector_expression<X> is not useful. You have to
> create an instance of the correct type. For (b) it is:
> vector_unary< V, scalar_minus<V> > (see vector_expression.hpp)
Thanks, that makes sense! My knowledge of ET and CRTP is far from complete, but
I understand what you mean.
> I'd like to add another general comment: If you write functions using ublas
> classes, it is better to define them for vector_expressions and
> matrix_expression instead of vector and matrix (return types should be
> vectors and matrices is most cases).
That's what I was trying to do, right? Anyways, it's a good comment, and I've
started doing so since a while.
Why do the return types have to be vectors/matrices in most cases? Isn't that
potentially expensive (many data copies) ? Is it a bad thing to try to write
functions that return vector/matrix expressions?
> template <class VECTOR, class MATRIS>
> VECTOR do_something(const vector_expression<VECTOR>& x, const
> matrix_expression<MATRIX>& m) ...
> and if you have to operate on subranges, you can even use
>
> template <class VECTOR, class MATRIS>
> VECTOR do_something(const vector_expression< vector_range<VECTOR> >& x, const
> matrix_expression<MATRIX>& m) ...
Why is this necessary? vector_range is a model of vector_expression, so why
wouldn't the first example work?
Thanks for all your comments!
nico
 nico galoppo von borries  address: 119 fidelity st., carrboro comp. graphics phd. student  north carolina, 27510 USA UNC, chapel hill  phone: +1 (919) 9621898 (office)  +1 (919) 9424388 (home)  email: nico at cs dot unc dot edu  homepage: http://www.ngaloppo.org