dolfin team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Mon, Jan 17, 2011 at 06:07:19PM +0000, Garth N. Wells wrote:
>
>
> On 17/01/11 18:03, Anders Logg wrote:
> > On Mon, Jan 17, 2011 at 04:30:23PM +0000, Garth N. Wells wrote:
> >>
> >>
> >> On 17/01/11 16:28, Anders Logg wrote:
> >>> The following doesn't seem to work in Python any longer:
> >>>
> >>> A = Matrix(10, 10)
> >>>
> >>> Is matrix initialization broken?
> >>>
> >>
> >> Probably not broken - more like
> >>
> >>   A = Matrix(10, 10)
> >>
> >> is not supported. Since a Matrix is in general sparse, it doesn't make
> >> sense to initialise it as above (some backends even insist on the
> >> sparsity being defined at construction).
> >>
> >> Garth
> >
> > It would be good to allow simple initialization of a Matrix without
> > requiring to go through all the hassle of creating a SparsityPattern.
> >
> > I generally don't think we should disallow certain operations just
> > because they are potentially slow.
>
> It's not just that they're slow. They cannot be supported by all backends.

Then we throw an error.

> > Some operations (like Matrix index
> > access) will only be performed for toy problems or while testing and
> > then speed is not very important (since the problem is small anyway).
> >
>
> If it's made available, it will be used. This type of operation is main
> reason for things breaking in parallel. We have getitem and setitem
> which are sufficiently unfriendly that hopefully users get the idea that
> they're for testing only.

Just because something can be misused, it shouldn't be disallowed.

It would be easy (and more helpful) to add a message the first time
getitem/setitem is used in a program:

  Warning: Index access to matrix/vector values is potentially very
  slow and it breaks in parallel. To disable this warning, set the
  parameter "warning_index_access" to false.

--
Anders