dolfin team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Mon, Jun 23, 2008 at 01:18:29PM +0200, Martin Sandve Alnæs wrote:
> 2008/6/23 Garth N. Wells <gnw20@xxxxxxxxx>:
> >
> >
> > Anders Logg wrote:
> >> On Sat, Jun 21, 2008 at 05:29:14AM -0400, Hatef Monajemi wrote:
> >>> Hi
> >>> Isn't this code supposed to work? It gives me "invalid lvalue in
> >>> assignment" error while compiling. How should I assign the elements of
> >>> a Matrix???
> >>>
> >>> //-------------------------------
> >>> int main()
> >>> {
> >>>
> >>>
> >>> for(i=0; i<nens; i++)
> >>> .
> >>> .
> >>> .
> >>> real* values = new real[nstat];
> >>> u1.interpolate(values);
> >>>
> >>> Matrix S(nstat,nens);
> >>>
> >>> for(int j=0;j<nstat;j++)
> >>>   {
> >>>     S(j,i)=values[j];
> >>>   }
> >>>
> >>> }
> >>
> >> Sorry, this doesn't work yet.
> >>
> >> We can easily fix this in the GenericMatrix interface by adding an
> >> auxiliary class MatrixEntry (which just holds (i, j) and a reference
> >> to the matrix) and have operator() return that, then overload
> >> operator= in MatrixEntry to assign to the matrix.
> >>
> >> We had this in DOLFIN 2-3 years back but the linear algebra has gone
> >> through several reimplementations since then...
> >>
> >> Until this has been implemented, you can use the setitem() function.
> >>
> >
> > I don't know if we want this because the operation is ill-suited to some
> > linear algebra back-ends and adding it encourages its use. If
> > element-wise access is needed, the uBlas matrix class should be used.
> >
> > Garth
> 
> I agree with Garth here. There's also no chance at all that this will
> be efficient. It's just not within the scope of the dolfin linear
> algebra classes.

It's very natural to be able to set values in a matrix. I'm not sure
we must always make it impossible to do operations which are not
efficient. We can easily discourage it by issuing a warning when
matrices are accessed item by item.

-- 
Anders

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