On Jun 23 2008, Anders Logg wrote:
>On Mon, Jun 23, 2008 at 01:18:29PM +0200, Martin Sandve Alnæs wrote:
>> 2008/6/23 Garth N. Wells <[EMAIL PROTECTED]>:
>> >
>> >
>> > 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.
It's not impossible to set values - a user just needs to choose the
appropriate class, which in this case is the uBlas matrix.
Garth
>
>
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