dolfin team mailing list archive

["Martin Sandve Alnæs" <martinal@xxxxxxxxx>]

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.

--
Martin