Anders Logg wrote:
> On Mon, Apr 28, 2008 at 02:24:38PM +0200, Anders Logg wrote:
>
>> On Mon, Apr 28, 2008 at 03:00:26PM +0200, Marie Rognes wrote:
>>
>>> Error:
>>>
>>> Traceback (most recent call last):
>>> File "test.py", line 33, in ?
>>> u0.assign(w)
>>> File
>>>
>>> "/home/meg/src/dolfin/dolfin/local/lib/python2.4/site-packages/dolfin/dolfin.py",
>>> line 3172, in assign
>>> return _dolfin.cpp_Function_assign(*args)
>>> RuntimeError: *** Error: Unable to find finite element in library:
>>> "Mixed finite element: [Crouzeix-Raviart finite element of degree 1
>>> on a triangle, Crouzeix-Raviart finite element of degree 1 on a
>>> triangle]".
>>> Segmentation fault
>>>
>>>
>>> Code:
>>>
>>> from dolfin import *
>>>
>>> # Geometry of the domain and mesh
>>> shape = "triangle"
>>> mesh = UnitSquare(16, 16)
>>>
>>> # Setting up the elements
>>> V = VectorElement("CR", shape, 1)
>>>
>>> u0 = Function(V, mesh, 0.0)
>>>
>>> v = TestFunction(V)
>>> u = TrialFunction(V)
>>> a = dot(grad(u), grad(v))*dx
>>> L = dot(u0, v)*dx
>>>
>>> pde = LinearPDE(a, L, mesh)
>>> w = pde.solve()
>>> u0.assign(w)
>>>
>>>
>>> Easy fix?
>>>
>> Probably not. The message comes from the copy constructor (in C++) of
>> DiscreteFunction, which looks in the list of precompiled elements and
>> that list does not include vector-valued CR elements.
>>
>> I'll see if I can find the problem. You are doing this from Python so
>> it should be possible to get around this problem and not need to
>> create the element from precompiled code.
>>
>
> ok, I found a solution (temporary).
>
> If you change the line
>
> u0 = Function(V, mesh, 0.0)
>
> to
>
> u0 = Function(V, mesh, Vector())
>
> it should work.
>
>
Perfect. Thanks!
> Then both w and u0 will be discrete functions (i.e. storing their
> values in a vector). They will also be defined on the same space so
> then just the values in the vector need to get copied and that works
> nicely (in operator= in DiscreteFunction).
>
> The problem with your code is that u0 was not a discrete function, but
> instead a "constant function" which does not store its values in a
> vector. So then what happens is that a new discrete function must be
> created (including a new finite element) and then DOLFIN must (in C++)
> look in its library of precompiled elements (which is limited to
> reduce compile times).
>
> We can fix this and related issues when we rework/cleanup the function
> classes (now that the linear algebra seems to be ok).
>
>
Good.
--
Marie E. Rognes
Ph.D Fellow,
Centre of Mathematics for Applications,
University of Oslo
http://folk.uio.no/meg
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