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Anders Logg wrote:
On Fri, Nov 13, 2009 at 07:28:07AM +0100, Johan Hake wrote:On Thursday 12 November 2009 21:15:51 Anders Logg wrote:I have received some complaints on the new Expression class. It works reasonably well from C++ but is still confusing from Python: 1. Why does a function space need to be specified in the constructor? f = Expression("sin(x[0])", V=V) Does this mean f is a function on V? (No, it doesn't.) 2. The keyword argument V=foo is confusing: f = Expression(("sin(x[0])", "cos(x[1])"), V=V) g = Expression("1 - x[0]", V=Q) The reason for the function space argument V is that we need to know how to approximate an expression when it appears in a form. For example, when we do L = dot(v, grad(f))*dx we need to interpolate f (locally) into a finite element space so we can compute the gradient. Sometimes we also need to know the mesh on which the expression is defined: M = f*dx This integral can't be computed unless we know the mesh which is why one needs to do assemble(M, mesh=mesh) My suggestion for fixing these issues is the following: 1. We require a mesh argument when constructing an expression. f = Expression("sin(x[0])", mesh) 2. We allow an optional argument for specifying the finite element. f = Expression("sin(x[0])", mesh, element)
We could also have
f = Expression("sin(x[0])", mesh, k)
where k is the order of the continuous Lagrange basis since that's the
most commonly used.
3. When the element is not specified, we choose a simple default element which is piecewise linear approximation. We can derive the geometric dimension from the mesh and we can derive the value shape from the expression (scalar, vector or tensor).
This is bad. If a user increases the polynomial order of the test/trial functions and f remains P1, the convergence rate will not be optimal.
A better solution would be to define it on a QuadratureElement by default. This, I think, is the behaviour that most people would expect. This would take care of higher-order cases.
This will remove the need for the V argument and the confusion about whether an expression is defined on some function space (which it isnot).
But it is when it's used in a form since it's interpolated in the given space.
It also removes the need for an additional mesh=mesh argument when assembling functionals and computing norms. This change will make the Constant and Expression constructors very similar in that they require a value and a mesh (and some optional stuff). Therefore it would be good to change the order of the arguments in Constant so they are the same as in Expression: f = Constant(0, mesh) f = Expression("0", mesh)
Yes.
And we should change Constant rather than Expression since Expression might have an optional element argument: f = Expression("0", mesh, element) Does this sound good?Yes, I think so. I suppose you mean f = CompiledExpression("0", mesh) Just referring to the Blueprint about the simplification of the Expression class in PyDOLFIN.I'm not so sure anymore. Calling it Expression looks simpler.
Agree. Garth What
were the reasons for splitting it into Expression and CompiledExpression? Is it the problem with the non-standard constructor when implementing subclasses? It's just that the most common use of expressions in simple demos will be stuff like f = Expression("sin(x([0])", mesh) so one could argue that this should be as simple as possible (and just be named Expression).Should an Expression in PyDOLFIN then always have a mesh? This will make an Expression in PyDOLFIN and DOLFIN different, which is fine with me.Yes, to avoid needing to pass the mesh to assemble() and norm() in some cases and to automatically get the geometric dimension.If others agree, can you add it to the Blueprint, mentioned above, and I can do the change some time next week, or after a release (?).Let's hear some more comments first. -- Anders ------------------------------------------------------------------------ _______________________________________________ DOLFIN-dev mailing list DOLFIN-dev@xxxxxxxxxx http://www.fenics.org/mailman/listinfo/dolfin-dev
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