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[Chong Luo <luo.chong@xxxxxxxxx>]

Thank you for referring me to this demo. That is exactly what I want.

Best,
Chong Luo


________________________________
From: Anders Logg <logg@xxxxxxxxx>
To: fenics-dev@xxxxxxxxxx
Sent: Friday, November 14, 2008 12:58:29 AM
Subject: Re: [FEniCS-dev] initial function

Take a look at demo/function/eval/cpp which does an interpolation.

-- 
Anders



On Tue, Nov 11, 2008 at 07:44:33PM -0800, Chong Luo wrote:
> My program is implemented in C++, so I'm looking for the equivalent C++ code of
> your Python code. When I look at dolfin-0.8.1/dolfin/function/ProjectL2.cpp, I
> found it's not implemented yet:
> 
> //-----------------------------------------------------------------------------
> void dolfin::projectL2(Mesh& meshB, Function& fA, Function& fB, FiniteElement&
> element)
> {
>   error("dolfin::projectL2 is disabled. Will be fixed when new Function is in
> place.");
>   /*
>   Form* a = ProjectionLibrary::create_projection_a(element.signature());
>   Form* L = ProjectionLibrary::create_projection_L(element.signature(), fA);
> 
>   // Compute projection
>   // FIXME: LinearPDE should not own memory from fB, allocate on heap for now
>   LinearPDE* pde = new LinearPDE(*a, *L, meshB);
>   pde->solve(fB);
>   */
> }
> 
> 
> Could you point me a way to do the projection or interpolation in C++?
> 
> Thank you.
> 
> Best,
> Chong Luo
> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
> From: Anders Logg <logg@xxxxxxxxx>
> To: fenics-dev@xxxxxxxxxx
> Sent: Friday, November 7, 2008 12:52:16 PM
> Subject: Re: [FEniCS-dev] initial function
> 
> On Fri, Nov 07, 2008 at 10:04:05AM -0800, Chong Luo wrote:
> > I'm trying to use Newton's method to solve an elasticity problem (using
> FEniCS,
> > of course). I want to set the initial displacement u to be a given function
> > (for example, u0(x,y) = (x, -0.5*y)). How I can initialise the degrees of
> > freedom vector x of the displacement u, so that it takes the values of the
> > given initial function u0(x,y)? If there is no direct way to do that, maybe
> my
> > question can be reduced to: given a component of the degrees of freedom
> vector
> > x of some discrete function u, how to get the corresponding coordinates in
> the
> > mesh?
> >
> > Thank you!
> >
> > Best,
> > Chong Luo
> 
> With the current (stable) version of FEniCS, I'd recommend projecting
> or interpolating whatever function you have into the finite element
> space. In Python, this can be done simply by
> 
>   f = InitialValue(...)
>   u0 = project(f, element)
>   u0 = interpolate(f, element)
> 
> With the current development version, you should be able to just
> define u0 as the initial value, and then in subsequent iterations
> (when you solve for the degrees of freedom), u0 will automatically
> change behavior from being defined by an expression to being defined
> by its degrees of freedom.
> 

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