dolfin team mailing list archive

[Patrick Riesen <priesen@xxxxxxxxxxxxxxx>]

Anders Logg wrote:
> On Fri, Mar 06, 2009 at 04:30:56PM +0100, Patrick Riesen wrote:
> > Anders Logg wrote:
> > > On Thu, Mar 05, 2009 at 04:55:38PM +0100, Patrick Riesen wrote:
> > > > hi all
> > > >
> > > > i'm stuck on the following problem:
> > > >
> > > > i'd like to compute a term as
> > > >
> > > > eta = (gamma)^(-2/3), or general eta = gamma^(n) and n is real
> > > >
> > > >
> > > > in my code, gamma is dependent on a velocity function from a previous 
> > > > interation, so gamma = gamma(u0), and how to compute gamma stems from a 
> > > > form-file, so i do:
> > > >
> > > > _M_gamma = new gammaFunctional(u0);
> > > >
> > > > Function  f_gamma(&mesh, *M_gamma, 0);
> > > >
> > > >
> > > > my question is now
> > > >
> > > > how can i create the other function f_eta = f_gamma^(2/3) in a nice way?
> > > >
> > > > what i thought about is to create a derived function class Eta where 
> > > > overload the evaluation function in a way as
> > > >
> > > > double eval(const double* x) const
> > > > {
> > > >    return (gamma.eval(x)*gamma.eval(x))^(-1/3)
> > > > }
> > > >
> > > > please tell me if this is a possible approach, or else advise me how to 
> > > > handle this :-)
> > > >
> > > >
> > > > thanks a lot for the support,
> > > >
> > > > patrick
> > > >
> > > > (btw i do this together with dolfin-grade2 so i use dolfin0.8.1 )
> > > I'd suggest operating on the vectors of dofs. Send in the gamma
> > > function to the constructor of your Function subclass, pick out the
> > > vector of dofs, iterate over the dofs and just insert the values into
> > > the vector using pow(x[i], 2.0/3.0).
> > >
> > > Use the get/set methods in Vector to first pick out arrays, operate on
> > > the arrays and then put them back.
> > >  
> > hi anders,
> > i understand how to create the f_eta class and hand in the f_gamma but 
> > i'm uncertain on what to do inside f_eta exactly. can you give me some 
> > more help, e.g. in a few pseudocode statements?
> > thanks a lot for the help,
> > patrick
>
> In the constructor of EtaFunction, do something like this:
>
>   const Vector& gamma_vector = gamma.vector();
>   const Vector& eta_vector = vector();
>
>   for (uint i = 0; i < eta_vector.size(); i++)
>     eta_vector.setitem(i, pow(gamma_vector[i], 3.0));

now i see the link between the gamma function and the Eta function.

many thanks for the instruction,

patrick