dolfin team mailing list archive

[Johan Hake <johan.hake@xxxxxxxxx>]

On Friday February 11 2011 09:54:39 Anders Logg wrote:
> On Fri, Feb 11, 2011 at 08:08:32AM -0800, Johan Hake wrote:
> > On Friday February 11 2011 01:16:31 Anders Logg wrote:
> > > How is DOLFIN_EPS treated in Expressions and boundary conditions
> > > (using compile_subdomains)?
> > > 
> > > I thought that DOLFIN_EPS was known to the JIT compiler.
> > > 
> > > Here are somethings I noted while debugging a code:
> > > 
> > > 1. f = Expression("sin(x[0] + DOLFIN_EPS)") does not compile.
> > > 
> > > The log file pointed to by the JIT compiler says "syntax error" and the
> > > generated code in .i file found in the same directory contains this:
> > > 
> > > class Expression_5d66d4e2b02f54f6088fc61767e40073: public Expression
> > > {
> > > 
> > > public:
> > >   double DOLFIN_EPS;
> > >   Expression_5d66d4e2b02f54f6088fc61767e40073():Expression()
> > >   {
> > >   
> > >     DOLFIN_EPS = 0.0;
> > >   
> > >   }
> > >   
> > >   void eval(dolfin::Array<double>& values, const
> > >   dolfin::Array<double>& x) const
> > >   {
> > >   
> > >     values[0] = sin(x[0] + DOLFIN_EPS);
> > >   
> > >   }
> > > 
> > > };
> > > 
> > > So there are two problems here:
> > > 
> > > 1.a) The JIT compiler fails to compile the Expression f which looks
> > > like a valid expression to me
> > > 
> > > 1.b) DOLFIN_EPS is set to zero.
> > 
> > Now it doesn't.
> 
> Thanks!
> 
> > > 2. When trying something similar with compile_subdomains, it seems
> > > that
> > > 
> > >   compile_subdomains("x[0] < DOLFIN_EPS")
> > > 
> > > compiles and produces correct code. In particular the .i file does not
> > > contain DOLFIN_EPS = 0.0.
> > 
> > subdomain treats DOLFIN_EPS and on_boundary seperately.
> > 
> > > But the magic for creating variables is a bit too clever. I had a typo
> > > which said "DOFLIN_EPS" instead of "DOLFIN_EPS" which compiles without
> > > any warnings and DOFLIN_EPS is set to zero. That's a litle dangerous.
> > 
> > You cannot blame your typos on compile expressions ;)
> 
> I just did! ;-)
> 
> Would it be possible (and desirable) to require default values for all
> variables in an Expression?
> 
> f = Expression("a*x[0]", a=2.0)
> f.a = 3.0

A good point. It would probably require to hide the kwarg: element, cell, 
degree, into a **kwargs, making them less transparent. We could also add the 
default value **kwargs after the above mention kwargs. If we go for this we 
can end up with syntax like:

  f = Expression("a*x[0]", element=some_element, a=2.0)

instead of:

  f = Expression("a*x[0]", a=2.0, element=some_element)

I think the last syntax makes more sense, but it requires more hidden logics.

> The current implementation is impressively clever to figure out variable
> names automatically, but it also means that all variable names that
> are misspelled are automatically set to zero without warning.

It is called a feature ;)

Johan