ufl team mailing list archive

[Patrick Riesen <priesen@xxxxxxxxxxxxxxx>]

Garth N. Wells wrote:
> On 06/07/10 13:25, Patrick Riesen wrote:
> > hello,
> >
> > i want to take the gradient of a coefficient with respect to some other
> > quantity than the spatial coordinates (i.e. some other coefficient).
> >
> > i tried to construct a custom operator derived from diff() and the
> > SpatialCoordinate/Derivative classes in ufl but after some point beyond
> > my knowledge it always gets evaluated to zero by FFC then stops
> > compiling with FFC : division by zero!
> >
> > does somebody of you experts know an approach to this?
>
> Did you try using 'variable'?
>
>     . . .
>     v  = variable(v)
>     f  = v*v
>     df = diff(f, v)   
yes, but if f is a coefficient and not an expression of v, this won't 
work, i.e.,

f = Coefficient(element)
f = variable(f)
v = SpatialCoordinates(cell)
v = variable(v)
df = diff(f, v)

will end up as Type Zero, although this is equivalent to grad(f). that's 
the one case, and it v = Coefficient(element) as well, it will also be zero.
if would be nice if where not zero :-)

patrick

> Garth
>
> > many thanks and best regards,
> > patrick
> >
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