dolfin team mailing list archive

[Johan Hake <johan.hake@xxxxxxxxx>]

On Sunday 24 January 2010 00:03:41 Garth N. Wells wrote:
> Johan Hake wrote:
> > On Saturday 23 January 2010 14:55:08 Garth N. Wells wrote:
> >> Johan Hake wrote:
> >>> On Saturday 23 January 2010 08:42:14 Garth N. Wells wrote:
> >>>> Is it correct that behind the scenes that
> >>>>
> >>>>   U0 = Function(V)
> >>>>   U  = Function(V)
> >>>>   U0.vector()[:] = U.vector()[:]
> >>>>
> >>>> involves a GenericVector::get(..) call and a GenericVector::set(..)
> >>>> call? If so, it isn't ideal since it introduces unnecessary new/delete
> >>>> operations and unnecessary copying of data.
> >>>
> >>> None of GenericVector::get(..) or GenericVector::set(..) are invoked,
> >>> see __getslice__ and __setslice__ in la_post.i.
> >>>
> >>>    U0.vector()[:]
> >>>
> >>> involves
> >>>
> >>>   GenericVector::operator =(..)
> >>>
> >>> and
> >>>
> >>>   U.vector()[:]
> >>>
> >>> involves
> >>>
> >>>   GenericVector::copy()
> >>>
> >>> However the latter is unnecessary as you instead can do:
> >>>
> >>>   U0.vector()[:] = U.vector()
> >>>
> >>> invoking the assignment operator of U0's vector with U's vector.
> >>
> >> What happens if I do
> >>
> >>   x = U.vector()[:]
> >
> > It just triggers the copy method of GenericVector, which is the same
> > behavior as for other itterable Python types.
> >
> >> ? Is x a numpy array?
> >
> > No you need to call array() to accomplish that.
> 
> OK. What I'm trying to do is
> 
>     # Get vectors
>     u_vec  = u.vector()[:]
>     u0_vec = u0.vector()[:]
>     v0_vec = v0.vector()[:]
>     a0_vec = a0.vector()[:]

You should not need to make a copy of the vectors here.

>     a_vec = 1.0/(2.0*beta)*((u_vec-u0_vec - v0_vec*dt)/(0.5*dt*dt) \
>          - (1.0-2.0*beta)*a0_vec)
>     v_vec = dt*((1.0-gamma)*a0_vec + gamma*a_vec) + v0_vec

The expression cause a lot of intermediate vectors, but should work, if beta, 
gamma and dt are all scalars.

>     # Update (U0 <- U0)
>     v0.vector()[:] = v_vec
>     a0.vector()[:] = a_vec

This should work.

Johan

> I guess I need to call array() to make this work?
> 
> Garth
> 
> > Johan
> >
> >> Garth
> >>
> >>> Johan
>