[petsc-users] Creating a 2d geometry by using vector

[Alan Wei]

Thanks Barry,

^_^
Alan

On Fri, Aug 19, 2011 at 3:51 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:

>
> On Aug 19, 2011, at 2:55 PM, Alan Wei wrote:
>
> > Dear Barry,
> >     In ex19, the X is a kinda of global vec, while the x is a local vec.
> Since DAVecGetArray makes the x and X sharing the same actual vector data,
> the vec X is, actually, a global copy of the vec x, isnt it?
>
>   Not really. If you pass a local (nonparallel) vector to the VecGetArray()
> the resulting array covers all the entries in the vector.  If you pass a
> global (parallel) vector then the resulting array only gives you access to
> the values that are on that one process.
>
>   Barry
>
> >
> > best,
> > Alan
> >
> > On Thu, Aug 18, 2011 at 10:12 PM, Barry Smith <bsmith at mcs.anl.gov>
> wrote:
> >
> > On Aug 18, 2011, at 9:40 PM, Alan Wei wrote:
> >
> > > Dear Barry,
> > >     That is really a great idea, and it is something that I want. I've
> never noticed that the structure in C is so marvelous. However, I have a
> further question. In that example, a variable x is defined like Field **x,
> and DAVecGetArray(da,X,&x). I wonder if there is any relationship between
> da, X and x after using DAVecGetArray. Or it just a prerequisite for using
> the nice notation of 'x'.
> >
> >   The only real relationship is that x looks into the X data structure to
> allow you to access the data as if it were in simple arrays. In other words
> the x and X share the same actual vector data.
> >
> >    Barry
> >
> >
> > >
> > > thanks,
> > >
> > > Alan
> > >
> > > On Thu, Aug 18, 2011 at 9:09 PM, Barry Smith <bsmith at mcs.anl.gov>
> wrote:
> > >
> > >  Alan,
> > >
> > >   I'm not sure what you are getting at but we commonly use tricks like
> (see src/snes/examples/tutorials/ex19.c)
> > >
> > > typedef struct {
> > >  PetscScalar u,v,omega,temp;
> > > } Field;
> > >
> > >  Then in this case use a DMDAVecGetArray() to get f and x and then have
> nice notation like
> > >
> > >      f[j][i].u     = x[j][i].u;
> > >      f[j][i].v     = x[j][i].v;
> > >      f[j][i].omega = x[j][i].omega - (x[j][i+1].v - x[j][i].v)*dhx;
> > >      f[j][i].temp  = x[j][i].temp;
> > >
> > > note that this means the values are stored interlaced in the vector.
> > >
> > >   Barry
> > >
> > > On Aug 18, 2011, at 9:03 PM, Alan Wei wrote:
> > >
> > > > Dear All,
> > > >     I'm trying to 'draw' a 2-D cylinder in PETSc. It simply restore
> the x and y coordinate values in two arrays. I wonder if I can use Vec in
> PETSc to implement it (i.e. defining a vec circle) so that I can call its x
> or y coordinate by circle.x or circle.y. Is this possible? also, is this a
> better way than just defining it as two arrays, i.e. x[ ] and y[ ].
> > > >
> > > > thanks in advance,
> > > > Alan
> > >
> > >
> >
> >
>
>
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