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
>
>
