I am looking at some UDF code that someone gave me and I am a litte confused, it is show below. My questionis why would a cell index (c1) be assigned a negative value (i.e. < 0). Any help would be super.

DEFINE_UDS_FLUX(conv_fluxes, f, thread, eqn)

{

cell_t c0, c1;

Thread *tc0, *tc1;

c0 = F_C0(f, thread);

tc0 = THREAD_T0(thread);

c1 = F_C1(f, thread);

tc1 = THREAD_T1(thread);

if( c1 < 0 )

{

return one thing

}

else

{

return another

}

}

According to me the cell_t index goes from 0 to n-1 cells. So the first if will never be executed. Luca

I ran this code with a Message if c1<0 and it produced results. so at some point c1 does adopt a negative value. When this happens c1 adopts the value of -1 according to the output line. I am guessing this must be something to do with the boundary, but some more insight would be very helpful.

Try to discover where is the cell with index -1. For example export its node position with the macro NODE_X(node). I had never come across a negative index. Maybe it means something... Is it the only negative index you found? Please let me know. Luca

AH! Maybe C1<0 means that no c1 cell exist. If you're applying your function on a boundary face-thread, there's no C1 cell adjacent to it, only a C0 cell. I used the macro BOUNDARY... reported on the UDF guide. Let me know what you discover.Luca

-1 is the only negative index retuned and from returning F_CENTROIDS, it looks like you are right. if c == -1 then it appears to be a boundary face and hence c1 dosen't exist.

Thanks.

OK then, we learned something more about UDF programming... Have a good day,Luca