Pick 1st Value and last Value of 1D cellVariable while running in parallel

[ACamposG]

I have a 1D cellVariable 'X' of 100 elements, with uniformGrid1D mesh type. During execution, on every single time step I need to take the first value X[0] and the last value X[-1] and subtract them (for some purpose) : X[0] - X[-1]

When I run in parallel, I use a conditional statement with procID=0 and procID=Nproc-1 to locate the values I need as follows:

if parallel.procID == 0:
Xi=X[0]
if parallel.procID == (parallel.Nproc - 1):
Xf=X[-1]

Subtraction= Xi-Xf

However, Xi and Xf are empty, it seems that outside the conditional the variables are reset.

I tried to broadcast the value of X[0] in the first core but it does not work. It seems that sometimes the last core is executed before the first core so X[0] is not there yet.

Is there a workaround or a way to do it?

Thanks

[guyer]

It's not that Xi and Xf are reset, but that they are only defined on procID == 0 and procID == (Nproc - 1), respectively.

You were on the right track with broadcasting, but not with X[0]. Rather, try

Xi = None
Xf = None
if parallel.procID == 0:
    Xi=X[0]
if parallel.procID == (parallel.Nproc - 1):
    Xf=X[-1]
Xi = parallel.bcast(Xi, root=0)
Xf = parallel.bcast(Xf, root=parallel.Nproc - 1)

Welcome to the unintuitive mysteries of parallel race conditions.

[ACamposG]

Thanks a lot John, it worked very nice

[guyer]

Happy to help