[petsc-users] petsc4py Vec: how to copy from one index to multiple indices of the same vector?

[Barry Smith]

On Oct 2, 2010, at 5:08 PM, Amal Alghamdi wrote:

> Dear all,
> 
> I would like to ask about the proper way to copy a value in vector x, let us say x[0] into places in the same vector, let us say x[1], x[100]. I want to do this using petsc4py.

  In this case you can just make a scatter that comes from the vector and goes to the same vector. So create an IS with the entry 0 in it twice and another with the entry 1 and 100 now create the VecScatter with those two index sets and do the scatter. The resulting code will work on any number of processes

   Barry

> 
> Actually I have tried the method getValue in Vec class. But I received an error telling me that this method "Can only get local values", so I tried to follow example 10 in the manual, which is posted below, but when trying to create sequential vector I get the error
> "
> [1] Invalid argument
> [1] Cannot create VECSEQ on more than one process
> "
> 
> 
> Vec p, x; /* initial vector, destination vector */
> VecScatter scatter; /* scatter context */
> IS from, to; /* index sets that define the scatter */
> PetscScalar *values;
> int idx_from[] = {100,200}, idx_to[] = {0,1};
> VecCreateSeq(PETSC COMM SELF,2,&x);
> ISCreateGeneral(PETSC COMM SELF,2,idx from,&from);
> ISCreateGeneral(PETSC COMM SELF,2,idx to,&to);
> VecScatterCreate(p,from,x,to,&scatter);
> VecScatterBegin(scatter,p,x,INSERT VALUES,SCATTER FORWARD);
> VecScatterEnd(scatter,p,x,INSERT VALUES,SCATTER FORWARD);
> VecGetArray(x,&values);
> ISDestroy(from);
> ISDestroy(to);
> VecScatterDestroy(scatter);
> 
> 
> Thank you very much
> Amal