[petsc-users] How to copy vectors to/from petsc?

[Satish Balay]

On Thu, 28 Sep 2017, Karl Rupp wrote:

> Hi Klaus,
> 
> there are several options, e.g.:
>  *
> http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Vec/VecCreateSeqWithArray.html
> (sequential)

For parallel http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Vec/VecCreateMPIWithArray.html

>  *
> http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Vec/VecSetValues.html
> (use with multiple MPI ranks if you cannot easily determine the local parts)
>  *
> http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Vec/VecGetArray.html
> (get a pointer to the data and assign values yourself)
> 
> Best regards,
> Karli
> 
> 
> On 09/28/2017 02:43 PM, Klaus Burkart wrote:
> > Hello,
> > 
> > I want to solve a physics problem with support of petsc i.e. solve the
> > linear system using petsc. So far I have been using another linear algebra
> > library. I am using c++. Now I am struggling to load the rhs vector/array
> > from the physics application to petsc and copying the result x back to the
> > physics application. I found information about how to load data from
> > binaries or from files but both is not the case here. The computation will
> > take place on a multi core CPU using MPI.
> > 
> > This is how far I am and includes my questions:
> > 
> > ...
> > #include <petscvec.h>
> > 
> > 
> > // vector x, rhs size n; "matrix" is the matrix from the physics application
> > PetscMPIInt n = matrix.diag().size();
> > 
> > Vec            rhs; // Question 1: How to set the value type to double?

PETSc provides configure options --with-precision=single,double,.. --with-scalar-type=real,complex

We default to double precision real values. The corresponding datatype is PetscScalar


> > VecSetSizes(rhs, n, PETSC_DECIDE); // Question 2: What is "global size"?

Sum of local sizes - when running parallely.

Satish

> > 
> > Vec            x;
> > VecSetSizes(x, n, PETSC_DECIDE);
> > 
> > 
> > 
> > 
> > // Question 3:
> > // How to load petsc rhs vector with data from vector (= pointer array)
> > named rhs_source?
> > // Current load/copy syntax is:    fast_copy(rhs_source.begin(),
> > rhs_source.end(), rhs.begin());
> > 
> > 
> > 
> > // Question 4:
> > // How to export petsc result vector x i.e. load the result x into array
> > named x_result?
> > // Current load/copy syntax is:    fast_copy(x.begin(), x.end(),
> > x_result.begin());
> > 
> > 
> > 
> > // destroy petsc vectors
> > VecDestroy(&rhs);
> > VecDestroy(&x);
> > PetscFinalize();
> > 
> > // What's the best way to copy rhs to petsc and to copy the result x back to
> > the physics application?
> > 
>