2:  #include <petsc/private/matimpl.h>

  4: PetscErrorCode MatConvert_Shell(Mat oldmat, MatType newtype,MatReuse reuse,Mat *newmat)
  5: {
  6:   Mat            mat;
  7:   Vec            in,out;
  9:   PetscInt       i,m,n,M,N,*rows,start;
 10:   MPI_Comm       comm;
 11:   PetscScalar    *array;

 14:   PetscObjectGetComm((PetscObject)oldmat,&comm);

 16:   MatGetOwnershipRange(oldmat,&start,NULL);
 17:   MatCreateVecs(oldmat,&in,&out);
 18:   MatGetLocalSize(oldmat,&m,&n);
 19:   MatGetSize(oldmat,&M,&N);
 20:   PetscMalloc1(m,&rows);
 21:   for (i=0; i<m; i++) rows[i] = start + i;

 23:   MatCreate(comm,&mat);
 24:   MatSetSizes(mat,m,n,M,N);
 25:   MatSetType(mat,newtype);
 26:   MatSetBlockSizesFromMats(mat,oldmat,oldmat);
 27:   MatSetUp(mat);
 28:   VecSetOption(in,VEC_IGNORE_OFF_PROC_ENTRIES,PETSC_TRUE);

 30:   for (i=0; i<N; i++) {
 31:     VecZeroEntries(in);
 32:     VecSetValue(in,i,1.,INSERT_VALUES);
 33:     VecAssemblyBegin(in);
 34:     VecAssemblyEnd(in);

 36:     MatMult(oldmat,in,out);

 38:     VecGetArray(out,&array);
 39:     MatSetValues(mat,m,rows,1,&i,array,INSERT_VALUES);
 40:     VecRestoreArray(out,&array);

 42:   }
 43:   PetscFree(rows);
 44:   VecDestroy(&in);
 45:   VecDestroy(&out);
 46:   MatAssemblyBegin(mat,MAT_FINAL_ASSEMBLY);
 47:   MatAssemblyEnd(mat,MAT_FINAL_ASSEMBLY);

 49:   if (reuse == MAT_INPLACE_MATRIX) {
 50:     MatHeaderReplace(oldmat,&mat);
 51:   } else {
 52:     *newmat = mat;
 53:   }
 54:   return(0);
 55: }