2: static char help[] = "Tests MatGetSubmatrix() in parallel.";

  4: #include <petscmat.h>

  8: int main(int argc,char **args)
  9: {
 10:   Mat            C,A;
 11:   PetscInt       i,j,m = 3,n = 2,rstart,rend;
 12:   PetscMPIInt    size,rank;
 14:   PetscScalar    v;
 15:   IS             isrow,iscol;

 17:   PetscInitialize(&argc,&args,(char*)0,help);
 18:   MPI_Comm_rank(PETSC_COMM_WORLD,&rank);
 19:   MPI_Comm_size(PETSC_COMM_WORLD,&size);
 20:   n    = 2*size;

 22:   MatCreate(PETSC_COMM_WORLD,&C);
 23:   MatSetSizes(C,PETSC_DECIDE,PETSC_DECIDE,m*n,m*n);
 24:   MatSetFromOptions(C);
 25:   MatSetUp(C);

 27:   /*
 28:         This is JUST to generate a nice test matrix, all processors fill up
 29:     the entire matrix. This is not something one would ever do in practice.
 30:   */
 31:   MatGetOwnershipRange(C,&rstart,&rend);
 32:   for (i=rstart; i<rend; i++) {
 33:     for (j=0; j<m*n; j++) {
 34:       v    = i + j + 1;
 35:       MatSetValues(C,1,&i,1,&j,&v,INSERT_VALUES);
 36:     }
 37:   }

 39:   MatAssemblyBegin(C,MAT_FINAL_ASSEMBLY);
 40:   MatAssemblyEnd(C,MAT_FINAL_ASSEMBLY);
 41:   PetscViewerSetFormat(PETSC_VIEWER_STDOUT_WORLD,PETSC_VIEWER_ASCII_COMMON);
 42:   MatView(C,PETSC_VIEWER_STDOUT_WORLD);

 44:   /*
 45:      Generate a new matrix consisting of every second row and column of
 46:    the original matrix
 47:   */
 48:   MatGetOwnershipRange(C,&rstart,&rend);
 49:   /* Create parallel IS with the rows we want on THIS processor */
 50:   ISCreateStride(PETSC_COMM_WORLD,(rend-rstart)/2,rstart,2,&isrow);
 51:   /* Create parallel IS with the rows we want on THIS processor (same as rows for now) */
 52:   ISCreateStride(PETSC_COMM_WORLD,(rend-rstart)/2,rstart,2,&iscol);

 54:   MatGetSubMatrix(C,isrow,iscol,MAT_INITIAL_MATRIX,&A);
 55:   MatGetSubMatrix(C,isrow,iscol,MAT_REUSE_MATRIX,&A);
 56:   MatView(A,PETSC_VIEWER_STDOUT_WORLD);

 58:   ISDestroy(&isrow);
 59:   ISDestroy(&iscol);
 60:   MatDestroy(&A);
 61:   MatDestroy(&C);
 62:   PetscFinalize();
 63:   return 0;
 64: }