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

  4:  #include <petscmat.h>

  6: int main(int argc,char **args)
  7: {
  8:   Mat            C,A;
  9:   PetscInt       i,j,m = 3,n = 2,rstart,rend;
 10:   PetscMPIInt    size,rank;
 12:   PetscScalar    v;
 13:   IS             isrow,iscol;

 15:   PetscInitialize(&argc,&args,(char*)0,help);if (ierr) return ierr;
 16:   MPI_Comm_rank(PETSC_COMM_WORLD,&rank);
 17:   MPI_Comm_size(PETSC_COMM_WORLD,&size);
 18:   n    = 2*size;

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

 25:   /*
 26:         This is JUST to generate a nice test matrix, all processors fill up
 27:     the entire matrix. This is not something one would ever do in practice.
 28:   */
 29:   MatGetOwnershipRange(C,&rstart,&rend);
 30:   for (i=rstart; i<rend; i++) {
 31:     for (j=0; j<m*n; j++) {
 32:       v    = i + j + 1;
 33:       MatSetValues(C,1,&i,1,&j,&v,INSERT_VALUES);
 34:     }
 35:   }

 37:   MatAssemblyBegin(C,MAT_FINAL_ASSEMBLY);
 38:   MatAssemblyEnd(C,MAT_FINAL_ASSEMBLY);
 39:   PetscViewerPushFormat(PETSC_VIEWER_STDOUT_WORLD,PETSC_VIEWER_ASCII_COMMON);
 40:   MatView(C,PETSC_VIEWER_STDOUT_WORLD);

 42:   /*
 43:      Generate a new matrix consisting of every second row and column of
 44:    the original matrix
 45:   */
 46:   MatGetOwnershipRange(C,&rstart,&rend);
 47:   /* Create parallel IS with the rows we want on THIS processor */
 48:   ISCreateStride(PETSC_COMM_WORLD,(rend-rstart)/2,rstart,2,&isrow);
 49:   /* Create parallel IS with the rows we want on THIS processor (same as rows for now) */
 50:   ISCreateStride(PETSC_COMM_WORLD,(rend-rstart)/2,rstart,2,&iscol);

 52:   MatCreateSubMatrix(C,isrow,iscol,MAT_INITIAL_MATRIX,&A);
 53:   MatCreateSubMatrix(C,isrow,iscol,MAT_REUSE_MATRIX,&A);
 54:   MatView(A,PETSC_VIEWER_STDOUT_WORLD);

 56:   ISDestroy(&isrow);
 57:   ISDestroy(&iscol);
 58:   MatDestroy(&A);
 59:   MatDestroy(&C);
 60:   PetscFinalize();
 61:   return ierr;
 62: }