[petsc-users] Incoherent data entries in array from a dense sub matrix

[medane.tchako...@univ-fcomte.fr]

Dear all,

I have been experiencing incoherent data entries from this code below, when 
printing the array. Maybe I’am doing something wrong. 

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     PetscInt nlines = 8; // lines
    PetscInt ncols = 4;  // columns
    PetscMPIInt rank;
    PetscMPIInt size;

    // Initialize PETSc
    PetscCall(PetscInitialize(&argc, &args, NULL, NULL));
    PetscCallMPI(MPI_Comm_rank(MPI_COMM_WORLD, &rank));
    PetscCallMPI(MPI_Comm_size(MPI_COMM_WORLD, &size));

    Mat R_full;
    Mat R_part;
    PetscInt idx_first_row = 0;
    PetscInt idx_one_plus_last_row = nlines / 2;
    PetscCall(MatCreateDense(PETSC_COMM_WORLD, PETSC_DECIDE, PETSC_DECIDE, 
nlines, ncols, NULL, &R_full));

    // Get sub matrix
    PetscCall(MatDenseGetSubMatrix(R_full, idx_first_row, 
idx_one_plus_last_row, PETSC_DECIDE, PETSC_DECIDE, &R_part));
    // Add entries to sub matrix
    MatSetRandom(R_part, NULL);
    //View sub matrix
    PetscCall(MatView(R_part, PETSC_VIEWER_STDOUT_WORLD));

    // Get array from sub matrix and print entries
    PetscScalar *buffer;
    PetscCall(MatDenseGetArray(R_part, &buffer));
    PetscInt idx_end = (nlines/2) * ncols;

    for (int i = 0; i < idx_end; i++)
    {
        PetscPrintf(PETSC_COMM_SELF, "buffer[%d] = %e \n", i, buffer[i]);
    }

    //Restore array to sub matrix
    PetscCall(MatDenseRestoreArray(R_part, &buffer));
    // Restore sub matrix
    PetscCall(MatDenseRestoreSubMatrix(R_full, &R_part));
    // View the initial matrix
    PetscCall(MatView(R_full, PETSC_VIEWER_STDOUT_WORLD));

    PetscCall(MatDestroy(&R_full));

    PetscCall(PetscFinalize());
    return 0;

----------------


Thanks
Medane