This is expected. For dense matrices, MatDenseGetSubMatrix() does not duplicate
the memory. You should interpret the array as a two-dimensional column-major
array: buffer[i+j*lda] where i,j are row and column indices, and lda can be
obtained with MatDenseGetLDA().
Jose
> El 7 feb 2025, a las 11:49, medane.tchako...@univ-fcomte.fr escribió:
>
> Re:
> Please find below the output from the previous code, running on only one
> processor.
>
> Mat Object: 1 MPI process
> type: seqdense
> 7.2003197397953400e-01 3.9777780919128602e-01 9.8405227390177075e-01
> 1.4405427480322786e-01
> 6.1793966542126100e-02 7.3036588248200474e-02 7.3851607000756303e-01
> 9.9650445216117589e-01
> 1.0022337819588500e-02 1.0386628927366459e-01 4.0114727059134836e-01
> 1.0677308875937896e-01
> 1.4463931936456476e-01 2.5078039364333193e-01 5.2764865382548720e-01
> 9.8905332488367748e-01
>
> buffer[0] = 7.200320e-01
> buffer[1] = 6.179397e-02
> buffer[2] = 1.002234e-02
> buffer[3] = 1.446393e-01
> buffer[4] = 0.000000e+00
> buffer[5] = 0.000000e+00
> buffer[6] = 0.000000e+00
> buffer[7] = 0.000000e+00
> buffer[8] = 3.977778e-01
> buffer[9] = 7.303659e-02
> buffer[10] = 1.038663e-01
> buffer[11] = 2.507804e-01
> buffer[12] = 0.000000e+00
> buffer[13] = 0.000000e+00
> buffer[14] = 0.000000e+00
> buffer[15] = 0.000000e+00
>
> Mat Object: 1 MPI process
> type: seqdense
> 7.2003197397953400e-01 3.9777780919128602e-01 9.8405227390177075e-01
> 1.4405427480322786e-01
> 6.1793966542126100e-02 7.3036588248200474e-02 7.3851607000756303e-01
> 9.9650445216117589e-01
> 1.0022337819588500e-02 1.0386628927366459e-01 4.0114727059134836e-01
> 1.0677308875937896e-01
> 1.4463931936456476e-01 2.5078039364333193e-01 5.2764865382548720e-01
> 9.8905332488367748e-01
> 0.0000000000000000e+00 0.0000000000000000e+00 0.0000000000000000e+00
> 0.0000000000000000e+00
> 0.0000000000000000e+00 0.0000000000000000e+00 0.0000000000000000e+00
> 0.0000000000000000e+00
> 0.0000000000000000e+00 0.0000000000000000e+00 0.0000000000000000e+00
> 0.0000000000000000e+00
> 0.0000000000000000e+00 0.0000000000000000e+00 0.0000000000000000e+00
> 0.0000000000000000e+00
>
>
> I was expecting to get in “buffer”, only the data entries from R_part.
> Please, let me know if this is the excepted behavior and I’am missing
> something.
>
> Thanks,
> Medane
>
>
>
>> On 7 Feb 2025, at 11:34, Pierre Jolivet <pie...@joliv.et> wrote:
>>
>>
>>
>>> On 7 Feb 2025, at 11:05 AM, medane.tchako...@univ-fcomte.fr wrote:
>>>
>>>
>>> Dear all,
>>>
>>> I have been experiencing incoherent data entries from this code below, when
>>> printing the array. Maybe I’am doing something wrong.
>>
>> What is incoherent?
>> Everything looks OK to me.
>>
>> Thanks,
>> Pierre
>>
>>> ----------------
>>>
>>> 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
>>
>>
>
