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

[medane.tchakorom at univ-fcomte.fr]

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 <pierre at joliv.et> wrote:
> 
> 
> 
>> On 7 Feb 2025, at 11:05 AM, medane.tchakorom at 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
> 
>