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

[Matthew Knepley]

On Fri, Feb 7, 2025 at 8:20 AM medane.tchakorom at univ-fcomte.fr <
medane.tchakorom at univ-fcomte.fr> wrote:

> 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.
>

As Jose already pointed out, SubMatrix() does not copy. It gives you a Mat
front end to the same data, but with changed sizes. In this case, the LDA
is 4, not 2, so when you iterate over the values, you skip over the ones
you don't want.

  Thanks,

    Matt


> 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
> >
> >
>
>

-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener

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