[petsc-users] How to get values from a matrix and set again?

[Barry Smith]

  Presumably you are using a MPIAIJ matrix. You can use

  Mat x,A,B;
  MatMPIAIJGetSeqAIJ(x,&A,&B,NULL);

  PetscScalar *a;
  MatSeqAIJGetArray(A,&a);
  MatInfo info;
  MatGetInfo(A,MAT_LOCAL,&info);
  for (i=0; i<info.nz_used; i++) {
     a[i] = yourfunction(a[i])
  }
  MatSeqAIJRestoreArray(A,&a);
  /* do same thing for B matrix */

> On Dec 7, 2015, at 12:11 AM, Bibrak Qamar <bibrakc at gmail.com> wrote:
> 
> Hi,
> 
> I am trying to apply a function on each element of a matrix. The function is the sigmoid function which transforms each element of the matrix.
> 
> I am trying to approach it like this but get error. What could be the problem? Or is there a better way to do this?
> 
> int sigmoid(Mat x){
> printf("in sigmoid\n");
> 
> PetscErrorCode ierr;
> PetscInt       Istart, Iend, Ii, Ji, rows, cols;
> 
> PetscScalar     v = 0.0;
> ierr = MatGetOwnershipRange(x,&Istart,&Iend);CHKERRQ(ierr);
> MatGetSize(x,&rows,&cols);
> 
> for (Ii=Istart; Ii<Iend; Ii++) {
>      for (Ji=0; Ji<cols; Ji++){
>     ierr = MatGetValues(x,1,&Ii,1,&Ji,&v);CHKERRQ(ierr);
>     v = mysigmoid(v);
>     ierr = MatSetValues(x,1,&Ii,1,&Ji,&v,INSERT_VALUES);CHKERRQ(ierr);
>     }
>   }
> 
> MatAssemblyBegin(x,MAT_FINAL_ASSEMBLY);
> MatAssemblyEnd(x,MAT_FINAL_ASSEMBLY);
> 
> 
> MatView(x,PETSC_VIEWER_STDOUT_WORLD);
> }
> 
> 
> error msg is:
> 
> [0]PETSC ERROR: --------------------- Error Message --------------------------------------------------------------
> [0]PETSC ERROR: Object is in wrong state
> [0]PETSC ERROR: Not for unassembled matrix
> [0]PETSC ERROR: See http://www.mcs.anl.gov/petsc/documentation/faq.html for trouble shooting.
> [0]PETSC ERROR: Petsc Release Version 3.6.3, Dec, 03, 2015 
> [
> [0]PETSC ERROR: #1 MatGetValues() line 1780 in /gpfs/home/petsc-3.6.3/src/mat/interface/matrix.c
> [0]PETSC ERROR: #2 sigmoid() line 21 in neural.c
> 
> 
> Thanks
> Bibrak Qamar
>