Actual source code: shellcnv.c
petsc-3.10.3 2018-12-18
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      2:  #include <petsc/private/matimpl.h>

      4: PETSC_INTERN PetscErrorCode MatConvert_Shell(Mat oldmat, MatType newtype,MatReuse reuse,Mat *newmat)
      5: {
      6:   Mat            mat;
      7:   Vec            in,out;
      9:   PetscInt       i,M,m,*rows,start,end;
     10:   MPI_Comm       comm;
     11:   PetscScalar    *array,zero = 0.0,one = 1.0;

     14:   PetscObjectGetComm((PetscObject)oldmat,&comm);

     16:   MatGetOwnershipRange(oldmat,&start,&end);
     17:   VecCreateMPI(comm,end-start,PETSC_DECIDE,&in);
     18:   VecDuplicate(in,&out);
     19:   VecGetSize(in,&M);
     20:   VecGetLocalSize(in,&m);
     21:   PetscMalloc1(m+1,&rows);
     22:   for (i=0; i<m; i++) rows[i] = start + i;

     24:   MatCreate(comm,&mat);
     25:   MatSetSizes(mat,m,M,M,M);
     26:   MatSetType(mat,newtype);
     27:   MatSetBlockSizesFromMats(mat,oldmat,oldmat);
     28:   MatSetUp(mat);

     30:   for (i=0; i<M; i++) {
     31:     VecSet(in,zero);
     32:     VecSetValues(in,1,&i,&one,INSERT_VALUES);
     33:     VecAssemblyBegin(in);
     34:     VecAssemblyEnd(in);

     36:     MatMult(oldmat,in,out);

     38:     VecGetArray(out,&array);
     39:     MatSetValues(mat,m,rows,1,&i,array,INSERT_VALUES);
     40:     VecRestoreArray(out,&array);

     42:   }
     43:   PetscFree(rows);
     44:   VecDestroy(&in);
     45:   VecDestroy(&out);
     46:   MatAssemblyBegin(mat,MAT_FINAL_ASSEMBLY);
     47:   MatAssemblyEnd(mat,MAT_FINAL_ASSEMBLY);

     49:   if (reuse == MAT_INPLACE_MATRIX) {
     50:     MatHeaderReplace(oldmat,&mat);
     51:   } else {
     52:     *newmat = mat;
     53:   }
     54:   return(0);
     55: }