[petsc-users] question on VecPointwiseMax

[Xiangdong]

Hello everyone,

When I call VecPointwiseMax(x,x,y) , I got an error message "object is in
wrong state. Vec is locked read only, argument #1."

However, In the online manual of output parameters of VecPointwiseMax, it
says:

http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Vec/VecPointwiseMax.html
Output Parameter*w *-the result

Notes: any subset of the x, y, and w may be the same vector. For complex
numbers compares only the real part
However, in the implementation of VecPointwiseMax_Seq,
http://www.mcs.anl.gov/petsc/petsc-current/src/vec/vec/impls/seq/bvec2.c.html#VecPointwiseMax_Seq
,

it seems that xin is read locked, and win and xin cannot be the same
vectors.

 *PetscErrorCode
<http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Sys/PetscErrorCode.html#PetscErrorCode>
VecPointwiseMax_Seq(Vec
<http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Vec/Vec.html#Vec>
win,Vec <http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Vec/Vec.html#Vec>
xin,Vec <http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Vec/Vec.html#Vec>
yin)* 17: { 19:   PetscInt
<http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Sys/PetscInt.html#PetscInt>
      n = win->map->n,i; 20:   PetscScalar
<http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Sys/PetscScalar.html#PetscScalar>
   *ww,*xx,*yy; /* cannot make xx or yy const since might be ww */
 23:   VecGetArrayRead
<http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Vec/VecGetArrayRead.html#VecGetArrayRead>(xin,(const
PetscScalar <http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Sys/PetscScalar.html#PetscScalar>**)&xx);
24:   VecGetArrayRead
<http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Vec/VecGetArrayRead.html#VecGetArrayRead>(yin,(const
PetscScalar <http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Sys/PetscScalar.html#PetscScalar>**)&yy);
25:   VecGetArray
<http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Vec/VecGetArray.html#VecGetArray>(win,&ww);
 27:   for (i=0; i<n; i++) ww[i] = PetscMax
<http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Sys/PetscMax.html#PetscMax>(PetscRealPart(xx[i]),PetscRealPart(yy[i]));
 29:   VecRestoreArrayRead
<http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Vec/VecRestoreArrayRead.html#VecRestoreArrayRead>(xin,(const
PetscScalar <http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Sys/PetscScalar.html#PetscScalar>**)&xx);
30:   VecRestoreArrayRead
<http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Vec/VecRestoreArrayRead.html#VecRestoreArrayRead>(yin,(const
PetscScalar <http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Sys/PetscScalar.html#PetscScalar>**)&yy);
31:   VecRestoreArray
<http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Vec/VecRestoreArray.html#VecRestoreArray>(win,&ww);
32:   PetscLogFlops
<http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Profiling/PetscLogFlops.html#PetscLogFlops>(n);
33:   return(0); 34: }

Can w and x really be same vector in the VecPointwiseMax? Thanks.

Best,
Xiangdong
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