Remove usage of VecGetValues

It fails with the newest versions of PETSc if the vector is not assembled

diff --git a/source/lac/petsc_vector_base.cc b/source/lac/petsc_vector_base.cc
index 17c50e09b688055c11d67cf19920922405af5841..309fb0e3613e0bc50848f20ae3848a7cb1112f69 100644 (file)
--- a/source/lac/petsc_vector_base.cc
+++ b/source/lac/petsc_vector_base.cc
@@ -108,9 +108,12 @@ namespace PETScWrappers
                     (index < static_cast<size_type>(end)),
                   ExcAccessToNonlocalElement(index, begin, end - 1));
 
-      PetscInt    idx = index;
-      PetscScalar value;
-      ierr = VecGetValues(vector.vector, 1, &idx, &value);
+      const PetscScalar *ptr;
+      PetscScalar        value;
+      ierr = VecGetArrayRead(vector.vector, &ptr);
+      AssertThrow(ierr == 0, ExcPETScError(ierr));
+      value = *(ptr + index - begin);
+      ierr  = VecRestoreArrayRead(vector.vector, &ptr);
       AssertThrow(ierr == 0, ExcPETScError(ierr));
 
       return value;