CGAL users discussion list

[Josip Dzolonga <>]

Hello everyone again :),

I have started constructing a minimal counterexample and noticed that the problem arises when I try to "dereference" a Facet_circulator. Namely this piece of code

// current - edge, best - facet
Facet_circulator c = T.incident_facets(current, best);
Facet_circulator d(c);
std::list<Facet> candidates;

do {
                std::cout << "1\n";
                *c;
                std::cout << "2\n";
} while(++c != d);

The 1 prints and then I get the assertion error described in the previous e-mail. I couldn't find any preconditions that a Facet_circulator has to satisfy before being dereferenced. Any ideas? I am aware that this code piece does not provide enough information for the problem, however I was just curious if this is a common problem;


Best,
Josip

On Sun, Nov 15, 2009 at 10:30 PM, Josip Dzolonga <> wrote:

is_valid() succeeds both before and after the loop. The kernel is exactly that one, Exact_predicates_inexact_constructions_kernel. I will try to come up with a minimal (as minimal as possible) example that crashes and post it.

Thank you all for the help,
Josip

On Sun, Nov 15, 2009 at 10:23 PM, Andreas Fabri <> wrote:

ok, I missed the 'break'.

The code sniplet you show should be ok then, so my guess
is that you there is some bug earlier.

Just the usual question: Do uou use an Exact_predicates_inexact_constructions_kernel
or something comparable ?

Add assert(T.is_valid()); right before the for loop.

andreas

Josip Dzolonga wrote:

I was referring to this

while(!seen.empty()) {
         for(Finite_vertices_iterator vit = T.finite_vertices_begin();
               vit != T.finite_vertices_end(); ++vit) {
               if(seen.find(vit->point()) != seen.end()) {
                   seen.erase(vit->point());
                   T.remove(vit);
                   break;
               }
         }
}

I break the loop after a point is removed. I also tried the code that you provided, but that unfortunately did not solve the problem.

Best,
Josip

On Sun, Nov 15, 2009 at 10:08 PM, Andreas Fabri < <mailto:>> wrote:

   Hi Josip,

   I am not sure you understood my explanantion, but it can also be
   that I didn't understand yours.

   It is a mistake to do the ++vit, AFTER having called T.remove(vit),
   and that is what your for loop does.

   My version makes a copy of thr iterator, advances the iterator
   and only then removes the vertex.

   andreas


   Josip Dzolonga wrote:

       Hi Andreas,

       I corrected myself in my latest reply, but that did not solve
       the problem.

       Josip

       On Sun, Nov 15, 2009 at 9:07 PM, Andreas Fabri
       <
       <mailto:>
       <mailto:
       <mailto:>>> wrote:

          Hi Josip,

          You shouldn't expect that ++vit works correctly after that
       you called
          T.remove(vit).

          You better write this


          for(Finite_vertices_iterator vit = T.finite_vertices_begin();
                          vit != T.finite_vertices_end();) {

                          if(seen.find(vit->point()) != seen.end()) {
                              seen.erase(vit->point());
                              Vertex_handle vh = vit;
                              ++vit;
                              T.remove(vh);
                              break;
                          }
                    }


          Note that the same would be true for a std::list.

          andreas

          Josip Dzolonga wrote:

              Hi Nico,

              I thought of that, but at no point I am reusing old cell
              handles/vertex handles. I thought that while removing them, I
              might be causing the problem - but even if I change to
       code to
              the following, the problems remains

              while(!seen.empty()) {
                       for(Finite_vertices_iterator vit =
              T.finite_vertices_begin();
                             vit != T.finite_vertices_end(); ++vit) {
                             if(seen.find(vit->point()) != seen.end()) {
                                 seen.erase(vit->point());
                                 T.remove(vit);
                                 break;
                             }
                       }
              }

              That is, I am iterating again over all vertices.

              Thanks,
              Josip

              On Sun, Nov 15, 2009 at 8:42 PM, Nico Kruithof
              < <mailto:>
       <mailto: <mailto:>>
              <mailto:
       <mailto:> <mailto:
       <mailto:>>>>

              wrote:

                 Hi Josip,

                 The assertion says that the vertex is not contained in
       the Cell.
                 When you remove vertices, cells get removed and added when
                 re-triangulating. Internally, CGAL will reuse the
       memory of
              deleted
                 cells for the new cells. This might be the reason you
       are getting
                 the assertion.
                 -- Nico


                 On Sun, Nov 15, 2009 at 6:26 PM, Josip Dzolonga
              < <mailto:>
       <mailto: <mailto:>>
                 <mailto:
       <mailto:> <mailto:
       <mailto:>>>> wrote:

                     Hello everyone,

                     I am trying to remove some certain points from the
       Delaunay
                     triangulation. I had some problems even before,
       and got some
                     advice from here, but the problem still pertains. The
                     problematic code follows

                     for(Finite_vertices_iterator vit =
       T.finite_vertices_begin();
                                 vit != T.finite_vertices_end(); ++vit) {
                                 if(seen.find(vit->point()) !=
       seen.end()) {
                                     seen.erase(vit->point());
                                     T.remove(vit);
                                }
                     }

                     Where seen is of type std::set<Point> and has been
       already
                     filled. Later on when I try to use the
       triangulation, I
              get the
                     following error

                     CGAL error: assertion violation!
                     Expr: v == V[3]
                     File: /usr/include/CGAL/Triangulation_ds_cell_base_3.h
                     Line: 98
                     terminate called after throwing an instance of
              'std::logic_error'
                       what():  basic_string::_S_construct NULL not valid
                     Aborted

                     Which comes from the function  int index(const
       Vertex_handle&
                     v). I can not remove the same vertex twice, since I am
              iterating
                     over the finite vertices and this is the only
       place where
              vertex
                     removal is taking place. Any ideas would be highly
              appreciated!

                     Best,
                     Josip





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