CGAL users discussion list

["Lev Borovoi" <>]

There's one more thing I'm afraid of. If the arrangement changes, can I
be sure that I won't iterate over the same edge twice? Or, conversely,
miss one of the edges even though it's not deleted?

-----Original Message-----
From: Efraim Fogel 
[mailto:]
 
Sent: Thursday, August 23, 2007 10:43 AM
To: 

Cc: Ron Wein
Subject: Re: [cgal-discuss] 2D Arrangements

Here are the answers to your questions assembled by Ron:

In general, as long as you do not remove things, your iterators stay 
valid. Removing an edge will of course invalidate the iterators/handles 
to the two deleted halfedges, but it can also
happen that this causes two faces to merge, and in such a case also some

face handle is invalidated.

Regarding the specific program, there is a problem with it, as the loop 
goes over the halfedges. When you delete a halfedge, you also deleted 
its twin, which is in general the next halfedge in the container, so you

get an invalid iterator. The solution is easy: Use an edge iterator
instead.

Arrangement arr;
Arrangement::Edge_iterator iter = arr.edges_begin();

while (iter != arr.edges_end())
{
  if (SomeCondition(*iter))
  {
    Arrangement::Edge_iterator t = iter;
    iter++;
    arr.remove_edge(t);
  }
  else
    iter++;
}

One face is deleted in case of a merge. If the arrangement faces are 
extended with external data, you can use an observer to keep track of 
the changes the arrangement undergoes and maintain the external data 
accordingly (implement before/after_merge_face() in this case). There is

an example in the manual showing how to use an observer for this
purpose.