CGAL users discussion list

[Efraim Fogel <>]

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.


 wrote:
> Hi
>
> Can I modify a 2D arrangement while traversing it? 
>
> For example:
>
> Arrangement arr;
> Arrangement::Halfedge_iterator iter = arr.halfedges_begin();
>
> while (iter != arr.halfedges_end())
> {
>   if (SomeCondition(*iter))
>   {
>     Arrangement::Halfedge_iterator t = iter;
>     iter++;
>     arr.remove_edge(t);
>   }
>   else
>     iter++;
> }
>
> If I do this, can I be sure that I'll iterate over each edge exactly once 
> unless it's deleted?
> And btw, if two faces are merged, what happens to their data?
>
> Thanks
>   
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