CGAL users discussion list

[Efi Fogel <>]

You can be sure.

Lev Borovoi wrote:
> 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.
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