Re: [Boost-users] [BGL] Problem with color maps for undirected_dfs_visit

From: Tarjei Knapstad (tarjeik_at_[hidden])
Date: 2003-11-17 09:19:35

On Fri, 2003-11-14 at 17:02, Jeremy Siek wrote:
> Hi Tarjei,
> 
> On Friday, November 14, 2003, at 05:33 AM, Tarjei Knapstad wrote:
> > On Thu, 2003-11-13 at 17:09, Jeremy Siek wrote:
> >> Hi Tarjei,
> >>
> >> You need to initialize the edge index property map (only the vertex
> >> index
> >> map is automatically initialized, and then only for VertexList=vecS).
> >>
> >
> > Hmm, ok, but I still don't quite understand. I thought that
> > initializing
> > the std::vector holding the colors was sufficient...? To my
> > understanding an iterator_property_map is just a property map which
> > iterates through some other container (that can provide a
> > RandomAccessIterator) actually holding the properties?
> 
> Yes, your edge color map is initialized just fine, but you also need to
> initialize the edge index map.
> 
> Here's the reason why: your edge color map uses the edge index map
> to go from an edge descriptor to an integer offset, which is then used
> to
> access into the color vector.
> 

> Currently, your edge index map is initialized to random garbage, so you
> are going out-of-bounds when accessing the edge color vector.
> 
> See the example file exterior_properties.cpp,
> 
> BTW, you don't need to declare an edge_color property for the
> adjacency_list because you are using an external edge color map.
> 

Ahh, I see. (*feeling slightly stupid*)

> >
> > Obviously there's some connection I don't quite get here... I also
> > don't
> > quite understand the EColorValue uv_color = get(edge_color, *ei); call
> > in undir_dfv_impl. It asks for the edge_color at the edge_descriptor
> > *ei, but from where?
> >
> > Anyway, I don't really even need an edge_index property here (or do
> > I?),
> > it was more or less to try to get this working.
> 
> Yes, you need it for the reason given above.
> 
> BTW, if you switch to using an internal edge color property, then you
> won't
> need the edge index property, or the edge color vector.
> 

Thanks Jeremy, it's all a lot clearer now!

Cheers,
--
Tarjei