Re: [Boost-Users] Large graph: how to use internal property maps in connected_components?

From: Douglas Paul Gregor (gregod_at_[hidden])
Date: 2003-03-24 09:09:47

On Mon, 24 Mar 2003, Martin Okrslar wrote:

> Thanks a lot for your help!
>
> But (just in order to learn more about my beloved BGL):
>
> >>The connected components algorithm uses a color map for internal storage. If
> >>you don't supply a color map, it tries to build a default color map using the
> >>vertex index property. That's where this is failing: when the vertex list
> >>type is listS, there is no default vertex_index property, so the code fails.
> >>
> Is there any way I could have known that from the documentation? I mean,
> this is by no means an accusation, but I'd just like to be able to learn
> to work through the BGL without having to anoy the list with lots of
> question. I still think that I missed something.

It's there, but it's not obvious. Under the "named parameters" section
there is a UTIL parameter called color map. UTIL parameters are neither
input nor output, but are needed for the algorithm to execute. The default
for color_map is an iterator_property_map that uses the IN
parameter vertex_index_map. The default for vertex_index_map is
get(vertex_index, g), which returns an invalid property map for your
graph. Not obvious at all :)

        Doug