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Subject: Re: [Boost-users] Confused with Depth-First-Search method, forwardor cross edge found on a undirected graph ?
From: lizy10b (lizy10b_at_[hidden])
Date: 2013-06-24 22:54:39


Hi Jeremiah Willcock,
Thanks for your reply.
Another question is the vertex color property also required in undirected_dfs()? or only edge color property is enough?
Thanks.

Zhiyu Li


2013-06-25



lizy10b



·¢¼þÈË£º Jeremiah Willcock
·¢ËÍʱ¼ä£º 2013-06-25 00:44:05
ÊÕ¼þÈË£º lizy10b
³­ËÍ£º boost-users
Ö÷Ì⣺ Re: Re: [Boost-users] Confused with Depth-First-Search method,forwardor cross edge found on a undirected graph ?
 
On Tue, 25 Jun 2013, lizy10b wrote:
> Hi Jeremiah Willcock ,
> ?
> It seems undirected_dfs() works. I have not check the result according to the graph, but at least there is no
> forward edge or cross edge output.
> Attached is the revised cpp file using undirected_dfs() .
> My question is why edge_color_map(get(edge_color, g))?is required when calling the undirected_dfs() .
> Thanks
It looks like what is going wrong with using directed DFS is that edges
are examined twice (once in each direction), and so you can have forward
edges even in a symmetric graph (which is how undirected graphs are
treated in directed-graph algorithms in BGL). An example is the triangle
0 -- 1 -- 2 -- 0: starting at 0 and processing the edges out of a vertex
in numerical order, 0 -> 1 is a tree edge, 1 -> 0 is a back edge, 1 -> 2
is a tree edge, 2 -> 0 is a back edge, 2 -> 1 is a back edge, then 0 -> 2
is a forward edge. An undirected DFS would mark 2 -> 0 as a back edge
then ignore 0 -> 2 since it is the same as 2 -> 0. The edge colors in
undirected_dfs are used to do this filtering-out of multiple directions
for the same edge.
-- Jeremiah Willcock



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