Re: [Boost-users] BGL: Problem with dijkstra_shortest_paths

30 Jun 2011

      On 6/30/2011 8:05 AM, David Doria wrote:
...
On Wed, Jun 29, 2011 at 6:07 PM, Stephen Woodbridge
  wrote:
...
Hi all,
I am C programmer floundering in a C++ world!
I have the function that I call from C that builds a graph, runs
dijkstra_shortest_paths(), then extracts the results and passes them back to
a C array of structs. Code is in pastebin:
http://pastebin.com/xi7vLpTx
This seems to work and everything looks ok, EXCEPT the edge ids are not
correct. I suspect this is because I'm not doing something correctly and I
have been messing with this for a couple weeks without success, so I think
its time to ask for help.
So in the function:
graph_add_edge(G&graph, int id, int source, int target, float8 cost)
id is the edge id
source is the node id at the start of the edge
target is the node_id at the end of the edge
cost is the cost to traverse the edge from source to target
I would really appreciate it if someone could look at this and point out
what I might be doing wrong.
Thanks in advance,
  -Steve
There are a few Dijkstra examples here, do they help?
http://programmingexamples.net/index.php?title=Boost#Boost_Graph_Library_.28...
David,

Thanks, I've been through the examples and some other code. I need to be 
able to call this code from a C application and basically the code seems 
to be working. Where I'm having problems is that I need to be able to 
tag the edges with an ID when I build the graph and to be able to 
identify those edges by ID in the results. So how do I do this?

My current code builds the graph with this helper function:

struct Edge
{
   int id;
   float8 cost;
};

struct Vertex
{
   int id;
   int edge_id;
};

template 
static void
graph_add_edge(G &graph, int id, int source, int target, float8 cost)
{
   E e;
   bool inserted;

   if (cost < 0) // edges are not inserted in the graph if cost is negative
     return;

   tie(e, inserted) = add_edge(source, target, graph);

   graph[e].cost = cost;
   graph[e].id = id;

   typedef typename graph_traits<G>::vertex_descriptor Vertex;
   Vertex s = vertex(source, graph);
   Vertex t = vertex(target, graph);
   graph[s].id = source;
   graph[t].id = target;
   graph[s].edge_id = id;
   graph[t].edge_id = id;

}

So the edge id is set on the Vertex when it is added, but since the same 
vertex can be on multiple edges, I assume only the edge id of the last 
vertex is kept and that is what I am seeing in my results.

So the question is:

1. How to I define an edge id and set it when I build my graph?
2. Given a vertex id and a predecessor id, How do I get the edge id?

Thanks,
   -Steve