Re: [Boost-users] BGL - changing internal values of a graph

From: Aaron Windsor (aaron.windsor_at_[hidden])
Date: 2008-01-31 07:46:06

On Jan 30, 2008 3:14 PM, Renato Golin <renato_at_[hidden]> wrote:
> Hi,
>
> I'm trying to write a simple graph:
>
> // Link and Edge types
> struct Link {
>         std::string name;
> };
>
> struct Edge {
>         long count;
> };
>
> // directed graph
> typedef boost::adjacency_list < boost::vecS, boost::vecS,
> boost::directedS, Link, Edge > graph_t;
> // Vertex and Edge descriptors
> typedef boost::graph_traits < graph_t >::vertex_descriptor link_d;
> typedef boost::graph_traits < graph_t >::edge_descriptor edge_d;
> // Map with all vertexes and links (deduplicate)
> typedef std::map < std::string, link_d > link_map_t;
>
> And my method to add a vertex (which is working well) is:
>
> link_d
> Chain::addVertex (string& name)
> {
>         link_map_t::iterator pos;
>         link_d descriptor;
>         Link link;
>         bool inserted;
>
>         // Insert into map
>     boost::tie(pos, inserted) = links.insert(make_pair(name, link_d()));
>
>         // Insert into graph (or get descriptor)
>         if (inserted) {
>                 link.name = name;
>                 descriptor = add_vertex(link, graph);
>                 pos->second = descriptor;
>                 cerr << "Added new vertex: " << graph[descriptor].name << endl;
>         } else {
>                 descriptor = pos->second;
>                 cerr << "Reused old vertex: " << graph[descriptor].name << endl;
>         }
>
>         return descriptor;
> }
>
>
> And my method to connect the edges (which is not working) is:
>
> void
> Chain::addConnection (string subject, string object)
> {
>     link_d sub, obj;
>     edge_d pred;
>         Edge e;
>         bool inserted, found;
>
>         // Add vertices
>         sub = addVertex(subject);
>         obj = addVertex(object);
>
>         // Connect through predicate and increment counter
>         boost::tie(pred, found) = edge (sub, obj, graph);
>         if (found) {
>                 e = graph[pred];
>                 cerr << "Old, pred: " << pred << " count: " << e.count << endl;
>                 e.count++;
>         } else {
>                 e.count = 1;
>         }
>         boost::tie(pred, inserted) = add_edge(sub, obj, e, graph);
>         e = graph[pred];
>
>         // Print to see if it was correct
>         cerr << "New, pred: " << pred << " count: " << e.count << "\n" << endl;
> }
>
> By reading a text line "ABABAB." and taking letter by letter and
> inserting into sub and obj (A,B), (B,A) ... I have the following output:
>
> line [ABABAB.]
> Added new vertex: first
> Added new vertex: A
> New, pred: (0,1) count: 1
>
> Reused old vertex: A
> Added new vertex: B
> New, pred: (1,2) count: 1
>
> Reused old vertex: B
> Reused old vertex: A
> New, pred: (2,1) count: 1
>
> Reused old vertex: A
> Reused old vertex: B
> Old, pred: (1,2) count: 1
> New, pred: (1,2) count: 2

<snip>

>
> The creation and reuse of vertices is working pretty well as you can see
> but the edge, even when reused doesn't have its value updated on a
> second time.
>
> It may be a very stupid problem (not even in BGL) but can you help me
> find out what's going on?
>

Hi Renato,

The sequence of statements:

        Edge e;
        e = graph[pred];
        e.count++;

copies the Edge structure from graph[pred] to e by value, then makes a
modification to the copy in e, which is then thrown away when e goes
out of scope. Instead, try:

        graph[pred].count++;

Regards,
Aaron