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

31 Jan 2008

      On Jan 30, 2008 3:14 PM, Renato Golin  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

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

Aaron Windsor