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

30 Jan 2008

      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

Reused old vertex: B
Reused old vertex: A
Old, pred: (2,1) count: 1
New, pred: (2,1) count: 2

Reused old vertex: A
Reused old vertex: B
Old, pred: (1,2) count: 1
New, pred: (1,2) count: 2

Reused old vertex: B
Added new vertex: .
New, pred: (2,3) count: 1

Reused old vertex: .
Added new vertex: last
New, pred: (3,4) count: 1

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?

Basically, the following part works while inside the function:

	if (found) {
		e = graph[pred];
		cerr << "Old, pred: " << pred << " count: " << e.count << endl;
		e.count++;
	}

I'm getting it again from the graph in order to assure it changed:

	boost::tie(pred, inserted) = add_edge(sub, obj, e, graph);
	e = graph[pred];

And even when using a new variable:

	boost::tie(pred, inserted) = add_edge(sub, obj, e, graph);
	Edge f;
	f = graph[pred];

My cerr still produces the correct value. But when I get it a few
iterations later the value is 1 again. What's going on? Or better, how
to change the value of the structure inside the graph?

thanks,
--renato

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

Renato Golin