Re: [Boost-users] bgl - add_vertex during breadth first

6 Sep 2005

      Hi Jeremy,

Many thanks for the response. I was hoping I could add vertices inside
the algorithm visitor based on some conditions and these same conditions
will be applied when the newly added vertices are visited by calling the
algorithm only once (a kind of recursive effect). Now it seems that I
have to call the algorithm many times and introduce some sort of my own
implementation of colour or add a property to each vertex which flags
whether it is already expanded or not. Is there any better idea?

Cheers,
Abebaw
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Message: 5
Date: Mon, 5 Sep 2005 17:19:21 -0500
From: Jeremy Siek 
Subject: Re: [Boost-users] bgl - add_vertex during breadth first
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To: boost-users@lists.boost.org
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Content-Type: text/plain; charset=US-ASCII; delsp=yes; format=flowed
Hi Abebaw,
Adding vertices and edges is a dangerous thing to do inside an
algorithm visitor. This is because doing that invalidates
iterators that the algorithm is using.  Also, the algorithm
creates a mapping from vertices to colors (to mark
which vertices have been processed), and adding vertices
will mess up that mapping. Can you just record that
you want to add vertices and do it after the BFS?
Cheers,
Jeremy
On Sep 4, 2005, at 7:43 AM, Abebaw Wubshet wrote:
...
Hi all,
I have started using the boost graph library recently. I want to
add vertices
and edges to a graph through an algorithm visitor. I chose the
'discover_vertex'
event and added some code to add a vertex/edge. But the compiler
could not
determine the apporpriate the matching add_vertex/add_edge
function. I tried
fully specifiying the template arguments with no success. (I am
using gcc 3.3.1
on SuSE Linux.)
I tried another alternative which does not work either. I created a
pointer to a
graph object which points to my graph insided the
'discover_vertex'. But then I
ended up having a run time error after the vertex is added.
Many thanks,
Abebaw Wubshet
Here is a sample code
/

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...
**/
#include 
#include <iostream>
#include 
#include 
#include 
using namespace boost;
class VertexProperties
{
 public:
    std::string _name;
    VertexProperties () {}
    VertexProperties (std::string name) : _name(name) {}
};
//OPTION ONE
class bfs_extend_graph1 : public default_bfs_visitor
{
public:
    template
    void discover_vertex (Vertex v, Graph& g)
        {
            std::cout<<"\ndiscover";
            Vertex v_new;
            //the compile gets confused here !!!!!
            v_new = add_vertex(VertexProperties("C"),g);
            add_edge(v,v_new,g);
        }
};
//OPTION TWO
class bfs_extend_graph2 : public default_bfs_visitor
{
public:
    template
    void discover_vertex (Vertex v, Graph& g)
        {
            std::cout<<"\ndiscover";
            Vertex v_new;
            typedef adjacency_list< vecS, vecS, directedS,
VertexProperties>
graph_t;
            //a pointer to
            shared_ptr ptr_g;
            *ptr_g = g; //reference to the actual graph
            v_new = add_vertex(VertexProperties("C"),*ptr_g);
            add_edge(v,v_new,*ptr_g);
        }
};
int main()
{
    typedef adjacency_list< vecS, vecS, directedS,
VertexProperties> graph_t;
    typedef graph_traits::vertex_descriptor Vertex;
    typedef graph_traits::vertex_iterator vertex_iterator;
    typedef property_map::type VP_map_t;
graph_t g;
    Vertex v1,v2;
v1 = add_vertex(VertexProperties("A"),g);
    v2 = add_vertex(VertexProperties("B"),g);
    add_edge(v1,v2,g);
//get the VP map
    VP_map_t VP_map = get(&VertexProperties::_name,g);
bfs_extend_graph1 bfs_e1;
    breadth_first_search(g,v1,visitor(bfs_e1));
bfs_extend_graph2 bfs_e2;
    breadth_first_search(g,v1,visitor(bfs_e2));
//check if everything is okay
    std::pair vp;
    std::cout<
return 0;
}
/

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**/