Dear BGL users,
I am trying to produce a simple routine to extract the subgraph of a
graph, by using the vertex names of the subgraphs. That is, for two
graphs G and H, I want to define the following operator:
G -= H;
Here, the resulting subgraph of G should have all the vertices that H
doesn't have. During this process, the incident edges of the removed
vertices are also removed. I have checked the subgraph class in BGL, but
this refers to a substantially different concept. I have tried the
following code, which works, but is extremely slow for the type of
application that I had in mind:
//////////////////////////////////////////////////
class Graph
{
public:
typedef adjacency_list > UndirectedGraph;
Graph();
~Graph();
int Nv(); // Return number of vertices
Rcpp::IntegerVector V(); // Return vector of NAMES of vertices as
integers.
Graph& operator-= (Graph& H);
private:
UndirectedGraph G;
};
/////////////////////////////////////////////////////////////////////////
// Overloaded Operators.
Graph& Graph::operator-= (Graph& H){
typedef graph_traits<UndirectedGraph>::vertex_descriptor Vdescriptor;
typedef property_map::type VertexNameMap;
typedef iterator_property_map
name_to_vertex_map;
Rcpp::IntegerVector vec = H.V();
VertexNameMap NameIndex;
for(ivIter t=vec.begin(); t!=vec.end(); ++t){
// Create/Update Iterator map (LValuePropertyMap).
int x[this->Nv()]; NameIndex = get(vertex_name,G);
for(int i=0; i < this->Nv(); ++i) x[NameIndex[i]]=i;
name_to_vertex_map NVmap(x);
// Remove vertex from name of *t in H.
Vdescriptor u=NVmap[*t];
clear_vertex(u,G); remove_vertex(u,G);
}//t
return *this;
}//-=
//////////////////////////////////////////////////
The problem seems to be that I re-create an iterator map (from vertex
NAMES to vertex INDICES) every time I remove a single vertex. This seems
counter-efficient. I would need to be able to create a reference to a
similar map that is automatically updated every time I remove a vertex.
However, since vertices in BGL are indexed from 0 to Nv-1, I don't see a
way to produce a LValuePropertyMap from vertex's names to vertex's indices.
Any help/opinion would be greatly appreciated,
--
Cedric Ginestet
Centre for Neuroimaging Sciences (L3.04)
NIHR Biomedical Research Centre
Department of Neuroimaging
Institute of Psychiatry, Box P089
King's College London
De Crespigny Park
London
SE5 8AF
