[Boost-users] [BGL] python bindings build graph from edge list

From: John Reid (j.reid_at_[hidden])
Date: 2006-06-01 04:02:47

Hi,

I was trying to build a graph from an edge list and ran into some 
problems. The following code throws an exception on write_graphviz(). I 
think it is because there's no property map called node_id in the graph.

import boost.graph as bgl
edges = [ ( 'a', 'b' ), ( 'b', 'c' ), ( 'c', 'a' ) ]
g = bgl.Graph( edges )
g.write_graphviz( 'bind-copy.dot' )

I get the following:
Traceback (most recent call last):
   File "<stdin>", line 1, in ?
KeyError: 'node_id'

I couldn't manage to generate the node_id property map automatically. So 
after you have built the graph with the edge list constructor how do you 
know which vertex is which?

I ended up using the following code to create the graph:

def build( edges ):
        
        # Create a graph to build
        g = bgl.Graph()
        
        # Construct some property maps
        node_ids = g.vertex_properties[ 'node_id' ] = g.vertex_property_map( 
'string' )
        color = g.vertex_properties[ 'color' ] = g.vertex_property_map( 'string' )
        centrality = g.vertex_properties[ 'centrality' ] = 
g.vertex_property_map( 'float' )
        weight = g.edge_properties[ 'weight' ] = g.edge_property_map( 'float' )
        length = g.edge_properties[ 'len' ] = g.edge_property_map( 'float' )
        
        # create the edges and vertices
        name_2_vertex = { }
        for ( name1, name2 ) in edges:
                if not name_2_vertex.has_key( name1 ):
                        v1 = name_2_vertex[ name1 ] = g.add_vertex()
                        node_ids[ v1 ] = name1
                else:
                        v1 = name_2_vertex[ name1 ]
                if not name_2_vertex.has_key( name2 ):
                        v2 = name_2_vertex[ name2 ] = g.add_vertex()
                        node_ids[ v2 ] = name2
                else:
                        v2 = name_2_vertex[ name2 ]
                e = g.add_edge( v1, v2 )
                weight[ e ] = 0.1
                length[ e ] = 4
        
        # Color articulation points red
        for v in bgl.biconnected_components( g ):
                color[ v ] = 'red'
        
        # Assign centrality values to property map
        bgl.brandes_betweenness_centrality( g, centrality )
        
        return g

This code seems to works fine. So I thought I'd post this just to see if 
I was missing something obvious and/or in the hope someone else might 
find it useful.

Best,
John.