for a simple graph, trace_faces() gives the expected answer for the faces of a
planar graph, as shown below.
import networkx as nx
lat = nx.Graph()
lat.add_edge(1,2)
lat.add_edge(2,3)
lat.add_edge(2,5)
lat.add_edge(3,4)
lat.add_edge(3,5)
lat.add_edge(4,5)
nodes_dict = {}
nodes_dict[1]=(0,0)
nodes_dict[2]=(0,1)
nodes_dict[3]=(0,2)
nodes_dict[4]=(0,3)
nodes_dict[5]=(1,2)
S = Graph(lat)
S.show(vertex_size = 600, pos = nodes_dict)
S.is_planar(set_embedding = True)
s_emb = S.get_embedding()
faces = S.trace_faces(s_emb)
print faces:
[[(1, 2), (2, 3), (3, 4), (4, 5), (5, 2), (2, 1)], [(5, 4), (4, 3), (3,
5)], [(2, 5), (5, 3), (3, 2)]]
There are three faces: one between nodes 2,3 and 5, another between nodes 3,4
and 5,and the outside face of all nodes.
but when I extend the graph:
lat = nx.Graph()
lat.add_edge(1,2)
lat.add_edge(2,3)
lat.add_edge(2,5)
lat.add_edge(3,4)
lat.add_edge(3,5)
lat.add_edge(3,9)
lat.add_edge(4,5)
lat.add_edge(4,6)
lat.add_edge(4,7)
lat.add_edge(4,8)
lat.add_edge(4,9)
lat.add_edge(5,6)
lat.add_edge(6,7)
lat.add_edge(7,8)
lat.add_edge(8,9)
nodes_dict = {}
nodes_dict[1]=(0,0)
nodes_dict[2]=(0,1)
nodes_dict[3]=(0,2)
nodes_dict[4]=(0,3)
nodes_dict[5]=(1,2)
nodes_dict[6]=(1,3)
nodes_dict[7]=(0,4)
nodes_dict[8]=(-1,4)
nodes_dict[9]=(-1,3)
S = Graph(lat)
S.show(vertex_size = 600, pos = nodes_dict)
S.is_planar(set_embedding = True)
s_emb = S.get_embedding()
faces = S.trace_faces(s_emb)
print faces
[[(6, 4), (4, 7), (7, 6)], [(3, 2), (2, 5), (5, 3)], [(5, 4), (4, 6),
(6, 5)], [(7, 8), (8, 9), (9, 3), (3, 5), (5, 6), (6, 7)], [(1, 2), (2,
3), (3, 4), (4, 5), (5, 2), (2, 1)], [(4, 9), (9, 8), (8, 4)], [(7, 4),
(4, 8), (8, 7)], [(3, 9), (9, 4), (4, 3)]]
the face that should exist between just nodes 3,4 and 5 is not found, it's
replaced with a face around nodes 1,2,3,4,5. Any ideas what is wrong, or
other ways of getting an accurate list of the faces in a planar graph?
I'm using 'Sage Version 5.13, Release Date: 2013-12-15'
I realize that trace_faces has been deprecated to just faces() per this
discussion: http://trac.sagemath.org/ticket/15551
but when I run
S.faces()
I get the error
AttributeError: 'Graph' object has no attribute 'faces'
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