The problem is not in the trace_faces() method, but in the is_planar()
calculation. The embedding of the second graph is not correct:
S = Graph(lat)
S.show(vertex_size = 600, pos = nodes_dict)
S.is_planar(set_embedding = True)
s_emb = S.get_embedding()
print s_emb
{1: [2], 2: [1, 3, 5], 3: [4, 9, 5, 2], 4: [5, 6, 7, 8, 9, 3], 5: [2, 3,
6, 4], 6: [7, 4, 5], 7: [8, 4, 6], 8: [9, 4, 7], 9: [3, 4, 8]}
This should be a clockwise listing of the neighbors of each node, i.e. node 1
has one neighbor, 2, node 2 has 3 neighbors that are (clockwise) 1,3,5. Node
3, however, has the correct nodes but are not in a clockwise ordering.
On Wednesday, March 26, 2014 5:34:53 PM UTC-6, Christa Brelsford wrote:
> 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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