#12165: Unexpected behavior with weighted graph
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Reporter: akm | Owner: jason, was
Type: defect | Status: new
Priority: major | Milestone: sage-4.8
Component: graphics | Keywords:
Work_issues: | Upstream: N/A
Reviewer: | Author:
Merged: | Dependencies:
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This bug is about a non-obvious, wierd behavior when plotting a certain
type of weighted graph.
I posted this on Google groups here: http://groups.google.com/group/sage-
support/browse_thread/thread/fc6d0a7e2911984c/22914ce6373dd5c0
This code results in a graph plotted with the (single) edges labeled with
the weight between each pair of vertices:
M = Matrix([[0,1,-1,5],[1,0,-1/2,-1],[-1,-1/2,0,2],[5,-1,2,0]]) G =
Graph(M,sparse=True) G.plot(edge_labels=True)
Whereas this code does not and instead has multi-edges labeled "None":
M = Matrix([[0, 1, 0, 0], [1, 0, 1, 2], [0, 1, 0, 0], [0, 2, 0, 0]]) G =
Graph(M,sparse=True) G.plot(edge_labels=True)
Here was the answer I got for reference:
I think it's something like this (take with grain of salt, I'm bad at
multitasking): Briefly, it's not interpreting the second matrix as a
weighted adjacency matrix. If you specifically tell it that it is, say
by M = Matrix([[0, 1, 0, 0], [1, 0, 1, 2], [0, 1, 0, 0], [0, 2, 0, 0]])
G = Graph(M,weighted=True) it should behave. Less briefly: the code in
graph.py uses the following logic to decide whether these two are
weighted or not:
if format == 'adjacency_matrix':
entries = uniq(data.list()) for e in entries:
try:
e = int(e) assert e >= 0
except:
if weighted is False:
raise ValueError("Non-weighted graph's"+ " adjacency matrix
must have only nonnegative"+ " integer entries")
weighted = True if multiedges is None: multiedges = False break
IOW, to decide if it should set weighted to True, it loops over each
entry and (1) tries to convert it to an integer, and (2) asserts that
it's non-negative. If either of those fail, then it decides that
weighted should be True (in this case weighted starts as None). So the
first graph works because the nonnegative check fails (or the integral
check fails, I'm too lazy to confirm what direction the matrix entries
are iterated in..) The second graph's matrix is integral and
nonnegative, so it sneaks through, and things get silly after that.
Would you like to open a bug report on trac? This should be easy to fix.
Doug
--
Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/12165>
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