Opened 13 years ago

Closed 12 years ago

degree() reports the degree of a self-loop vertex as contributing 1 to total degree

Reported by: Owned by: mvngu rlm minor sage-4.6.2 graph theory jason, ncohen sage-4.6.2.alpha0 Minh Van Nguyen Robert Miller, Minh Van Nguyen N/A

Note: When this ticket is closed, make sure to also close ticket #9809.

From sage-devel:

[mvngu@sage mvngu]\$ sage
----------------------------------------------------------------------
| Sage Version 4.3.3, Release Date: 2010-02-21                       |
| Type notebook() for the GUI, and license() for information.        |
----------------------------------------------------------------------
sage: G = Graph({1:[1]}); G
Looped graph on 1 vertex
sage: sum(G.degree())
1
sage: G.size()
0
sage: G = Graph({1:[1]}, loops=True); G
Looped graph on 1 vertex
sage: sum(G.degree())
1
sage: G.size()
0
sage: G = Graph({1:[1]}, loops=True, multiedges=True); G
Looped multi-graph on 1 vertex
sage: sum(G.degree())
1
sage: G.size()
0

The size of G is 1 because there is one edge, i.e. the single
self-loop. As shown by the above session, Sage reports the size of G
as 0. I believe this is a bug.

See also the discussion at this other sage-devel thread. This also happens in the C graph backends for sparse and dense graphs:

[mvngu@sage ~]\$ sage
----------------------------------------------------------------------
| Sage Version 4.3.5, Release Date: 2010-03-28                       |
| Type notebook() for the GUI, and license() for information.        |
----------------------------------------------------------------------
sage: from sage.graphs.base.sparse_graph import SparseGraphBackend
sage: S = SparseGraphBackend(7)
sage: S.loops(True)
sage: S.has_edge(1, 1, None)
True
sage: list(S.iterator_edges(range(7), None))
[(1, 1)]
sage: S.degree(1, directed=False)
1
sage:
sage: reset()
sage:
sage:
sage: from sage.graphs.base.dense_graph import DenseGraphBackend
sage: D = DenseGraphBackend(78)
sage: D = DenseGraphBackend(7)
sage: D.loops(True)
sage: D.has_edge(1, 1, None)
True
sage: list(D.iterator_edges(range(7), None))
[(1, 1)]
sage: D.degree(1, directed=False)
1

Notice that degree() reports the degree of a self-loop as one, when in fact it should be 2. That's because both SparseGraphBackend and DenseGraphBackend inherit the same degree() function from CGraphBackend. I think the implementation of degree() in CGraphBackend needs to take into account the existence of self-loops.

comment:1 Changed 13 years ago by mvngu

Cc: jason ncohen added modified (diff)

comment:2 Changed 12 years ago by mvngu

Description: modified (diff)

comment:3 Changed 12 years ago by mvngu

Authors: → Minh Van Nguyen modified (diff) new → needs_review

Changed 12 years ago by rlm

apply after previous patch

comment:4 Changed 12 years ago by rlm

Minh,

Your patch looks good to me. If you approve of mine, please set this to positive review.

Thanks!

comment:5 Changed 12 years ago by mvngu

Reviewers: → Robert Miller, Minh Van Nguyen needs_review → positive_review

comment:6 Changed 12 years ago by mvngu

Description: modified (diff)

comment:7 Changed 12 years ago by jdemeyer

Milestone: sage-4.6.1 → sage-4.6.2

comment:8 Changed 12 years ago by jdemeyer

Merged in: → sage-4.6.2.alpha0 → fixed positive_review → closed
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