Ticket #8395(closed defect: fixed)

Opened 3 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 N/A Robert Miller, Minh Van Nguyen Minh Van Nguyen sage-4.6.2.alpha0

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.

Change History

comment:1 Changed 3 years ago by mvngu

• Description modified (diff)

comment:2 Changed 2 years ago by mvngu

• Description modified (diff)

comment:3 Changed 2 years ago by mvngu

• Status changed from new to needs_review
• Description modified (diff)
• Authors set to Minh Van Nguyen

Changed 2 years ago by rlm

apply after previous patch

comment:4 Changed 2 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 2 years ago by mvngu

• Status changed from needs_review to positive_review
• Reviewers set to Robert Miller, Minh Van Nguyen

comment:6 Changed 2 years ago by mvngu

• Description modified (diff)

comment:7 Changed 2 years ago by jdemeyer

• Milestone changed from sage-4.6.1 to sage-4.6.2

comment:8 Changed 2 years ago by jdemeyer

• Status changed from positive_review to closed
• Resolution set to fixed
• Merged in set to sage-4.6.2.alpha0
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