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From "Simone Tripodi (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (SANDBOX-337) Wrong value for Vertex degree
Date Sun, 03 Jul 2011 18:26:21 GMT
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[ https://issues.apache.org/jira/browse/SANDBOX-337?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13059251#comment-13059251
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Simone Tripodi commented on SANDBOX-337:
----------------------------------------

Thanks for the patch! :)

I'm not sure the modification you are proposing is 100% right, looks like for Directed graphs
there are different opinions: take a look at this [article|http://www.utm.edu/departments/math/graph/glossary.html]:

{quote}
*degree*

The degree (or valence) of a vertex is the number of edge ends at that vertex. For example,
in this graph all of the vertices have degree three.

In a digraph (directed graph) the degree is usually divided into the in-degree and the out-degree
(*whose sum is the degree* of the vertex in the underlying undirected graph).
{quote}

Take also a look at this [samples|http://reference.wolfram.com/mathematica/ref/VertexDegree.html]
with directed graphs: for Vertex {{2}}, that has {{deg+ = 2}} and {{deg- = 1}}, the degree
is {{2}}

In the [book|http://www.algoritmica.org/] I'm reading (sorry, in Italian only) it is reported
the following:

{quote}
Il grado in uscita di un nodo è pari al numero di archi uscenti da esso, mentre il grado
in ingresso e dato dal numero di archi entranti. Il grado è la somma del grado d'ingresso
e di quello d'uscita
{quote}

> Wrong value for Vertex degree
> -----------------------------
>
>                 Key: SANDBOX-337
>                 URL: https://issues.apache.org/jira/browse/SANDBOX-337
>             Project: Commons Sandbox
>          Issue Type: Bug
>          Components: Graph
>            Reporter: Marco Speranza
>            Priority: Minor
>         Attachments: VertexDegreeTestCase.patch
>
>
> Hi folk, I'm doing a tests case for the class BaseMutableGraph, in order to upgrade our
testcase suite. I think that our implementation of vertex degree is wrong.
> "The degree, or valency, dG(v) of a vertex v in a graph G is the number of edges incident
to v, with loops being counted twice. A vertex of degree 0 is an isolated vertex. A vertex
of degree 1 is a leaf. In the labelled simple graph example, vertices 1 and 3 have a degree
of 2, vertices 2, 4 and 5 have a degree of 3, and vertex 6 has a degree of 1. If E is finite,
then the total sum of vertex degrees is equal to twice the number of edges."
> so for a complete graph with 5 nodes, each vertex has a degree of 4. Instead our implementation
returns 8.
> IMHO this is wrong. WDYT?
> Have a nice week end

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