Re: [jgrapht-users] BlockCutpointGraph - getBlock

[H.N. de R. <hnr...@gr...>]

On Fri, Nov 29, 2013 at 04:06:48PM +0100, Carl Witt wrote:

Hello Carl,

> I have a question concerning the public UndirectedGraph 
> <http://jgrapht.org/javadoc/org/jgrapht/UndirectedGraph.html><V 
> <http://jgrapht.org/javadoc/org/jgrapht/alg/BlockCutpointGraph.html>,E 
> <http://jgrapht.org/javadoc/org/jgrapht/alg/BlockCutpointGraph.html>> 
> *getBlock*(V 
> <http://jgrapht.org/javadoc/org/jgrapht/alg/BlockCutpointGraph.html> vertex) 
> method.
> The documentation says
> 
>      Returns the vertex if vertex is a cutpoint, and otherwise returns 
> the block (biconnected component) containing the vertex.
> 
> But the example given in the introductory section of the 
> BlockCutpointGraph class clarifies that the a vertex can be part of more 
> than one block
>
> Is the documentation of the getBlock method being imprecise (should it 
> read "returns a block containing the vertex") or is the underlying 
> definition of a block not the one from Mathworld?

I don't know how it could happen, but cutpoint is one of the very few
concepts in graph theory that have only one meaning. A cutpoint is indeed in
more than one block (x is a cutpoint iff x is in more than one block), but
non-cutpoints are in precisely one block.

So getBlock(x) returns x if x is a cutpoint and the unique block that
contains x, otherwise.

Hope this clears things up,
   Ernst

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