From: Horst Duchene <horst@...>
Date: 2002-09-23T21:33:06+09:00
Subject: Re: rgl can't produce disconnected graphs


----- Original Message -----
From: "Phlip" <phlip_cpp@yahoo.com>

> Being a rather simple person, I decided to start by producing the simplest
> graph I could possibly make; a graph with no vertices and no edges.
>
> This proved trivial, so I decided to go to the next level: A graph with one
> vertex and no edges:
>
>         dg=RGL::DirectedAdjacencyGraph.new #[1,2 ,2,3 ,2,4, 4,5, 6,4, 1,6]
>         dg.add_vertex('yo')
>         dg.write_to_graphic_file('jpg')
>         system('kview graph.jpg &')
>
> This produces a graph containing a title, and nothing else. There is no
> single vertex. (Graphviz, the back-end for 'write_to_graphic_file', knows
> to produce a single vertex here.)
>
There is one vertex:

irb(main):073:0> dg.vertices
["yo"]

write_to_graphic_file has a bug. It only shows edges. I fixed it in the latest CVS of rgl/dot.rb. By
the way: Support for dot is rather rudimentary. Debugging graph datastructures should better be done
using inspect or the graph accessors for vertices and edges.

> RGL either has a bug, or its author thinks single vertices are not graphs.
> In the above example, if we uncomment the "[1,2 ,2,3 ..." stuff and take
> out the .new, we get a graph with 6 vertices. But the 7th is missing.
>

A graph can have isolated vertices as the above example with the single vertex "yo" shows.

The expression

    dg = RGL::DirectedAdjacencyGraph[1,2, 2,3, 2,4, 4,5, 6,4, 1,6]

creates a graph with edges (1-2) (2-3) (2-4) (4-5) (6-4) (1-6), one for each pair in the array given
as parameter for the singeleton method RGL::DirectedAdjacencyGraph.[]

    dg.edges.sort.to_s => "(1-2)(1-6)(2-3)(2-4)(4-5)(6-4)"
    dg.vertices.sort => [1, 2, 3, 4, 5, 6]

The constructor does not take vertices as parameter but edges. Vertices are created internally for
the source and target of each edge.

Horst