Re: [igraph] how to plot a very basic MST with igraph

[Steve Boudreault]

>
> Anyway, igraph is not optimal for this, then. igraph does MSTs, not EMSTs.
>


Ok.

To put you in the context, some of the works done in my field using MST
were done using and IDL routine.

For some reasons related to the job I have to do, I wanted to use R, and
this is why I have tried igraph in the first place.

I have to admit that, before your emails, I didn't knew that what I needed
was an EMST. Perhaps I should have pointed out that I was new to R and to
the used of minimum spanning tree.


>
> So you expected us to guess that you wanted an EMST, without you telling
> us? :)
>


Well *sorry* ... I *thought* I have explained clearly what I wanted.

I think we can close this topic : I will try to get help somewhere else,
or to use another tool.

Thank you anyway.


>
> On Wed, Mar 5, 2014 at 1:50 PM, Steve Boudreault
> <address@hidden>wrote:
>> > Maybe you want this:
>> > http://en.wikipedia.org/wiki/Euclidean_minimum_spanning_tree
>> > Do you?
>> Yes, this is correct.
> Anyway, igraph is not optimal for this, then. igraph does MSTs, not
EMSTs.
> As I wrote in my very first email, you can create a full graph, set edge
weights to Euclidean distances, and then you can use
> minimum.spanning.tree() in igraph. But this is very suboptimal. It might
work if you have a couple of hundred points, maybe a couple of
thousands.
> Btw. this naive algorithm is the first one discussed at the wikipedia
page:
> http://en.wikipedia.org/wiki/Euclidean_minimum_spanning_tree#Algorithms_for_computing_EMSTs_in_two_dimensions
>> In the link you gave me, I see the following sentence : << [...] an
EMST
>> (Euclidean minimum spanning tree) connects a set of dots using lines
such
>> that the total length of all the lines is minimised and any dot can be
reached from any other by following the lines. >>
>> Isn't that the rule? i.e. that my objects in the RA vs DEC place are
connected such that the total length of all the lines is minimised?
> So you expected us to guess that you wanted an EMST, without you telling
us? :)
> I can add that all my objects have the same weight.
> I have no idea what you mean. You don't need to assign weights to your
objects. Please read that wikipedia page.
> G.
>> > On Wed, Mar 5, 2014 at 11:17 AM, Steve Boudreault
>> > <address@hidden
>> >> wrote:
>> >> > But I'd prefer you tell us want you want to do, and then we don't
>> need
>> >> > to
>> >> > guess.
>> >> Please look at the following image :
>> >> http://inspirehep.net/record/833443/files/mst.png
>> >> These are "minimum spanning tree" (right?) of stars distributed in X
>> (i.e.
>> >> right ascension, RA) and Y (i.e. declination, DEC). You have this
for
>> three open clusters : IC2391, M34, M11.
>> > Again. The minimum spanning trees in igraph are defined for graphs
>> (=networks), not for points:
>> > http://en.wikipedia.org/wiki/Minimum_spanning_tree
>> > Maybe you want this:
>> > http://en.wikipedia.org/wiki/Euclidean_minimum_spanning_tree
>> > Do you?
>> > This *is* what I want to do. I want to do a plot like this.
>> > This is not an explanation, I am sorry. What is the rule to connect
>> the
>> dots? Maybe this is evident for people in your field, but not to me. G.
>> > [...]
>> --
>> ============================================================
>> Steve Boudreault, Ph.D.
>> CNRS UMR 8111 / GÉPI
>> Bâtiment 11 - Hipparque     Email: address@hidden
>> Observatoire de Paris       Phone : +33 (0) 145077868
>> 5 Place Jules Janssen       Fax: +33 (0) 145077878
>> 92190 Meudon, France        Mobile : +33 (0) 604530082
>> ============================================================


-- 
============================================================
Steve Boudreault, Ph.D.
CNRS UMR 8111 / GÉPI
Bâtiment 11 - Hipparque     Email: address@hidden
Observatoire de Paris       Phone : +33 (0) 145077868
5 Place Jules Janssen       Fax: +33 (0) 145077878
92190 Meudon, France        Mobile : +33 (0) 604530082
============================================================