Re: [igraph] rewire() and self loops

[Tamás Nepusz]

Hi Dov,  

I have recently fixed this in the development branch. If you wanna give it a go 
and don't mind compiling it yourself, check out the nightly build tomorrow:

http://code.google.com/p/igraph/downloads/list

Revision 3006 of the 0.7-main tree should include all the changes -- this will 
be uploaded around 6am GMT tomorrow.

Best,
--  
T.


On Thursday, 11 October 2012 at 21:43, Gábor Csárdi wrote:

> Hi Dov,
>  
> I think this is simply a bug. He have this in the C code for rewire:
>  
> if (!igraph_vector_search(neis[0], 0, d, NULL) &&
> !igraph_vector_search(neis[1], 0, b, NULL) &&
> b!=c && a!=d && a!=b && c!=d) {
> do the rewiring here
> }
>  
> So, yes, if a=b or a=d, or b=c or c=d, the rewiring is not performed,
> I am not sure why. I have submitted a bug report for this:
> https://bugs.launchpad.net/igraph/+bug/1065681
>  
> Thanks, Best,
> Gabor
>  
>  
> On Thu, Oct 11, 2012 at 3:29 PM, Dov Pechenick <address@hidden 
> (mailto:address@hidden)> wrote:
> > Hello,
> >  
> > I'm using igraph in R, and I'm trying to randomize the edges of directed
> > graphs while preserving in- and out-degree distributions. Since my graphs
> > are directed, I don't use degree.sequence.game(). Instead, I've been using
> > rewire(). This function correctly preserves the degree distribution,
> > however as far as I can tell it completely ignores self loop edges during
> > the rewiring process. That is to say, every single self loop in my original
> > network is also preserved in all my rewired networks. I did not expect this
> > based on the documentation:
> >  
> > "simple rewiring algorithm which chooses two arbitrary edges in each step
> > (namely (a,b) and (c,d)) and substitutes them with (a,d) and (c,b) if they
> > don't yet exist"
> >  
> > While true that self loops aren't explicitly mentioned, the most general
> > form of the algorithm allows a==b and c==d. Thus, this algorithm should
> > allow for the generation and destruction of self-loops. For example, a->a
> > and b->b can be replaced with a->b and b->a if they don't yet exist. The
> > reverse should also be a valid swap. I tested this on the following
> > networks, just to be sure:
> >  
> > a <- graph(c(0,0,1,1,2,2,3,3)) # 4 nodes, only self loops
> > b <- graph(c(0,1,1,0,2,3,3,2,0,2,2,0,0,3,3,0,1,2,2,1,1,3,3,1)) # 4 node
> > complete graph, no self loops
> >  
> > Rewiring using rewire() does nothing to the above graphs. For a, self loops
> > are never destroyed, and for b, they are never created. If I am missing
> > something, please let me know. Otherwise, is there a way to rewire directed
> > graphs that both preserves degree distribution and allows for the generation
> > and destruction of self-loops? (Ideally it would be the same function, with
> > an option to allow self-loops.)
> >  
> > Thanks!
> > Dov
> >  
> > _______________________________________________
> > igraph-help mailing list
> > address@hidden (mailto:address@hidden)
> > https://lists.nongnu.org/mailman/listinfo/igraph-help
>  
>  
>  
>  
>  
> --  
> Gabor Csardi <address@hidden (mailto:address@hidden)> MTA KFKI RMKI
>  
> _______________________________________________
> igraph-help mailing list
> address@hidden (mailto:address@hidden)
> https://lists.nongnu.org/mailman/listinfo/igraph-help