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Re: [igraph] question re adding edges

From: Gábor Csárdi
Subject: Re: [igraph] question re adding edges
Date: Fri, 7 Sep 2012 15:21:15 -0400

Hi Carl, see answers below.

On Fri, Sep 7, 2012 at 11:14 AM, Carl Pearson <address@hidden> wrote:
> rg <- add.edges(rg, t(array(rg$faction[el], dim=dim(el))), faction=T)
> ## transposing
> But I feel like this isn't the "right" way; it doesn't seem to embrace
> the syntax provided by the library.  Am I trying to use the syntax
> incorrectly?  Is there a more "igraph"y way to do what I'm attempting
> here?

There reason for the need of the transpose is that edges() does not
know about edge list matrices. It assumes that the edges are given
just as a numeric or symbolic vector, or multiple vectors:

          The unnamed arguments of ‘edges’ are concatenated and used as
          vertex ids of the end points of the new edges. The named
          arguments will be added as edge attributes.


            g <- graph.empty() + vertices(letters[1:10]) +
            vertices("foo", "bar", "bar2", "foobar2")
            g <- g + edge("a", "b")
            g <- g + edges("foo", "bar", "bar2", "foobar2")
            g <- g + edges(c("bar", "foo", "foobar2", "bar2"),
color="red", weight=1:2)

And in R, matrices are just vectors with the 'dim' attribute set.
igraph currently simply ignores the 'dim' attribute.

I agree that it would be better to handle edge list matrices, within
edges(), or even without edges(), by just simply doing

rg <- rg + array(rg$faction[el], dim=dim(el))

so I'll see whether this is possible to implement in a clean way. (It
should be.)



Gabor Csardi <address@hidden>     MTA KFKI RMKI

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