I am not sure why you think the "length" edge attribute is used for anything. It is not, AFAIK.
G. On Sun, Mar 9, 2014 at 6:05 PM, aschmid1 <[email protected]> wrote: > Tamas, > Thank you. I suspected I wanted too much from igraph. Indeed one cannot > draw say a triangle with arbitrary edge lengths... But then I have a naive > question (I'm new to graphs): how the edge attribute "length" may be used > (providing there is also attribute "weight")? > Best, Alec > > > On 03/09/2014 3:32 PM, Tamás Nepusz wrote: > >> I thought that the commands >>> gm<-set.edge.attribute(gm, "weight", index=E(gm), value=weights) >>> gm<-set.edge.attribute(gm, "length", index=E(gm), value=weights) >>> ensure that plot(gm) yields the edge lengths corresponding to the values >>> of vector weights. >>> >> No, it doesn't. *Some* layout algorithms *might* take into account the >> weights of the edges to *some* extent (i.e. they would *try* to ensure >> that edges with larger weights end up being shorter than edges with >> smaller weights), but there is no layout algorithm that would >> guarantee that the lengths you specify are satisfied exactly. The main >> reason is that most of the length combinations are impossible to >> satisfy in the 2D plane; for instance, it is impossible to plot a full >> graph of size 4 if we want every single edge to be of length 1 (and >> there are infinitely many examples for this). >> >> As far as I know, the Fruchterman-Reingold layout algorithm (see >> layout.fruchterman.reingold) is the only one in igraph that supports >> weights; see http://igraph.sourceforge.net/doc/R/layout.html . But >> even for the Fruchterman-Reingold algorithm, the only thing that the >> weights do is that they make the attraction forces between the >> endpoints of the edges proportional to the weight of the edge. >> >> -- >> T. >> > > _______________________________________________ > igraph-help mailing list > [email protected] > https://lists.nongnu.org/mailman/listinfo/igraph-help >
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