[igraph] plotting planar graphs with igraph?

[Christian Jost]

Dear all,

I currently struggle with igraph to make it do things it doesn't seem to be designed to do. First, my networks are actually planar graphs representing tunneling networks (dug by ants in 2D sand disks) that I have digitised in Pajek format (see example below, vertex 1 is the origin of the digging activity). I can read these graphs without problem with read.graph, but when it comes to manipulating them I encounter problems. For the moment I am stuck with two problems.

a) plot the graph with the correct x and y coordinates (specifically, I tried to write a function doing that, successfully plotting vertices at the x,y coordinates, but I cannot extract the vertex numbers of each edge provided by E(g) and therefore cannot add the edges; but there may be a simpler way to draw such a graph).

b) I have to remove the degree-2 nodes in order to get a simpler network with only the bifurcations and the endpoints. I use degree(g) to detect the nodes with degree 2, but when I feed their identities to "delete.vertices()" I get as a result many disconnected vertices. This function does not seem to reconnect the neighbour vertices after removing the degree 2 node. Is this normal behaviour? How could I keep the network connected? The final goal will be to order the bifurcations (topological distance from first vertice/origin) and compute the geometrical distances between them and the origin (where I will need again the degree-2 vertices to compute the original tunnel length).

gCC = read.graph(file="testNetwork.net", format="pajek")

is.connected(gCC)

gCCtree = minimum.spanning.tree(gCC)

tmpDeg = degree(gCCtree)

tmpDeg2 = which(tmpDeg==2)

if (tmpDeg[1]==1) { # conserve node 0, the origin, by all means

tmpDeg2 = tmpDeg2[2:length(tmpDeg2)]

}

gCCtree2 = delete.vertices(gCCtree,tmpDeg2-1)

degree(gCCtree2)

which gives me 0-degree vertices. 

Any help/clarification would be most welcome.

Thanks in advance, Christian.

*Vertices 27

1 "'node'" 0.0699918 0.0010586 1

2 "'node'" 0.0737461 -0.0174669 1

3 "'node'" 0.0651648 -0.010586 1

4 "'node'" 0.0678464 -0.0153497 1

5 "'node'" 0.0576561 -0.0174669 1

6 "'node'" 0.0469294 -0.0142911 1

7 "'node'" 0.043175 -0.0206427 1

8 "'node'" 0.0394207 -0.0174669 1

9 "'node'" 0.0404933 0.00476371 1

10 "'node'" 0.0635558 0.0296408 1

11 "'node'" 0.0871545 -0.00793951 1

12 "'node'" 0.0839365 0.0031758 1

13 "'node'" 0.0769641 0.0111153 1

14 "'node'" 0.0796458 0.0174669 1

15 "'node'" 0.0313756 0.0100567 1

16 "'node'" 0.0120675 0.010586 1

17 "'node'" 0.0265486 0.026465 1

18 "'node'" 0.0292303 -0.015879 1

19 "'node'" 0.0227943 -0.021172 1

20 "'node'" 0.0378117 -0.0476371 1

21 "'node'" 0.0748188 -0.0243478 1

22 "'node'" 0.0962722 -0.0111153 1

23 "'node'" 0.0957359 -0.0015879 1

24 "'node'" 0.0925179 0.0164083 1

25 "'node'" 0.0581924 0.0407561 1

26 "'node'" 0.0640921 0.0497543 1

27 "'node'" 0.046393 0.0524008 1

*Edges

1 2 0.0189021

1 3 0.0126054

1 6 0.0277036

1 9 0.0297302

1 10 0.0292979

1 11 0.0193785

1 12 0.0141045

1 13 0.0122373

2 21 0.00696402

3 4 0.00546665

3 5 0.0101847

6 7 0.00737821

6 8 0.00815268

7 20 0.027522

8 18 0.0103133

9 15 0.0105427

10 25 0.0123416

11 22 0.00965496

12 23 0.0127247

13 14 0.00689451

13 24 0.0164297

15 16 0.0193153

15 17 0.0171036

18 19 0.00833297

25 26 0.0107598

25 27 0.0165778