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How to add an edge between two nodes of two different graphs?

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Asaf McRock
Asaf McRock on 15 Jan 2021
Edited: Asaf McRock on 3 Feb 2021 at 19:52
I'm trying to connect the nodes of graph g (Blue) with the nodes of graph h (Red).
so=[1 1 1 2 2 2 2 3 3 3 5];
ta=[2 3 4 3 4 5 6 6 7 5 7];
g=graph(so,ta);
p=plot(g);
x=p.XData;
y=p.YData;
z2=[0 0 0 0 0 0 0];
subplot(2,2,1);plot(g,'XData',x,'YData',y,'ZData',z2)
s=[1 1 1 2 2 3 3 3 3 4 4 5 5 ];
t=[2 5 6 3 4 7 4 5 6 7 5 7 6 ];
h=graph(s,t);
z=[-3 -3 -3 -3 -3 -3 -3];
hold on; subplot(2,2,1);plot(h,'XData',x,'YData',y,'ZData',z)
Is it something doable?
Thank you all for your continuous support!

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Accepted Answer

Christine Tobler
Christine Tobler on 18 Jan 2021
You could also add the edges of both graphs to one larger graph, and then use addedge to connect them. You would probably not get a clear separation visually of what used to be the first graph and the second graph this way, though.
so=[1 1 1 2 2 2 2 3 3 3 5];
ta=[2 3 4 3 4 5 6 6 7 5 7];
g=graph(so,ta);
p=plot(g);
x=p.XData;
y=p.YData;
z2=[0 0 0 0 0 0 0];
s=[1 1 1 2 2 3 3 3 3 4 4 5 5 ];
t=[2 5 6 3 4 7 4 5 6 7 5 7 6 ];
h=graph(s,t);
z=[-3 -3 -3 -3 -3 -3 -3];
gh = graph([so, s+7], [ta, t+7])
gh = addedge(gh, 2, 4+7)
plot(gh,'XData',[x, x],'YData',[y, y],'ZData',[z2, z2-3])

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Asaf McRock
Asaf McRock on 29 Jan 2021
Thanks for your reply, Dr. Tobler! Much appreciated.
I read a very nice scientific paper recently about temporal/ dynamic graph which was written by folks from twitter. And I like the idea that a social network ( graph) evolve with time. Users are nodes, relationship between users are the links of the graph. A new user could join the network at one time step and another one might leave at different time step. They use edge weights to represent time steps, and this was new to me and I didn't fully understand it. So, I said maybe it's better for me to play with some MATLAB code about dynamic graph, you know, just to have a better understanding.
To be quite honest with you I could not understand points a and b you mentioned, I'd appreciate if you can please elaborate, perhaps by a simple line of code.
Thanks again!
Christine Tobler
Christine Tobler on 2 Feb 2021
Here are some small examples of what I meant
a)
graphs = {graph(zeros(10)};
for ii=1:10 % time steps
graphs{ii+1} = addedge(graphs{ii}, randi(10), randi(10));
end
This adds a random new edge to the graph at each timestep, and stores the graphs for each timestep in the cell array graphs.
b)
Let's say we have a graph representing a train network, where each edge is the connection between two places. If the dynamic is only about how many trains are running each edge per day, this can simply be stored in a matrix numTrains(edgeID, dayID). If a connection isn't used at all in a day, instead of removing the edge we can simply set that entry in numTrains to zero.
This only works for simpler cases, but in those cases it makes manipulating the data much easier than working with a cell array of graphs.
Asaf McRock
Asaf McRock on 3 Feb 2021 at 19:50
OMG! I wonder when can I reach this level of creativity.
Very interesting small examples indeed. I will start practicing and learning from here.
Thanks for your time and continuous support!

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More Answers (1)

Asaf McRock
Asaf McRock on 16 Jan 2021
v1=[x(1) x(1)];
v2=[y(1) y(1)];
v3=[z2(1) z(1)];
j1=[x(2) x(4)];
j2=[y(2) y(4)];
j3=[z2(2) z(4)];
line(v1,v2,v3,'Color','red','LineStyle','--')
line(j1,j2,j3,'Color','red','LineStyle','--')

  1 Comment

Asaf McRock
Asaf McRock on 16 Jan 2021
It's not possible to add an edge between nodes of different graphs. So, this is the only way, I guess, to visualize an interlink between nodes of different graph. It might be inefficient though especially for larger number of nodes.

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