Remove edges in a subgraph SG using nodes IDs (numbers) from the graph G

Hi, I have a graph defined by node pairs (node IDs): s = [1 1 2 4 7 ...] t = [2 5 3 5 6 ...] Since the graph is huge and I would need to work only on a portion of it, I extracted a subgraph.

If I am not wrong, it looks like that subgraph is reordering the node IDs (right?), therefore, when I try to remove edges (and nodes) from the subgraph, by using the same node IDs of my graph contained in s = [...] and in t = [...], I get an error.. Any idea to fix this small issue? (I also tried to assign and employ "nodenames"..but without success)

Answer 1

[Walter Roberson]

Use node names instead of node numbers when you create the graph. Then in the sub-graph you can use findnode(). Or just give the node names directly in the rmedge() call.

[Sim]

Thanks a lot Walter! However, do you think there could be still a way to use the node numbers (node IDs), instead of node names as you kindly suggested? ...I tell you this since - in my application - I use the node IDs to "communicate" with another software... therefore, not using those node IDs, i.e. numbers, it would be quite complicated/difficult for me....Many thanks for your support!

[Walter Roberson]

Use string() on the node numbers to get node IDs that are, for example,

s = ["1" "1" "2" "4" "7"]

Then do the manipulation based on these node "names". When it comes time to communicate with outside software based upon numeric node IDs, extract the node names and str2double() them to get the numeric values.

[Sim]

Thanks Walter! ....I was trying to follow your indications by converting the arrays s and t using string()

s = string(s);
t = string(t);

.. but I think I got stuck.... Probably it is better if I show my initial attempt with an example, and then, where I got stuck....

Initial attempt

First I create a graph G:

% Load s, t, x, y, nodenames and create a graph G
load('graph_example.mat') % attached in this post
G = graph(s,t,[],nodenames2);

% Plot the graph G
h = plot(G,'XData',x,'YData',y,'linewidth',2,'MarkerSize',2);
% just for a clearer plot..
h.NodeLabel = [];
text(x,y, nodenames2, 'FontSize',12, 'FontWeight','bold', ...
    'HorizontalAlignment','left', 'VerticalAlignment','top');

....here below shown:

Then, I select the nodes composing the subgraph SG and I create the subgraph:

% Select nodes to keep for the subgraph and create the subgraph SG
nk = [45 24 32 35 10 48 39 4 26 42 5 25 14 2 29 34 41]; % nodes to keep
SG = subgraph(G,nk);

% Plot subgraph SG, together with the graph G
figure
hold on
h1 = plot(G,'XData',x,'YData',y);
h2 = plot(SG,'XData',x(nk),'YData',y(nk),'linewidth',2,'MarkerSize',2);
hold off
% just for a clearer plot..
h1.NodeLabel = [];
h2.NodeLabel = [];
text(x,y, nodenames2, 'FontSize',12, 'FontWeight','bold', ...
    'HorizontalAlignment','left', 'VerticalAlignment','top');

...which looks like this:

As third and last step, I select the edges that I want to remove and I try to plot the subgraph SG without the selected edges...

% Using the node IDs of G, select edges to be removed in SG: [2 29], [2 42], [5 42], etc..
s_remove = [2 2 5 10 10 10 42];
t_remove = [29 42 42 24 42 48 48];

% Remove edges in SG
SGrmedge = rmedge(SG,s_remove,t_remove);

% Plot the subgraph SG without selected edges
figure
h = plot(SGrmedge,'XData',x,'YData',y,'linewidth',2,'MarkerSize',2);
% for a clearer plot
h.NodeLabel = [];
text(x,y, nodenames2, 'FontSize',15, 'FontWeight','bold', ...
    'HorizontalAlignment','left', 'VerticalAlignment','top');

but I get this error:

Error using graph/validateNodeID (line 529)
Numeric node IDs must be positive integers not greater than the number of nodes in the graph (17).

Error in graph/findedge (line 70)
    s = validateNodeID(G, s, true); % true: allows categorical

Error in graph/rmedge (line 49)
    ind = findedge(G, s, t);

Error in graph_example (line 62)
Grmedge = rmedge(SG,s_remove,t_remove);

Second attempt using string() for arrays s and t

I use string() for arrays s and t:

% Load s, t, x, y, nodenames and create the graph G
load('graph_example.mat')
s = string(s);
t = string(t);
G = graph(s,t,[],nodenames2);

and I get this error...

Error using matlab.internal.graph.constructFromEdgeList (line 136)
Edge list contains a name not present in the node names argument.

Error in graph (line 300)
                matlab.internal.graph.constructFromEdgeList(...

Error in graph_example (line 32)
G = graph(s,t,[],nodenames2);

Would you be so nice to tell me where/what I am doing wrong please? Any further hint?

[Sim]

I think I found a workaround to my problem....

% Using the node IDs of G, select edges to be removed in SG: [2 29], [2 42], [5 42], etc..
s_rm = [2 2 5 10 10 10 42];
t_rm = [29 42 42 24 42 48 48];

% Convert the node IDs of G (contained in s_rm and in t_rm) into node IDs of SG
sgn = str2double(table2array(SG.Nodes));
conversion = [sgn (1:length(sgn))'];
[~,s_rmcv] = ismember(s_rm, conversion(:,1));
[~,t_rmcv] = ismember(t_rm, conversion(:,1));

% Remove edges in SG
SGrmedge = rmedge(SG,s_rmcv,t_rmcv);

% Plot the sub-graph SG without selected/removed edges, together with graph G
figure
hold on
h1 = plot(G,'XData',x,'YData',y);
h2 = plot(SGrmedge,'XData',x(sgn),'YData',y(sgn),'linewidth',2,'MarkerSize',2);
hold off
% for a clearer plot
h1.NodeLabel = [];
h2.NodeLabel = [];
text(x,y, nodenames2, 'FontSize',12, 'FontWeight','bold', ...
    'HorizontalAlignment','left', 'VerticalAlignment','top');

...which results as :

BTW, any other solution is really welcome! Thanks a lot :)