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

Question

Sim 2019-11-6

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/489625-remove-edges-in-a-subgraph-sg-using-nodes-ids-numbers-from-the-graph-g

编辑： Sim 2019-11-8

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)

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Answer 1

Walter Roberson 2019-11-7

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编辑：Walter Roberson 2019-11-7

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.

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Sim 2019-11-7

编辑：Sim 2019-11-8

在 MATLAB Online 中打开

graph_example.mat

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 2019-11-8

编辑：Sim 2019-11-8

在 MATLAB Online 中打开

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');