# Subgraph from selected edges

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Sim on 20 Aug 2021
Commented: Sim on 21 Aug 2021
Hi, is it possible to extract a subgraph by selecting the edges, instead of nodes? If Yes, how?
From the Matlab docs, I can see it is possible by selecting the nodes, but not the edges:
H = subgraph(G,nodeIDs)
H = subgraph(G,idx)

Wan Ji on 20 Aug 2021
Edited: Wan Ji on 21 Aug 2021
Note that all the endnodes of some given edges may be repeated, as commented by @Sim. Unique is a good way to solve such problem, to the example provided by Sim, now the code is modified as
clc; close all; clear all;
s = [1 1 1 1 2 2 2 2 2 2 2 2 2 2 15 15 15 15 15];
t = [2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20];
x = [1 2 0 0 0 1 7 8 9 10 1 2 4 3 5 6 7 8 9 10];
y = [0 1 0 1 2 2 0 1 2 3 4 5 3 5 6 3 4 6 7 8];
G = graph(s,t);
G.Nodes.X = x(:);
G.Nodes.Y = y(:);
EdgeNumber = [1 2 15] ; % the no. of egde
EndNodes = G.Edges.EndNodes(EdgeNumber,:)
H = subgraph(G, unique(EndNodes));
figure
hold on
plot(G,'XData',x,'YData',y,'EdgeColor','blue','LineWidth',1)
h = plot(H,'XData',H.Nodes.X,'YData',H.Nodes.Y,'EdgeColor','red','LineWidth',2);
h.NodeLabel = {}; It is found that this method includes one edge more, i.e. the edge (2,15) in G. This is OK, because what the method provide is the subgraph connected with these 5 nodes.
IF YOU WANT TO get subgraph really by edges. Then
s = [1 1 1 1 2 2 2 2 2 2 2 2 2 2 15 15 15 15 15];
t = [2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20];
x = [1 2 0 0 0 1 7 8 9 10 1 2 4 3 5 6 7 8 9 10];
y = [0 1 0 1 2 2 0 1 2 3 4 5 3 5 6 3 4 6 7 8];
G = graph(s,t);
G.Nodes.X = x(:);
G.Nodes.Y = y(:);
EdgeNumber = [1 2 15] ; % the no. of egde
EndNodes = G.Edges.EndNodes(EdgeNumber,:);
[q, ~, ic]= unique(EndNodes);
p = 1:1:numel(q);
st = p(reshape(ic,size(EndNodes)));
sub_Graph_by_Egde = graph(st(:,1), st(:,2));
sub_Graph_by_Egde.Nodes.X = G.Nodes.X(q,:);
sub_Graph_by_Egde.Nodes.Y = G.Nodes.Y(q,:);
figure
hold on
plot(G,'XData',x,'YData',y,'EdgeColor','blue','LineWidth',1)
h = plot(sub_Graph_by_Egde,'XData',sub_Graph_by_Egde.Nodes.X,...
'YData',sub_Graph_by_Egde.Nodes.Y,'EdgeColor','red','LineWidth',2); I think I have solved your problem
##### 2 CommentsShowHide 1 older comment
Sim on 21 Aug 2021
Fantastic @Wan Ji, you totally nailed it! Very grateful!
Issue solved! :-)

### More Answers (1)

Wan Ji on 20 Aug 2021
hi, friend, you can make EdgeNumber an array that contains all the edge numbers that you are interested in. and make the line H = subgraph(G, EndNodes); changed as H = subgraph(G, EndNodes(:));
Sim on 21 Aug 2021
many thanks @Wan Ji! :)