# How to highlight single edges in an undirected graph?

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laha_M on 8 Oct 2021
Commented: laha_M on 9 Oct 2021
I am plotting an undirected graph where there exist multiple edges between the same nodes. Now I want to highlight a few of the edges. But only a single edge needs to be highlighted (anyone among the multiple edges between the same node). How do I do that?
I am attaching one graph. You can see both the edges get highlighted in red if two edges exist connecting the same nodes. Instead, I want to highlight only one edge (anyone) and keep the rest of the edges unhighlighted. How to do it for an undirected graph?
Thanks.

Christine Tobler on 8 Oct 2021
Do you still want the multiple edges to be displayed? If not, you can use the simplify command to reduce all multiple edges to just one.
Otherwise, here is one way to do this:
g = graph([1 1 2 2 3 3], [2 2 3 3 1 1], 'omitselfloops');
p = plot(g);
highlight(p, 1, 2)
figure;
e = findedge(g, 1, 2)
e = 2×1
1 2
p = plot(g);
highlight(p, 'Edges', e(1))
##### 3 CommentsShow 1 older commentHide 1 older comment
Christine Tobler on 9 Oct 2021
In that case, you'll need to use the second output of findedge, which in the case of multiple edges tells us which node pair each edge id is coming from:
g = graph([1 1 2 2 3 3], [2 2 3 3 1 1], 'omitselfloops');
p = plot(g);
highlight(p, 1, 2);
s = [1 3];
t = [2 1];
figure;
[e, stID] = findedge(g, s, t)
e = 4×1
1 2 3 4
stID = 4×1
1 1 2 2
[~,uniqueSTid] = unique(stID)
uniqueSTid = 2×1
1 3
p = plot(g);
highlight(p, 'Edges', e(uniqueSTid));
laha_M on 9 Oct 2021
Thanks.

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