# [graph-tool] How can I know a specific edge from a Graph?

Tiago de Paula Peixoto tiago at skewed.de
Wed Jul 22 09:13:40 CEST 2020

```Am 22.07.20 um 00:12 schrieb spolo96:
> Hello everyone, I'm having a hard time dealing with multiple edges in a graph
> with the use of gt.shortest_path with negative weights. This is a simple
> code that creates a simple graph in order to show my problem:
>
> g70 = gt.Graph()
>
> edge_weight = g70.new_edge_property("double")
> g70.edge_properties["weight"] = edge_weight
>
> edges = [[0,1],[1,2],[0,2],[0,2]]
> weights = [-1,0,-2, 0]
>
> for i in range(len(edges)):
>     g70.ep.weight[e] = weights[i]
>
> for path in gt.shortest_path(g70, 0, 2, weights=g70.ep.weight,
> negative_weights=True):
>     print(path)
>
> gt.graph_draw(g70, vertex_text=g70.vertex_index, edge_text=g70.ep.weight)
>
> As you can see in the image, there are 2 edges from node 0 to node 2, the
> solution that appears before the image specifies: <Edge object with source
> '0' and target '2' at 0x7f2b70d49930> meaning that the shortest path from
> node 0 to node 2 is an edge from node 0 to node 2. However it doesn't
> specify which one of the two edges: (0,2) ->0, (0,2)->-2 is the solution
> edge.

Of course it does. The function returns both the vertices and the edges.
The edge descriptors map to a specific edges, which have their own index
and properties (such as weight). Just do the following:

for e in gt.shortest_path(g70, 0, 2, weights=g70.ep.weight,
negative_weights=True)[1]:
print(e, g70.edge_index[e], g70.ep.weight[e])

which will print:

(0, 2) 2 -2.0

Note that it's always obvious which of the parallel edges get selected:
it's always the one with the smallest weight, otherwise it would not be
part of the shortest path.

Best,
Tiago

--
Tiago de Paula Peixoto <tiago at skewed.de>

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