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Is there a very short expression in iGraph 0.6 for python 2.7 to see if two vertices specified by index are connected by an edge or not?

I found somewhere:

are_connected(v1, v2)

but in python I would get an error message: "NameError: global name 'are_connected' is not defined"

The above expression could be for R or just totally wrong. I don't know. R is not enough for what I'm trying to do with my project.

My graph is undirected and has many sequences of vertices and edges (vs and es) described in this tutorial: http://hal.elte.hu/~nepusz/development/igraph/tutorial/tutorial.html

Update: I've found http://packages.python.org/python-igraph/igraph.GraphBase-class.html#is_multiple is_multiple and is_mutual and I think each of them could do the trick, yet I still get the error: "NameError: global name 'are_mutual' is not defined".

On the internet I couldn't find an example of how to implement it correctly. I'm still looking.

TylerH
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Laci
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  • I've not heard of `igraph`; the standard graph library for Python IME is `networkx`. It has [`connected_components`](http://networkx.lanl.gov/_modules/networkx/algorithms/components/connected.html) – Katriel Dec 13 '12 at 08:42
  • CONNECTION IS DIFFERENT FROM ADJACENCY – Tommaso Guerrini Apr 22 '20 at 11:56

3 Answers3

15

For the record: are_connected (and also is_mutual and is_multiple that the poster has mentioned) are methods of the graph itself and not functions on their own, so the correct way to use them is as follows:

>>> g = Graph.GRG(100, 0.2)
>>> g.are_connected(0, 2)
False
Tamás
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  • in igraph 0.7.1.post6 this function tells whether the vertices are adjacent, not connected. Just saying that maybe a more proper name could be are_adjacent – Tommaso Guerrini Apr 22 '20 at 11:52
13

GraphBase class has function get_eid(v1, v2, directed=True, error=True) that returns arbitrary edge between vertices specified by their indices. In you call it like this:

g.get_eid(v1, v2, directed=False, error=False)

it will return -1 if the vertices are disconnected, and some edge otherwise.

Andrei
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    @phant0m, why is it so terrible? – Andrei Dec 13 '12 at 08:53
  • This worked perfectly for what I wanted to do. Mersi Andrei. This is the solution I was looking for. – Laci Dec 13 '12 at 09:21
  • @Andrei It's just unusual for a Python API to return error codes. – phant0m Dec 13 '12 at 09:32
  • @phant0m: if `error=False` is not given, `get_eid` raises an exception as expected. `error=False` explicitly requests `get_eid` to return -1 instead of raising an exception. – Tamás Dec 13 '12 at 10:16
  • @Tamás Interesting... In that case, I'd prefer False or better yet None over -1 though. – phant0m Dec 13 '12 at 10:41
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    @phant0m: the function returns -1 because most of igraph is written in C and the C code behind `get_eid` (i.e. `igraph_get_eid`) returns -1. I admit that `False` or `None` would be more natural. – Tamás Dec 13 '12 at 11:47
0

I have never heard about that module but, anyway, it seems like it is an import problem, try to import that function from the module, i.e.:

from igraph import are_connected

Otherwise, python will not recognize it. Another possibility is that the function must be called from a graph object you have declared first:

from module import MyGraphObject
...
MyGraphObject.are_connected(...)
juankysmith
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  • I will keep in mind the {from module import MyGraphObject ...} solution because for sure I will run in to many similar problems during my project, but first I did go with the idea of Andrei and it worked easy for me. Thank you anyway! – Laci Dec 13 '12 at 09:24