How do I find the number of vertices in a graph created by iGraph in python?

Question

I'm writing a function that receives a graph as input. The very first thing I need to do is determine the order of the graph (that is, the number of vertices in the graph).

I mean, I could use g.summary() (which returns a string that includes the number of vertices), but then I'd have parse the string to get at the number of vertices -- and that's just nasty.

To get the number of edges I'm using len(g.get_edgelist()) , which works. But there is no g.get_vertexlist() , so I can't use the same method.

Surely there is an easy way to do this that doesn't involve parsing strings.

Answer 1

g.vcount() is a dedicated function in igraph that returns the number of vertices. Similarly, g.ecount() returns the number of edges, and it is way faster than len(g.get_edgelist()) as it does not have to construct the full edge list in advance.

Answer 2

As some functions in igraph have been renamed in meantime, I found the answers here out of date. What the docs suggest now is calling

gorder(g)

which works for me. Analogy for ecount is

gsize(g)   # vcount(g) still works, but not g.vcount()

It's useful to note that help pages are cleverly redirected, so

?vcount

brings you to gorder docs etc.

Answer 3

g.vs should return the sequence of vertices as an igraph.VertexSeq object:

>>> from igraph import Graph
>>> g = Graph.Formula("A-B")
>>> g.vs["name"]
['A', 'B']
>>> len(g.vs)
2
>>> g.vcount()
2

Edit: As @Tamas mentions below, g.vcount() will also return the number of vertices. Example edited to account for this.