Re: [igraph] Question about fastgreedy.community

[Tamás Nepusz]

> At gengraph_mr-connected.cpp:146 : Cannot make a connected graph from
> the given degree sequence, Invalid value
> 
> Do you have any method to fix it?
You are trying to generate a graph from a degree sequence for which there 
exists no connected simple graph that satisfies the degree sequence. In other 
words, if you see this message, just generate a new degree sequence.

By looking at your code, I think that a possible way for this to happen is if 
deg[1] is equal to N-1 and the sum of degrees is odd. In this case, you add 1 
to deg[1], making it equal to N, and it is of course impossible to generate a 
simple graph on N vertices where a vertex has degree N. Of course there are 
many other possible ways to fail. The Erdos-Gallai theorem describes the 
necessary and sufficient condition for the degree sequence you are generating:

http://en.wikipedia.org/wiki/Erd%C5%91s%E2%80%93Gallai_theorem

When running degree.sequence.game, the VL algorithm first checks whether your 
degree sequence satisfies the conditions of the Erdos-Gallai theorem, and 
returns the error message you see if it does not.

-- 
T.