Re: [igraph] vertex ids and function igraph_vcount(const igraph_t *graph)

[Tamas Nepusz]

Hi Christian,

> Hello everybody
>    if you have a graph with six edges like this:
[...]
> How many memory is reserved for the vertex vector? --> 4 or 30000000  
> positions?

30000000. igraph graphs have numeric vertex IDs starting from zero and  
they are always continuous. The code you sent creates a graph for you  
that has 30000000 vertices and most of them are isolated (they have no  
edges). If you need only 4, you have to create a mapping from your  
numeric IDs to the range [0; 3], remap the vertex IDs in the edges  
vector to this range and possibly add the original numeric IDs as a  
numeric attribute to the vertices if you need them later. For more  
information on using attributes from the C core, see the following page:

http://igraph.sourceforge.net/doc/html/ch09s02.html

Regards,
--
Tamas