Hi Tiago,
It is consistent with how directionality is implement in graph-tool, which preserves the total number of edges. Furthermore, the concept of multigraph vs simple graph is orthogonal to directed vs undirected graphs.
So this is not really about the clustering coefficient computation.
I understand.
Besides, I think the way the undirected filter works is perfectly intuitive. What you seemed to be expecting was for a simple directed graph to become a simple undirected graph. But in this case how would property maps be handled? Suppose the incoming edge had a weight value and the outgoing edge had another, which one should be kept in the simple graph? How would a directed multigraph be handled when converted to undirected? Should it magically become a simple graph? Wouldn't that be a lot more surprising?
Yes, I was thinking that way because there were no attributes in the current graph, but you are right, of course. For instance igraph provides a keyword for edge coalescence to sum/take the max/other of the edge attributes; is there a similar way of doing this that is already available in graph-tool? Best, Tanguy