Question

I need to construct directed graph (at runtime) with cycles in Prolog and I am not sure know how to represent it. My requirement is that I need to get from one vertex to his neigbour in a constant time.

Is it possible to represent it as a tree, e.g.:

t(left_son,V,right_son)

but how to solve the cycles?

I can make a list of edges:

graph([a,b,c,d],[e(a,b),e(b,c),e(c,a),e(c,d)])

OR just

[a->[b],b->[c],c->[a,d],d->[]]

but how to avoid calling function ‘member’ on list while searching for neighbours, which cost linear time?

Thanks for your help

Answer 1

If your graph has less vertices than max arity allowed by your Prolog (for instance SWI-Prolog has unlimited arity), you could represent the edges as indices of arguments:

could be

G = graph(a-[2],b-[3],c-[1,4],d-[2]).

then use arg(Edge, G, Vert-Edges) to access neighbours.

Of course any Prolog will let you describe the graph using facts. This is also the preferred way for very large graphs.

:- dynamic edge/2.

edge(a,b).
edge(b,c).
edge(c,a).
edge(c,d).
edge(d,b).

edit The edge/2 relational representation also get the (potentially large) benefits of automatic indexing.

more edit I totally forgot RDF and Semantic Web. Really powerful, we are going toward that.