How can directed graphs be constructed from a list of lists of vertex pairs in Mathematica 7 by Graph or by MakeGraph?

Question

I let Mathematica 7 construct a list of lists of vertex pairs, e.g.:

s = { {{1,2},{2,3}}, {{1,2},{1,3}} }

The s of the example contains two directed graphs. How can I say Mathematica 7 to construct the corresponding list of Mathemtica graphs by Graph or by MakeGraph?

Answer 1

Mathematica 7 has no built-in support for graphs. You are probably referring to the separate Combinatorica package, which is described in the book Computational Discrete Mathematics. Do not confuse it with the built-in Graph and related functionality introduced in Mathematica 8.

The function to use from that package is FromOrderedPairs.

FromOrderedPairs[{{1, 2}, {2, 3}}]
(* -Graph[<2,3,Directed>]- *)

ShowGraph[%]

With this function, the graph edges are specified as a list of vertex-index pairs.

I think that using Combinatorica is quite difficult without having a copy of the book I mentioned above. There's some documentation that comes with it, but it's quite bare-bones.