Route finding

With the complete set of path/2 facts, we can start finding routes between nodes. We will revise what we learnt in the previous two modules.

We can find if there is a route between two immediately adjacent nodes:

     | ?- path(a, 1).

     yes

We can find which path or paths start or end with a particular node:

     | ?- path(2, Node).

     Node = 5 ?

     yes
     | ?- path(Node, 5).

     Node = 2 ?

     yes

We could even try a query such as path(Start, End).

We also saw how we could have more than one goal in a query. For instance we could find if there was a route between two nodes via an intermediate node.

     | ?- path(a, 1), path(1, 2).

     yes

A more interesting query would be one that used a variable to specify the intermediate node:

     | ?- path(a, Via), path(Via, 2).

     Via = 1 ?

     yes

Prolog has instantiated the first goal (path(a, Via)) against the database of facts and thus has to find a second goal of path(1, 2).

If we wanted to traverse n intermediate nodes, we would have to specify n+1 goals in our query.