Since we've just met facts and rules, we want to get some practice with using them.

The Rules

The program we wrote in the last tutorial was a fairly small one, so we won't be adding many rules.

Test your program by loading it into Prolog after each modification, and running the following queries against it:

• likes(john,X).
• likes(mary,X).
• likes(Y,food).
• likes(Y,wine).

(Do this now, before you change anything!)

Open the file in the text editor and try adding in rules to express the following:

• John likes anything that Mary likes
• John likes anyone who likes wine
• John likes anyone who likes themselves

Do these one at a time, testing the above queries each time

The Family Tree Example

Suppose that we want to represent a family tree, so that we can ask questions like "is john related to ...", or "list all john's sisters" and so on.

The basic entities will be people; the properties we will want to look at will be father, mother, brother, sister, ..... We choose three basic predicates, male, female and parent, which will describe a family by a series of facts.

Take the following family tree as an example:

                              James I
                                 |
                                 |
                +----------------+-----------------+
                |                                  |
             Charles I                          Elizabeth
                |                                  |
                |                                  |
     +----------+------------+                     |
     |          |            |                     |
 Catherine   Charles II   James II               Sophia
                                                   |
                                                   |
                                                   |
                                                George I

In Prolog we represent this as:

  male(james1).
  male(charles1).
  male(charles2).
  male(james2).
  male(george1).

  female(catherine).
  female(elizabeth).
  female(sophia).

  parent(charles1, james1).
  parent(elizabeth, james1).
  parent(charles2, charles1).
  parent(catherine, charles1).
  parent(james2, charles1).
  parent(sophia, elizabeth).
  parent(george1, sophia).

Start a new file in your text editor (call it "family.pl"), and copy and paste the above program into it.

We can now formulate some queries (try entering these yourself):

• Was George I the parent of Charles I?
  Query: parent(charles1, george1).
• Who was Charles I's parent?
  Query: parent(charles1,X).
• Who were the children of Charles I?
  Query: parent(X,charles1).

Try adding the following rules to the program, and check the results:

• M is the mother of X if she is a parent of X and is female
• F is the father of X if he is a parent of X and is male
• X is a sibling of Y if they both have the same parent.

Remember that "and" in Prolog is represented using a comma.

If you get this done, can you add rules for:

• "sister", "brother",
• "aunt", "uncle",
• "grandparent", "cousin"

Written by James Power
Revised: 7^th October 1998
Contact: James.Power@may.ie