1. Prolog programming Introduction to Prolog (part3)
CS 370

2. How to solve this problem?
Consider the following facts:
parent(pam,bob).
parent(tom,bob).
parent(tom,liz).
parent(bob,ann).
parent(bob,pat).
parent(pat,jim).
Is Liz a child of Tom?
?- child(liz, tom).
The answer will be “error “
WHY ? Because there are no facts in the program about a child.

3. Defining relations by rules.
To solve the previous child(liz, tom) relation problem
we have to either to :
Use the parent facts ( which were defined previously ) ,considering the parent fact as the inverse of child relation.
Ex : ?- parent (tom ,liz) . OR
Use the alternative way based of the following logical statements:
For all X and Y,
Y is a child of X if X is a parent of Y.

4. Defining relations by rules.
In prolog , we can define the offspring relation using the alternative logical statement in the following way:
child(Y,X) :- parent(X, Y)
This clause is called a rule
Each rule has two parts:
Condition. (The right hand side)
Conclusion. (The left hand side)
So, the child rule consist of :
child(Y,X) -> as a conclusion part.
parent(X,Y) > as a condition part.

5. Defining relations by rules.
The differences between rule and fact:
Fact:
is a something that always, unconditionally true.
Rule:
specify things that are true if some condition is satisfied.
It has two parts : condition and conclusion separated by “ :- “.

6. Defining relations by rules.
How rules are actually used by prolog program?
?- child(liz, tom).
X= tom, Y= liz.
The compiler will search for the initial goal which is : chlid(liz,tom).
If the rule was defined , the condition part will be executed: parent(tom,liz).
The condition can be found using the facts about parent relation.
After finding that the condition part is true, the conclusion part will be also true.

7. Anonymous Variables Do somebody have a child? Who is a parent ?
Example : Does Bob have a child ?
-> hasachild( X ) :- parent( X, _).
Do somebody have a child?
somebody_has_child :- parent( _, _).
somebody_has_child :- parent( X, X).
Who is a parent ?
?- parent( X, _).
In this query we are interested in people who have children, but not in the names of the children.

8. Lexical Scope
The lexical scope of A variable is one clause.
If X occurs in two clauses, then it signifies two different variables.
hasachild(X) :- parent( X, Y).
isapoint(X) :- point( X, Y, Z).
But each occurrence of X with in the same clause means the same variables.
hasachild( X) :- parent( X, Y).
An atom is always the same object in any clause throughout the whole program.

9. Class exercise(2)
Write a rule to describe the grandparent relation?

10. Home exercise(1)
Write a rule to describe the sister relation?