Logic Programming
(PROLOG)
Taif University
Fall 2009
Text Book
Prolog Programming, A First Course,
by Paul Brna, 2001
References:
PROLOG A Relational Language and its Applications,
by John Malpas, Prentice Hall
Using Turbo Prolog, QUE Corporation (For PROLOG),
by Yin K. and Solomon D
Course Description
This course will introduce
The theory of functional and logic based
approaches.
Declarative programming will be
introduced using the Prolog programming
language.
Course Objectives
Students should:
Become familiar with the basic syntax of
Prolog language.
Be able to give a declarative and procedural
reading of a Prolog program.
Be able to pursue any course that makes use of
predicate calculus or Prolog.
Course Policy
Class participation and regular attendance
is expected.
Students are responsible for bringing
themselves up-to-date on class material
and assignments.
Exams will be a combination of material
presented in lectures, and homework
problems.
Homework and assignments should be
completed by the due date.
Student Evaluation
Assignments and Homework 30%
Midterm exam
20%
Final examination
50%
Introduction
Types of programming languages
Imperative languages: Describe the steps that are
required to solve a given problem. The programmer
must know an algorithm that tells the computer what
to do. That is how to get the output from a given
input. (Examples: C++, C#, Pascal, Algol, … etc.)
Declarative languages: Declare the logic by which the
program solves a problem (the logic of problem
solving is declared in the program). The programmer
must know what relations hold between various
entities. (Examples: LISP, PROLOG, … etc.)
PROLOG
PROLOG: PROgramming in LOGic.
It was developed from a foundation of logical
theorem proving and originally used for research
in natural language processing
Its popularity has been mainly in the artificial
intelligence (AI), where it has been used in expert
systems, natural language, and intelligent
databases.
It is based on First Order Predicate Logic
Elements of a PROLOG program
Facts: A fact is an assertion that a particular
relation holds.
Examples on facts
Ali owns the book.
The relationship between Ali and the book is own.
Abdu likes apples.
The relationship between Abdu and apples is like.
Fatima is female.
The relationship between Fatimaand gender is female.
Every mother loves her children.
The relationship between mother and children is love.
Amina is the mother of Ali.
The relationship between Amina and Ali is mother.
Fact as a Relation
Relationship
Object 1
Object 2
Own
Ali
Book
Like
Abdu
Apples
Female
Fatima
Gender
Love
Mother
Child
Mother
Amina
Ali
Elements of a PROLOG program
Rules: A rule is a fact whose truth value
depends on the truth value of other facts..
Examples of rules
Ahmed likes something if Omer likes it.
(i.e. Ahmed likes anything that Omer
likes)
Two persons are sisters if both are female
and they have the same parents.
Suhaib will pass if he gets more than
60%.
Elements of a PROLOG program
Goals: A goal is what is to be determined.
PROLOG attempts to match the goal with the facts
and rules within the program. If the goal is a fact,
PROLOG responds by its truth value. If goal
contains variables, PROLOG responds with values
that constitute a solution.
Examples on goals
What does Abdu like?
What does Ahmed own?
Is Fatima a female?
Are Suha and Ruba sisters?
FACT and RULES = Knowledge Base
Facts: don’t need to be proved,
accepted as is.
Rules: Truth value depends on the
truth value of the facts on which it
depends.
Predicate logic in PROLOG
(i) " Every mother loves her children"
(ii) Amina is the mother of Ali.
From the above we can conclude:
(iii ) Amina loves Ali
We need to define a syntax to express the
facts and formalize the rules:
Predicates
Facts Syntax
Zahra is Female
female(Zahra).
Ali likes apples
likes(Ali, apples).
Ahmed owns the book
owns(ahmed, book).
Predicate name
Predicate arguments (objects)
Predicate End Indicator
Example Rules
Ahmed like anything Omer likes:
likes(Ahmed, X):-
likes(Omer, X).
(:- Stands for if)
X and Y are sisters if: X and Y are female and have same parents:
sister_of(X, Y):female(X),
female(Y),
parents(X, F, M),
parents(Y, F, M).
The (,) stands for and
Example Goals
likes(Ali, X).
owns(ahmed, Y).
female(Fatima).
sister_of(suha, ruba).
sunny.
This one can be a fact, a goal (a query!)