Artificial Intelligence CS 370 D Prolog programming Introduction

Artificial Intelligence CS 370 D Prolog programming Introduction to Prolog 1

INTRODUCTION TO PROLOG = PROgramming in LOGic n we defines facts and rules and give this to the logic program n Ask it what we want to know n It will look and reason, using available facts and rules, and then tells us an answer (or answers) 2

INTRODUCTION TO PROLOG = PROgramming in LOGic PRO LOG v Three basic mechanisms among others: 1. Pattern Matching, 2. Tree based data structuring 3. Back-Tracking. v Suitable for problems that involve structured objects and relations between them. v It allows symbolic computation. v Examples: 1. If object X is closer to the observer than object Y and object Y is closer than object Z than X is closer than Z. 3

INTRODUCTION TO PROLOG Prolog basic concepts by an example: n Ali Leyla Nour Omar Meriam khaled Zahra Let’s consider the Parent relation 4

INTRODUCTION TO PROLOG q DEFINING RELATIONS BY FACTS v This relation can be defined by the following Prolog proram: parent(leyla, omar). parent(ali, omar). parent(omar, meriam). parent(omar, khaled). parent(ali, nour). parent(khaled, zahra). v This program consists of 6 Clauses. Each clause declares one fact about the relation parent. v The clause parent(omar, meriam). Is an instance of the relation parent. A relation is defined as a set of instances. 5

INTRODUCTION TO PROLOG What can we do with this program? Let’s ask the system questions about the relation parent: Ø Question: is omar parent of khaled? In prolog ? - parent(omar, khaled). ü Answer: yes 6

INTRODUCTION TO PROLOG Ø Question: is leyla parent of meriam? In prolog ? - parent(leyla, meriam). ü Answer: no Ø Question: who is zahra parent? In prolog ? - parent(X, zahra). The system tells what is the value of X for which the statement is true. ü Answer: X= khaled 7

INTRODUCTION TO PROLOG Ø Question: who are omar children? In prolog ? - parent(omar, X). ü Answer: X= meriam X= khaled no Ø Question: Who is parent of whom? In prolog ? - parent(X, Y). ü Answer: X= leyla Y=omar; X=ali Y=omar; X=omar Y=meriam; X=omar Y= khaled; X=ali Y=nour; X=khaled y=zahra; 8

INTRODUCTION TO PROLOG Ø Question: who is grandparent of khaled? In prolog ? - parent(X, khaled) parent(Y, X). Note: the logical meaning remains the same if we change the order of the two requirements. ü Answer: X= omar Y= ali; X= omar Y=leyla Ø Question: Who are ali grandchildren? In prolog ? - parent(ali, X), parent (X, Y). ü Answer: X= omar y=khaled; X=omar Y=meriam; 9

INTRODUCTION TO PROLOG Ø Question: Do meriam and nour have a common parent? In prolog ? - parent(X, meriam) parent(X, nour). First who is parent of meriam X ? Second is (this same) X parent of nour ? ü Answer no 10

INTRODUCTION TO PROLOG n What can we conclude from this example: v It is easy in Prolog to define a relation such as parent by stating the n-tuples of objects that satisfy the relation. In this case we have defined a relation by a set of facts represented by clauses. v We can easily express queries in Prolog. v A Prolog program consists of clauses. Each clause terminates with a full stop. v The arguments of relations can be: concrete objects, constants, general objects such as X and Y. v Questions to the system consist of one or more goals. The answer can be positive (goal satisfiable) or negative (goal unsatisfiable). v If several answers satisfy the question than Prolog will find as many of them as desired by the user. 11