Artificial Intelligence

Transcription

1 Artificial Intelligence Dr Ahmed Rafat Abas Computer Science Dept, Faculty of Computers and Informatics, Zagazig University

2 Programming in Prolog An Introduction

3 What is Prolog? PROgramming in Logic. High-level interactive language. Logic programming language. Based on Horn Clauses (parent(x,z) ancestor(z,y)) ancestor(x,y)

4 What is Prolog? Programming languages are of two kinds: Procedural (BASIC, ForTran, C++, Pascal, Java); Declarative (LISP, Prolog, ML). In procedural programming, we tell the computer how to solve a problem. In declarative programming, we tell the computer what problem to be solved. In Prolog, we are often forced to give clues to the solution method.

5 What is Prolog used for? Good at Grammars and Language processing, Knowledge representation and reasoning, Unification, Pattern matching, Planning and Search. i.e. Prolog is good at Symbolic AI. Poor at: Repetitive number crunching, Representing complex data structures, Input/Output (interfaces).

6 Basic Elements of Prolog The program is a database of facts and rules. Some are always true (facts): father( john, jim). Some are dependent on others being true (rules): parent( Person1, Person2 ) :- father( Person1, Person2 ). To run a program, we ask questions about the database.

7 Prolog in English Example Database: John is the father of Jim. Jane is the mother of Jim. Jack is the father of John. Person 1 is a parent of Person 2 if Person 1 is the father of Person 2 or Person 1 is the mother of Person 2. Person 1 is a grandparent of Person 2 if some Person 3 is a parent of Person 2 and Person 1 is a parent of Person 3. Example questions: Who is Jim's father? Is Jane the mother of Fred? Is Jane the mother of Jim? Does Jack have a grandchild?

8 Prolog in Prolog Example Database: John is the father of Jim. Jane is the mother of Jim. Jack is the father of John. Person 1 is a parent of Person 2 if Person 1 is the father of Person 2 or Person 1 is the mother of Person 2. Person 1 is a grandparent of Person 2 if some Person 3 is a parent of Person 2 and Person 1 is a parent of Person 3. Example questions: Who is Jim's father? Is Jane the mother of Fred? Is Jane the mother of Jim? Does Jack have a grandchild? Example Database: father( john, jim ). mother( jane, jim ). father( jack, john ). parent( Person1, Person2 ) :- father( Person1, Person2 ). parent( Person1, Person2 ) :- mother( Person1, Person2 ). grandparent( Person1, Person2 ) :- parent( Person3, Person2 ), parent( Person1, Person3 ). Example questions:?- father( Who, jim ).?- mother( jane, fred ).?- mother( jane, jim ).?- grandparent( jack, _ ).

9 Visual Prolog Visual Prolog is object oriented, strictly typed and mode checked. We will focus on the core of the code, i.e. the Prolog code when disregarding classes, types and modes. For this purpose we will use the PIE example that is included in the Visual Prolog distribution. PIE is a "classical" Prolog interpreter, by using this you can learn and experiment with Prolog without at all being concerned with classes, types, etc.

10 Horn Clause Logic Visual Prolog and other Prolog language dialects are based on Horn Clause logic. Horn Clause logic is a formal system for reasoning about things and the way they relate to each other. In natural language I can express a statement like: John is the father of Bill. In Horn Clause Logic this could be formalized as: father( Bill, John ). father is a predicate/relation taking two arguments, where the second is the father of the first.

11 A rule like this: X is the grandfather of Z, if X is the father of Y and Y is the father of Z Could formalized in Horn clause Logic as: grandfather(person,grandfather):- father(person,father), father(father,grandfather). A theory is a collection of facts and rules.

12 Example of a theory: father("bill", "John"). father("pam", "Bill"). grandfather(person, GrandFather) :- father(person, Father), father(father, GrandFather).

13 It s required to answer the following questions: Is John the father of Sue? Who is the father of Pam? Is John the grandfather of Pam?... Such questions are called goals. They can be formalized as follows:

14 ?- father("sue", "John").?- father("pam", X).?- grandfather("pam", "John"). Such questions are called goal clauses or simply goals. Together facts, rules and goals are called Horn clauses, hence the name Horn Clause Logic. Some goals like the first and last are answered with a simple yes or no. For other goals like the second we seek a solution, like X = "Bill".

15 Some goals may even have many solutions. For example:?-father(x,y). has two solutions: X = "Bill", Y = "John". X = "Pam", Y = "Bill". A Prolog program = a theory + a goal. When the program starts it tries to find a solution to the goal in the theory.

16 Prolog Inference Engine (PIE) Now we will try the little example above in PIE, the Prolog Inference Engine that comes with Visual Prolog. Before we start you should install and build the PIE example. Select "Install Examples" in the Windows start menu (Start -> Visual Prolog -> Install Examples). Open the PIE project in the IDE and run the program. When the program starts Select File -> New and enter the father and grandfather clauses above:

17 //FILE0.PRO father("bill", "John"). father("pam", "Bill"). grandfather(person, GrandFather) :- father(person, Father), father(father, GrandFather). to save the contents use File -> Save. File -> Consult will load the disc contents of the file. Once you have "consulted" the theory, you can use it to answer goals. On a blank line in the Dialog window type a goal (without the?- in front). grandfather(x,y).

18 When the caret is placed at the end of the line, press the Enter key on your keyboard. PIE will now consider the text from the beginning of the line to the caret as a goal to execute. You should see a result like this: X= Pam. Y= John:.

19 Extending the family theory