Artificial Intelligence Introduction to Prolog Prolog Basics - Victoria ...

Comp 307: Artificial Intelligence

Lecture 2:2

Prolog (I) • Introduction to Prolog

COMP307

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Artificial Intelligence

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Facts, rules, queries Matching and backtracking

• The procedural meaning of Prolog – – –

“if” in Prolog “while” in Prolog Recursion in Prolog

Reading: • Prolog documentation on course home page

Xiaoying Gao Victoria University of Wellington

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The SWI-Prolog user’s manual • How to start Prolog on Unix, see 2.1 – edit a file: myfile.pl – command line “prolog” or “swi-prolog” – load a file and compile ?-[myfile].

• How to use it with emacs, see 2.5 Lecture 2:1

Comp 307: Artificial Intelligence

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Lecture 2:3

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Learning Prolog online course

Comp 307: Artificial Intelligence

Introduction to Prolog • A logic programming language human(john). human(mary).

Lecture 2:4

Prolog Basics • Program: –

Facts predicate(arguments).

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Rules Head:- Body.

alive(john). alive(mary). age(mary, 20).

• Arguments: – Constants begin with lower case letter, or in ‘ ’ – Variables begin with upper case letter or _ – Free Variable: _

breathe(X):- human(X), alive(X).

• Queries: the questions that a program answers

?- breathe(mary).

yes

Comp 307: Artificial Intelligence

Lecture 2:5

Comp 307: Artificial Intelligence

Prolog

Java vs Prolog

• Prolog Program – is not a sequence of commands. – like a knowledge base--a collection of assertions. • The Prolog system performs mechanical reasoning based on these assertions. – Matching & Backtracking ?-breathe(john). ?-human(john). yes

yes ?-human(jim). no. ?-human(X).

?-breathe(jim). no ?-breathe(X). X=john; X=mary; no

X=john ; X=mary ; no.

Lecture 2:6

Comp 307: Artificial Intelligence

Lecture 2:7

Procedural vs Declarative

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OOP Event-driven Array While If Recursion Data Fields & local variables Data type Files Input/Output Comments

Comp 307: Artificial Intelligence

The Procedural meaning of Prolog Input/Output, “call a method” in Prolog

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Procedural Numeric Programming = Data Structure + Algorithm

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Declarative Non-numeric, Symbolic Programming = facts + rules

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Java C, C++ Pascal Fortran Basic …

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Prolog

Lecture 2:8

/* rectangle example*/ rectangle_example:write('Enter the width: '), read(W), write('Enter the height: '), read(H), calculate_area(W, H, Area), format("The area is ~w~n", [Area]). calculate_area(W, H, X):X is W*H. |?-rectangle_example.

Comp 307: Artificial Intelligence

Lecture 2:9

Comp 307: Artificial Intelligence

“if” in Prolog

Lecture 2:10

“if– else” in Prolog Multiple clauses and “cut” instead of “if-else”

Multiple clauses instead of “if” rectangle_example:write('Enter the width: '), read(W), write('Enter the height: '), read(H), rectangle_or_square(W, H).

rectangle_example:write('Enter the width: '), read(W), write('Enter the height: '), read(H), rectangle_or_square(W, H).

rectangle_or_square(X, X):write('this is a square'). rectangle_or_square(X, Y):X=\=Y, write('this is a rectangle').

rectangle_or_square(X, X):-!, write('this is a square'). rectangle_or_square(_, _):write('this is a rectangle').

|?-rectangle_example.

|?-rectangle_example.

Comp 307: Artificial Intelligence

Lecture 2:11

“While” and Recursion Loop: Break problem into a sequence of sub-problems, all subproblems are the same. Recursion: Break problem into sub-problems, some of which are smaller versions of the same problem

Every loop is a recursion

Comp 307: Artificial Intelligence

Lecture 2:12

“while” in prolog “Tail Recursion” instead of “while” Base case: stop Normal case: do one step change the arguments call the same clause using different arguments

print_stars(0):-!. print_stars(N):write(‘*’), N1 is N-1, print_stars(N1).

|?-print_stars(5).

Comp 307: Artificial Intelligence

Lecture 2:13

Recursion in Java • n! = 1 * 2 * 3 *...* n n! = n * (n-1)! 0!=1

• Recursive method in Java: a method that calls itself public int factorial(int n) { if (n == 0) return 1; else { int f = factorial (n-1); return n * f; } }

Comp 307: Artificial Intelligence

Recursion in Prolog factorial(0, 1):-!. factorial(N, X):N1 is N-1, factorial(N1, X1), X is N * X1. |?-factorial(4, X). X=24. Base case: do something Normal case: change the arguments call the same clause using different arguments do one step

Lecture 2:14