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Prolog Programming Concepts

Learn the basics of Prolog, a declarative logic programming language. Explore predicate definitions, facts, rules, and more. Discover applications like expert systems, NLP, and databases.

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Prolog Programming Concepts

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  1. DM552: Part 2 ProgramingLogic Chapter 2: Prolog (Introduction and Basic Concepts) Dr Youcef Djenouri djenouri@imada.sdu.dk 2017-2018

  2. Introduction (1/2) • (imperative, functional, object-oriented, declarative) • Prolog is the logic declarative language. • Alain Colmeraeur & Philippe Roussel, 1971-1973 • With help from theorem proving folks such as Robert Kowalski • Original project: Type in French statements & question • Computer needed NLP and deductive reasoning

  3. Introduction (2/2) • Assists thinking in terms of objects and entities • Not good for number crunching • Useful applications of Prolog in • Expert Systems (Knowledge Representation and Reasoning) • Natural Language Processing • Relational Databases

  4. Basic Concepts (1/10) • Prolog programs are made up of facts and rules. • A fact asserts some property of an object, or relation between two or more objects. e.g. parent(jane,alan). Can be read as “Jane is the parent of Alan.” • Rules allow us to infer that a property or relationship holds based on preconditions. e.g. parent(X,Y) :- mother(X,Y). = “Person X is the parent of person Y if X is the mother of Y.”

  5. Basic Concepts (2/10)

  6. Basic Concepts (3/10) • Names of relationship and objects must begin with a lower-case letter. • Variables must begin with a higher-case letter. • Relationship is written first (typically the predicate of the sentence). • Objects are written separated by commas and are enclosed by a pair of parenthesis. • The full stop character ‘.’ must come at the end of fact and rules.

  7. Basic Concepts (4/10) • Both facts and rules are predicate definitions. • ‘Predicate’is the name given to the word occurring before the bracket in a fact or rule: parent(jane,alan). • By defining a predicate you are specifying which information needs to be known for the property denoted by the predicate to be true. Predicate name

  8. Basic Concepts (5/10) • Predicate definitions consist of clauses. = An individual definition (whether it be a fact or rule). e.g. mother(jane,alan).= Fact parent(P1,P2):- mother(P1,P2).= Rule • A clause consists of a head • and sometimes abody. • Facts don’t have a body because they are always true. body head

  9. Basic Concepts (6/10) • The symbol ‘,’ use as conjunction operator. • Example • Facts: • likes(mary,food). • likes(mary,tea). • likes(john,tea). • likes(john,mary) • likes(mary,X),likes(john,X).

  10. Basic Concepts (7/10) • And: «, » • OR: A new line, • Not: \+ • Equality: == • Anonymous variable: _

  11. Basic Concepts (8/10) • A Prolog program consists of predicate definitions. • A predicate denotes a property or relationship between objects. • Definitions consist of clauses. • A clause has a head and a body (Rule) or just a head (Fact). • A head consists of a predicate name and arguments. • A clause body consists of a conjunction of terms. • Terms can be constants, variables, or compound terms.

  12. Basic Concepts (9/10) mother_of(cathy,maria) An atom alone is a structure too. arguments atom Closing paren. Opening parenthesis Atom at the beginning is called functor

  13. Basic Concepts (10/10) • Clauses from the same predicate must be put in a group: How prolog reacts depends on the implementation mother(…). mother(…). father(…). father(…). mother(…). father(…). mother(…). father(…).

  14. Prolog reasoning • If we have this fact and rule • rainy(london). • rainy(bangkok). • dull(X):- rainy(X). • We can ask (or query) prolog on its command prompt • ?- dull(C). (is there a C that makes this predicate true?) • It will automatically try to substitute atoms in its fact into its rule such that our question gives the answer true • in this example, we begin with dull(X), so the program first chooses an atom for X, that is london (our first atom in this example) • The program looks to see if there is rainy(london). There is! • So the substitution gives the result “true” • The Prolog will answer • C= london • To find an alternative answer, type “;” and “Enter” • It’ll give C= bangkok • If it cannot find any more answer, it will answer “no”

  15. How to ask question • First, write a prolog program in a .pl file. • Then load the file, using a prolog interpreter. Or use the consult command: ?- consult(‘file.pl’). • To exit, use command: halt.

  16. Example (1\8) /* Clause 1 */ located_in(atlanta,georgia). /* Clause 2 */ located_in(houston,texas). /* Clause 3 */ located_in(austin,texas). /* Clause 4 */ located_in(toronto,ontario). /* Clause 5 */ located_in(X,usa) :- located_in(X,georgia). /* Clause 6 */ located_in(X,usa) :- located_in(X,texas). /* Clause 7 */ located_in(X,canada) :- located_in(X,ontario). /* Clause 8 */ located_in(X,north_america) :- located_in(X,usa). /* Clause 9 */ located_in(X,north_america) :- located_in(X,canada).

  17. Example (2/8) • To ask whether atlanta is in georgia: ?- located_in(atlanta,georgia). • This query matches clause 1. So prolog replies “yes”. ?- located_in(atlanta,usa). This query can be solve by calling clause 5, and then clause 1. So prolog replies “yes”.

  18. Example (3/8) ?-located_in(atlanta,texas). this query gets “no” as its answer because this fact cannot be deduced from the knowledge base. The query succeeds if it gets a “yes” and fails if it gets a “no”.

  19. Example (4/8) ?- located_in(X, texas). This is a query for prolog to find X that make the above query true. • This query can have multiple solutions: both houston and austin are in texas. • What prolog does is: find one solution and asks you whether to look for another. • ->

  20. Example (5/8) • The process will look like X = houston More (y/n)? y X = austin More (y/n)? y no Some implementations let you type semicolon without asking any question. Cannot find any more solution

  21. Example (6/8) • To get all the cities in texas: ?-located_in(X,texas), write(X), nl, fail. New line Rejects the current solution. Forcing prolog to go back and substitutes other alternatives for X.

  22. Example (7/8) ?- located_in(austin,X). ?- located_in(X, texas). ?- located_in(X,Y). Gets the names of regions that contain austin. Gets the names of the cities that are in texas. Gets all the pairs that of located_in that it can find or deduce.

  23. Example (8/8) ?-located_in(X,X). Forces the two arguments to have the same value, which will result in a fail.

  24. Classwork parent(michael, cathy). parent(melody, cathy). parent(charles_gordon, michael). parent(hazel, michael). male(michael). male(charles_gordon). female(cathy). female(melody). female(hazel). father(X,Y):- parent(X,Y), male(X). mother(X,Y):- parent(X,Y), female(X).

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