Programming Paradigms and Languages Introduction to Prolog. dr Robert Kowalczyk WMiI UŁ

Transcription

1 Programming Paradigms and Languages Introduction to Prolog dr Robert Kowalczyk WMiI UŁ

2 Prolog programming Prolog is a general purpose logic programming language associated with artificial intelligence and computational linguistics. dr Robert Kowalczyk WMiI UŁ 2

3 Origins of Prolog The Prolog is in the 70s of the twentieth century as a programming language used for automatic analysis of natural languages. Particularly well suited for programs related to artificial intelligence. Prolog in contrast to most of the popular languages is a declarative language. dr Robert Kowalczyk WMiI UŁ 3

4 Programming in logic Prolog language is an example of logic programming language. In this programming paradigm is administered according to a set, and the result of a program is proof of the theorem on the basis of these relationships. In other words, the result of the program is based on the relationships that define the program, and not a way to obtain these data. Writing a program in Prolog is not to describe the algorithm. Language of the fifth generation. dr Robert Kowalczyk WMiI UŁ 4

5 Prolog interpreter SWI-Prolog is an open source implementation of the programming language Prolog, commonly used for teaching and semantic web applications. It has a rich set of features, libraries for constraint logic programming, multithreading, unit testing, GUI, interfacing to Java, ODBC and others, literate programming, a web server, SGML, RDF, RDFS, developer tools (including an IDE with a GUI debugger and GUI profiler), and extensive documentation. dr Robert Kowalczyk WMiI UŁ 5

6 Main rules in Prolog programs Running PROLOG interpreter: plwin.exe (in Windows system) Each line has to finish with. sign: write('ala and cat'). To finish working with Prolog we need to write: halt. Scripts in Prolog are text files with extensions.pl:.pl Comments in Prolog we will do with % sign: % something dr Robert Kowalczyk WMiI UŁ 6

7 First program 1. Write file script.pl: % Simple database like(janek,zosia). like(zosia,tomek). like(tomek,marek). like(janek,magda). 2. Load file: [script.pl]. or [path to file script.pl']. 3. Check your path and eventually change your local directory with commands: pwd. cd. dr Robert Kowalczyk WMiI UŁ 7

8 1. Ask if Janek likes Zosia? Query to our first program?- like(janek,zosia). true 2. If Janek likes Tomek??- like(janek,tomek). false 3. Who likes Zosia??- like(x,zosia). X = janek. 4. Who does Janek like??- like(janek,x). X = zosia; X = magda. To get both answers we have to use the operator "; which in fact means or" in Prologu. dr Robert Kowalczyk WMiI UŁ 8

9 Additional commands - 1 Command listing. We can display the content of the loaded database, and with command: listing(predicate). Information about predicate, e.g.: listing(like). To get help we can use the command: help. And help with the exact predicate using: help(predicate). For example help(write). dr Robert Kowalczyk WMiI UŁ 9

10 Additional commands - 2 Pay attention to the predicate which in form of: for example predicate/n write/1 write/2 max/2 We can also use apropos(word). Which will cause the display of all the passwords (predicates) in which the word predicate occured. dr Robert Kowalczyk WMiI UŁ 10

11 Additional commands - 3 An interesting option is the possibility to use an emac editor which among others can colour syntax. We can start it with the command: emacs. or emacs(script). and also from the window SWI-Prolog: File/Edit script dr Robert Kowalczyk WMiI UŁ 11

12 Prolog example continue Lets go back to our script. We know that the question:?- like(janek,zosia). gives: true What about the question:?- like(zosia,janek). Of course it gives: false Shouldn t it give true. If one likes the other we can assume that the other likes the first person too. But it doesn t work in Prolog like that. We can correct the script like this. dr Robert Kowalczyk WMiI UŁ 12

13 Prolog example continue We can add to the script: like(zosia,janek). like(tomek,zosia). like(marek,tomek). like(magda,janek). The second way (the better one) is to add rules: like(x,y) :- like(y,x). From now on if person A likes person B, then person B also likes person A. Lets assume that we live in such a perfect world that even if person A likes person B and person B also likes person C then A likes C. What kind of rule should we now define to our database so that everyone starts liking each other. Maybe this: like(x,y) :- like(x,z), like(z,y). dr Robert Kowalczyk WMiI UŁ 13

14 Any Quetions? dr Robert Kowalczyk WMiI UŁ 14