A Tutoring Lab

for Logic Programming and Artificial Intelligence

Table of Contents

  1. How do I write a Prolog program and execute it ?
  2. Where can I find a Prolog interpreter?
  3. Why special symbols do not appear in the screen ?
  4. What is 'Warning: singleton variable' ?
  5. What do I do if I get 'Action (h for help)' ?

How do I write a Prolog program and execute it ?

Create a directory in your hard disk (e.g. C:\MyPrologPrograms). You can write your Prolog program in any text editor (e.g. notepad). If you are using SWI-Prolog, save it with an extension ".pl" in the directory with your programs, so that it can be recognised as a SWI-Prolog program. Leave the editor open. You may need it.

Click on SWI-Prolog icon. You are prompt with a "?-". Make sure you are working in the appropriate directory (where your program is stored). Type "?-pwd." to get the name of the current directory. If it is different from the one you need then type for example "?-cd('C:\MyPrologPrograms')." If you type "?-ls." you are going to get a list of files contained in this directory.

In order to load the file into the Prolog interpreter use consult/1 and type the name of the file, for example "?-consult(test)" if test has an extention .pl or the complete filename included in single quotes. Alternatively, you could use "?-[test].". Now you can pose any query.
 

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Where can I find a PROLOG interpreter ?

You can download a PROLOG interpreter to be used for running the programs that you will need to implement. We recommend that you download SWI-PROLOG from the site:
http://www.swi.psy.uva.nl/usr/jan/SWI-Prolog.html
In there you will find installation instructions. Installing it is quite straightforward. In case you have any other PROLOG available please contact us to discuss compatibility issues.
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Why special symbols do not appear on the screen ?

The easiest way to get around it is to set the option "Use document specified fonts" in the Preferences/Fonts option, if you are using Netscape Navigator.
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What is "Warning: Singleton Variable" ?

It is just a warning you get from the SWI-interpreter. Singleton is a variable that appears only once in a program clause and therefore there is no reason to have a name (it could have been anonymous "_" instead). However, you are prompt with this warning because you might  have mispelled the name of the variable. For example:
    father(Father,Child):- parent(Father,Child), male(Fathr).
In the above, Fathr is a singleton variable.
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What do I do if I get "Action (h for help)" ?

Well, you type "h" to get a menu for possible actions. Normally, you type "a" for aborting the execution and get back to the "?-". In any case to abort execution type "Ctrl-C".
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P.Kefalas.
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Revised: 12/3/99.