Prolog Problems

This is a remake of the P-99: Ninety-Nine Prolog Problems collection that I assembled over several years of teaching at the University of Applied Sciences (Berner Fachhochschule) at Biel-Bienne, Switzerland. The collection is structured into seven sections. I have renumbered the problems in order to get more freedom to rearrange things within the sections.

The purpose of this problem collection is to give you the opportunity to practice your skills in logic programming. Your goal should be to find the most elegant solution of the given problems. Efficiency is important, but logical clarity is even more crucial. Some of the (easy) problems can be trivially solved using built-in predicates. However, in these cases, you learn more if you try to find your own solution.

Every predicate that you write should begin with a comment that describes the predicate in a declarative statement. Do not describe procedurally, what the predicate does, but write down a logical statement which includes the arguments of the predicate. You should also indicate the intended data types of the arguments and the allowed flow patterns.

The problems have different levels of difficulty. Those marked with a single asterisk (*) are easy. If you have successfully solved the preceeding problems you should be able to solve them within a few (say 15) minutes. Problems marked with two asterisks (**) are of intermediate difficulty. If you are a skilled Prolog programmer it shouldn't take you more than 30-90 minutes to solve them. Problems marked with three asterisks (***) are more difficult. You may need more time (i.e. a few hours or more) to find a good solution.

You can find solutions of the problems at the end of each section. All programs have been tested on the SWI-Prolog system. SWI-Prolog offers a comprehensive Free Software Prolog environment, licensed under the Lesser GNU Public License. SWI-Prolog is widely used in research and education as well as for commercial applications.

If you find errors or better solutions, please send mail to: werner.hett(at)

First section: Prolog Lists