Event Report 15th Prolog Programming Contest Bart Demoen K.U.Leuven Belgium Editor: Enrico Pontelli After organizing the Prolog Programming Contest 14 times, you would think that we have seen it all. Not so: the 15th PPC had some new surprises. But let's start from the beginning ... Many years ago, it was quite a drag to find teams and at the ICLP welcome reception, we awere always trying to lure people into participating, for instance by trying to make them feel guilty for refusing to participate and also by refuting their petty excuses. At the end of the reception, we usually had enough teams (and drinks). In recent years, there is no more need for that: people show up willingly without having to pressurize them. Again so this year: we had ten teams. All teams had three members, except the team of Paul Tarau: it has one member only: this tradition started at the JICSLP in Manchester 1996.  After the initial hassle just before the contest  starts - people want their laptop connected to power and extension cords are always scarce - the questions are distributed: one set of questions for each team, except for Paul, who can get two copies. Similar for the laptops: it is stricktly one per team, but Paul can use two. And then the teams start programming, or rather, they first tear up the sheets with the problems, so that they can read them independently, and then decide which problem to tackle first. This is important, as speed can make the difference at the end.  Moreover, as soon as a team submits a solution, its members get chocolate. Until very recently, nobody realized that this first solution need not be correct ... Already after 10 seconds, a team pointed out that the "pixels" problem had an error in the example - it couldn't start better. At some point, a team asked whether it was allowed to use setarg/3 for the "pixels" problem: of course that is allowed, but why would you ? Eventually, three teams used setarg/3 and all three were punished by having to resubmit several times. Close to the end, several teams asked whether they could hand in the solution on paper .. now, that is new ! The prescribed method is on a memory stick, but since the keyboard is a bottleneck in this sort of competitions, teams have often more solutions on paper than typed in. So things went pretty normal, until one team decided to withdraw  during the competition: the contest rules do not provide such a withdrawing option, so we refused the withdrawal and disqualified the team. We had to disqualify two other teams for using assert and retract: that is explicitly forbidden - but who reads the rules ? We can't remember ever having disqualified any team before and suddenly this year, there were three! The other teams did quite good, but also made consistently the same errors. For instance, the "chain" problem could be solved by making partitions of a given number, where all members are mutually prime, but the teams invariably choose to generate all permutations of N elements, and then do a costly operation for each of them. That means that strictly speaking, no-one solved that problem, because even for small N, the timeout was reached, or a LOCAL STACK overflow. We were more lenient than that. Another problem was apparently that in the "pixels" problem, the two sentences: each square contains a number larger than (or equal to) 1 and the input always contains an even number of pixels with a particular number larger than 1 seemed to suggest that there would be no ones in the input.   It is great to make fun of programmers under pressure, isn't it ? But really, we have great admiration for anyone willing to risk his/her reputation in such a contest, and so should you. Now for the results: on average the teams did well. Each team solved two problems. During the ICLP banquet, the wrong team of Italians was announced as the runner up. So now for good: the second best team had Agostino Dovier, Andrea Formisano and Enrico Pontelli as members. They solved three problems. Agostino ended 3th last year, so it looks promising for next year. The winning team consisted of Marco Gavanelli, Tom Schrijvers and Peter Stuckey: they solved four problems. Since Peter has been winning this contest so many times now, it is worth analysing his technique and strategy, especially since he hardly knows any Prolog (as one of his team members told me). We have been unable to understand completely how he does it, but here are two essential ingredients: Peter hardly reads the problem statement and he starts programming before he has a clue about its solution. It is the true spirit of extreme programming, which seems to come naturally to him. One day, hopefully, Peter will write up his strategy in more detail! Let us finish by thanking people: Maria Garcia de la Banda and Enrico Pontelli for letting us organise the contest in their conference; Agostino Dovier for provding a good room, a good time slot, and even the only session that started in time; the sponsors of this contest: the ALP and in particular Gopal Gupta through his company Interoperate (http://www.interoperate.biz/). And finally the participants with the guts to subject themselves to potential humiliation. We hope to see you in the future and we promise to have better chocolates next time !