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| Syntactic Confectionery Delight | |
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Actually I think this is an isomorphism of Lewis Carroll's Zebra Puzzle.
You can read about the puzzle in Krzysztof Apt's lecture notes: http://www.cwi.nl/~apt/constraints/notes1.ps It starts at the bottom of page 8, which is really the 10th page if you include the two title pages... I solved this puzzle in two different ways in Prolog and Lisp. In Prolog I chose to first specify all the constraints on the domains of the five values, every constraint shrinking the domains and in the end you're left with only one solution. In Lisp I used streams to generate all permutations in a lazy way and I filtered out all the possibilities that clashed with the rules in the same way you'd sieve primes from the stream of integers. The biggest choice I think is if you see the house number as a value certain variables {colour, drink, pet, nationality, profession} can have or if you see everything as a symbol and reason about things occurring together or not. I'm very curious to know how fast your Perl solution is. I know it doesn't make much sense calling any numbers, since there are too many variables, but my Prolog version took 0.004 seconds, using swi-prolog on a Sun Ultra 5 and I didn't time the Lisp version. I'm just wondering if Perl will be in the same order of magnitude. I was also completely amazed that the Englishman doesn't drink tea and that the Italian doesn't drink coffee, but I assumed that was just to make things less obvious :) Pasted in here is the Prolog version, as it's the most readable of the two:
% find the Zebra and the Water, zebra_water(X).
% by Willem Robert van Hage, wrvhage@science.uva.nl
right(2,1).
right(3,2).
right(4,3).
right(5,4).
nextdoor(X,Y) :- right(X,Y).
nextdoor(X,Y) :- right(Y,X).
match([],_).
match([X|Xs],Ys) :-
member(X,Ys),
match(Xs,Ys).
zebra_water(X) :-
nextdoor(Horse,Diplomat),
nextdoor(Fox,Doctor),
nextdoor(Norwegian,Blue),
Y = [house( Green, _, green, _, _, _),
house( White, _, white, _, _, _),
house(Norwegian, norwegian, _, _, _, _),
house( Blue, _, blue, _, _, _),
house( Fox, _, _, _, fox, _),
house( Doctor, _, _, doctor, _, _),
house( Horse, _, _, _, horse, _),
house( Diplomat, _, _, diplomat, _, _),
house( 3, _, _, _, _, milk),
house( 1, norwegian, _, _, _, _),
house( _, englishman, red, _, _, _),
house( _, spaniard, _, _, dog, _),
house( _, japanese, _, painter, _, _),
house( _, italian, _, _, _, tea),
house( _, _, yellow, diplomat, _, _),
house( _, _, green, _, _, coffee),
house( _, _, _, sculptor, snails, _),
house( _, _, _, violinist, _, juice),
house( _, _, _, _, zebra, _),
house( _, _, _, _, _, water) ],
X = [ house(1,_,_,_,_,_),
house(2,_,_,_,_,_),
house(3,_,_,_,_,_),
house(4,_,_,_,_,_),
house(5,_,_,_,_,_) ],
right(Green,White),
match(Y,X).
It should come out nicely aligned when pasted into an ascii editor :) brother Willem (wrvhage) -- wrvh@xs4all.nl | http://wrvh.xs4all.nl In reply to I think it's Lewis Carrol's puzzle
by wrvhage
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