I chose to show a small example of the Zebra Puzzle and an implementation of the solution to it in F#.
The implementation is rather basic as it uses simple integers to assign the attributes to the house. A more type safe implementation using a discriminated union for each attribute type is a good idea, more to follow on that in a later blog post with an improved example. Please find the code below.
Credits to Tomas Petricek, whose implementation of the permutations of a list I used from Stack Overflow.
The solution is shown below or under this link.
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| //http://en.wikipedia.org/wiki/Zebra_Puzzle | |
| // Zebra Puzzle | |
| // | |
| //1 There are five houses. | |
| // | |
| //2 The Englishman lives in the red house. | |
| // | |
| //3 The Spaniard owns the dog. | |
| // | |
| //4 Coffee is drunk in the green house. | |
| // | |
| //5 The Ukrainian drinks tea. | |
| // | |
| //6 The green house is immediately to the right of the ivory house. | |
| // | |
| //7 The Old Gold smoker owns snails. | |
| // | |
| //8 Kools are smoked in the yellow house. | |
| // | |
| //9 Milk is drunk in the middle house. | |
| // | |
| //10 The Norwegian lives in the first house. | |
| // | |
| //11 The man who smokes Chesterfields lives in the house next to the man with the fox. | |
| // | |
| //12 Kools are smoked in a house next to the house where the horse is kept. | |
| // | |
| //13 The Lucky Strike smoker drinks orange juice. | |
| // | |
| //14 The Japanese smokes Parliaments. | |
| // | |
| //15 The Norwegian lives next to the blue house. | |
| // | |
| //Who drinks water? Who owns the zebra? | |
| // | |
| //Each house is painted a different color, and their inhabitants are of different nationalities, own different pets, drink different beverages and smoke different brands of American cigarettes. | |
| namespace ZebraDemo | |
| module programOrdered = | |
| let rec permutations inputlist taken = | |
| seq { if Set.count taken = List.length inputlist then yield [] else | |
| for l in inputlist do | |
| if not (Set.contains l taken) then | |
| for perm in permutations inputlist (Set.add l taken) do | |
| yield l::perm } | |
| let myperms = permutations [1;2;3] Set.empty | |
| Seq.length myperms | |
| Seq.toList myperms | |
| let splitToTuple = function | |
| | [fst; sec; thrd; fth; ffth] -> (fst, sec, thrd, fth, ffth) | |
| | _ -> failwith "wrong list format" | |
| let nextTo (frst, sec) = | |
| frst - 1 = sec || frst + 1 = sec | |
| let myHouses = [1..5] | |
| let myHousePerms = permutations myHouses Set.empty | |
| Seq.length myHousePerms | |
| let myOrderedAnswer = | |
| [for myCountryList in myHousePerms do | |
| let (England, Spain, Ukraine, Japan, Norway) = splitToTuple myCountryList | |
| if Norway = 1 then | |
| for myColorList in myHousePerms do | |
| let (red, green, ivory, yellow, blue) = splitToTuple myColorList | |
| if England = red && | |
| green - 1 = ivory && | |
| nextTo (Norway, blue) then | |
| for myDrinkList in myHousePerms do | |
| let (coffee, tea, milk, oj, WATER) = splitToTuple myDrinkList | |
| if Ukraine = tea && | |
| green = coffee && | |
| milk = 3 then | |
| for mySmokeList in myHousePerms do | |
| let (oldGold, kools, chesterfields, luckyStrike, parliaments) = splitToTuple mySmokeList | |
| if yellow = kools && | |
| luckyStrike = oj && | |
| Japan = parliaments then | |
| for myPetList in myHousePerms do | |
| let (dog, snail, horse, fox, ZEBRA) = splitToTuple myPetList | |
| if Spain = dog && | |
| oldGold = snail && | |
| nextTo(chesterfields, fox) && | |
| nextTo(kools, horse) then | |
| yield [("red", red); ("green", green); ("ivory", ivory); ("yellow", yellow); ("blue", blue); | |
| ("England", England); ("Spain", Spain); ("Ukraine", Ukraine); ("Japan", Japan); ("Norway", Norway); | |
| ("coffee", coffee); ("tea", tea); ("milk", milk); ("oj", oj); ("WATER", WATER); | |
| ("dog", dog); ("snail", snail); ("horse", horse); ("fox", fox); ("ZEBRA", ZEBRA); | |
| ("oldGold", oldGold); ("kools", kools); ("chesterfields", chesterfields); ("luckyStrike", luckyStrike); ("parliaments", parliaments)] ] | |
| let ans = List.head myOrderedAnswer | |
| let selectFromList houseNum = | |
| List.filter (fun (fst, snd) -> snd = houseNum) ans | |
| |> List.map (fun (fst, snd) -> fst) | |
| printfn "house 1:" | |
| printfn "%A" (selectFromList 1) | |
| printfn "house 2:" | |
| printfn "%A" (selectFromList 2) | |
| printfn "house 3:" | |
| printfn "%A" (selectFromList 3) | |
| printfn "house 4:" | |
| printfn "%A" (selectFromList 4) | |
| printfn "house 5:" | |
| printfn "%A" (selectFromList 5) |
.
