Question

I have been given an exercise to solve the zebra puzzle using a constraint solver of my choice, and I tried it using the Prolog clpfd library.

I am aware that there are other more idiomatic ways to solve this problem in Prolog, but this question is specifically about the clpfd package!

So the specific variation of the puzzle (given that there are many of them) I’m trying to solve is this one:

There are five houses

1. The Englishman lives in the red house
2. The Swedish own a dog
3. The Danish likes to drink tea
4. The green house is left to the white house
5. The owner of the green house drinks coffee
6. The person that smokes Pall Mall owns a bird
7. Milk is drunk in the middle house
8. The owner of the yellow house smokes Dunhill
9. The norwegian lives in the first house
10. The marlboro smoker lives next to the cat owner
11. The horse owner lives next to the person who smokes dunhill
12. The winfield smoker likes to drink beer
13. The norwegian lives next to the blue house
14. The german smokes rothmanns
15. The marlboro smoker has a neighbor who drinks water

I tried to solve it with the following approach:

Each attribute a house can have is modeled as a variable, e.g. “British”,
“Dog”, “Green”, etc. The attributes can take values from 1 to 5, depending on the house
in which they occur, e.g. if the variable “Dog” takes the value 3, the dog lives in the
third house.

This approach makes it easy to model neighbor constraints like this:

def neighbor(X, Y) :-
    (X #= Y-1) #\/ (X #= Y+1).

But somehow, the clpfd package does not yield a solution, even though (IMO) the problem is modeled correctly (I used the exact same model with the Choco constraint solver and the result was correct).

Here is the complete code:

:- use_module(library(clpfd)).

neighbor(X, Y) :-
    (X #= (Y - 1)) #\/ (X #= (Y + 1)).

solve([British, Swedish, Danish, Norwegian, German], Fish) :-

    Nationalities = [British, Swedish, Danish, Norwegian, German],
    Colors = [Red, Green, Blue, White, Yellow],
    Beverages = [Tea, Coffee, Milk, Beer, Water],
    Cigarettes = [PallMall, Marlboro, Dunhill, Winfield, Rothmanns],
    Pets = [Dog, Bird, Cat, Horse, Fish],

    all_different(Nationalities),
    all_different(Colors),
    all_different(Beverages),
    all_different(Cigarettes),
    all_different(Pets),

    Nationalities ins 1..5,
    Colors ins 1..5,
    Beverages ins 1..5,
    Cigarettes ins 1..5,
    Pets ins 1..5,

    British #= Red, % Hint 1
    Swedish #= Dog, % Hint 2
    Danish #= Tea, % Hint 3

    Green #= White - 1 , % Hint 4
    Green #= Coffee, % Hint 5,
    PallMall #= Bird, % Hint 6

    Milk #= 3, % Hint 7
    Yellow #= Dunhill, % Hint 8,
    Norwegian #= 1, % Hint 9

    neighbor(Marlboro, Cat), % Hint 10
    neighbor(Horse, Dunhill), % Hint 11
    Winfield #= Beer, % Hint 12

    neighbor(Norwegian, Blue), % Hint 13
    German #= Rothmanns, % Hint 14,
    neighbor(Marlboro, Water). % Hint 15

Did I misunderstand a concept within clpfd, or am I simply missing something obvious here? In case it helps, here you can find the same approach implemented using Choco and Scala.

---

Edit: The reason why I believe that the solver isn’t able to solve the problem ist that it never comes up with definite values for the variables, but only with ranges, e.g. “Fish 1..3\/5”.

Answer 1

running your code in SWI-Prolog, I get

?- solve(X),label(X).
X = [3, 5, 2, 1, 4].

Without label:

?- solve(X).
X = [3, _G3351, _G3354, 1, _G3360],
_G3351 in 4..5,
all_different([_G3351, _G3386, _G3389, 2, _G3395]),
all_different([3, _G3351, _G3354, 1, _G3360]),
_G3386 in 3..5,
all_different([_G3386, _G3444, 1, _G3450, _G3360]),
_G3389 in 1\/3..5,
_G3389+1#=_G3478,
_G3492+1#=_G3389,
_G3395 in 1\/3..5,
_G3478 in 2..6,
_G3444#=_G3478#<==>_G3529,
_G3444 in 2..5,
_G3444#=_G3556#<==>_G3553,
_G3444#=_G3568#<==>_G3565,
_G3444#=_G3492#<==>_G3577,
_G3450 in 2\/5,
all_different([_G3354, 4, 3, _G3450, _G3614]),
_G3360 in 2\/4..5,
_G3354 in 2\/5,
_G3614 in 1..2\/5,
_G3614+1#=_G3556,
_G3568+1#=_G3614,
_G3556 in 2..3\/6,
_G3553 in 0..1,
_G3565#\/_G3553#<==>1,
_G3565 in 0..1,
_G3568 in 0..1\/4,
_G3492 in 0..4,
_G3577 in 0..1,
_G3577#\/_G3529#<==>1,
_G3529 in 0..1.

If I change all_different to all_distinct I get the solution without label:

....
all_distinct(Nationalities),
all_distinct(Colors),
all_distinct(Beverages),
all_distinct(Cigarettes),
all_distinct(Pets),
....

?- solve(X).
X = [3, 5, 2, 1, 4].

As you see, the docs state stronger propagation for all_distinct vs all_different. Running the proposed sample help to understand the difference between those:

?- maplist(in, Vs, [1\/3..4, 1..2\/4, 1..2\/4, 1..3, 1..3, 1..6]), all_distinct(Vs).
false.

?- maplist(in, Vs, [1\/3..4, 1..2\/4, 1..2\/4, 1..3, 1..3, 1..6]), all_different(Vs).
Vs = [_G419, _G422, _G425, _G428, _G431, _G434],
_G419 in 1\/3..4,
all_different([_G419, _G422, _G425, _G428, _G431, _G434]),
_G422 in 1..2\/4,
_G425 in 1..2\/4,
_G428 in 1..3,
_G431 in 1..3,
_G434 in 1..6.