Prolog Room Adjacency Puzzle

Question

I am working on an assignment and I have to write a prolog program to solve a puzzle where there are 5 rooms and there are five people, Hunter, Laura, Arnie, Addiley and Ramey. Hunter cannot be in room 5, Laura cannot be in room 1, Arnie cannot be in room 1 or 5, Arnie cannot be adjacent to Laura or Addiley and Ramey is in a room with a higher index than Laura. I have seen various other answers for the exact same problem but no solution has worked thus far.

I have gotten most of the logic down, however the logic to check whether a person is adjacent to another person seems to not be working.

Here is my code:

layout([bedroom(_, 1), bedroom(_, 2), bedroom(_, 3), bedroom(_, 4), bedroom(_, 5)]).

adj(B,C):- B is C+1; B is C-1.

hallway(X) :- layout(X),
        member(bedroom(hunter, H), X),
        member(bedroom(laura, L), X),
        member(bedroom(arnie, N), X),
        member(bedroom(addiley, A), X),
        member(bedroom(ramey, R), X),
        H \= 5,
        L \= 1,
        N \= 1,
        N \= 5,
        N \= adj(N, L),
        N \= adj(N, A),
        R > L.

The resulting output is:

X = [bedroom(hunter,1),bedroom(laura,2),bedroom(arnie,3),bedroom(addiley,4),bedroom(ramey,5)]

However in this output, Arnie is adjacent to Laura and Addiley, seemingly breaking the rules that put into the adjacency logic. Why is it when I check for adjacency nothing seems to happen?

"N \= adj(N, L)" is wrong - should be e.g. "\+ adj(N, L)" – brebs Apr 20 '22 at 18:05 — brebs, Apr 20 '22 at 18:05

TessellatingHeckler · Accepted Answer · 2022-04-20T18:37:29.127

You are very close (and upvote having code and asking a specific question). The problem is here:

    N \= adj(N, L),
    N \= adj(N, A),

adj(N, L) is true if they are adjacent numbers, and false if they aren't. It doesn't return a value, it just is true or isn't true and the code continues forwards or stops and backtracks and changes member() and then tries again; so you just need:

    not(adj(N, L)),
    not(adj(N, A)),

to say that "it must not be true that Arnie and Laura have adjacent room numbers".

Compare that with your line in English: "Arnie's room number must not be equal to whether Arnie's room number is adjacent to Laura's", which makes no sense. (Or, more specifically since = is different in Prolog, your line says "Arnie's room number variable N must not unify with the term adj(N, L). Well, member() bound variable N to hold a room number at this point, so variable N can't also be bound to the line of code adj(N, L). So the number and the code are always different, the lines were always true and they weren't testing anything useful).

Prolog Room Adjacency Puzzle

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