CODEWITHSUNDEEP
prolog
user3514453
user3514453

Reputation: 35

Prolog program type error

This is one of the assignment questions which I'm currently working on (yes, this is homework):

moveSouth([_],_,_Num,_Moves,_Time).
moveSouth(Members,SouthGrp,Num,Moves,Time) :-
    member(X1/Y1,Members),
    member(X2/Y2,Members),
    X1 @< X2,
    (((NewTime is Time+Y1),(Y1>=Y2));((NewTime is Time+Y2),(Y1<Y2))),
    NewTime =< Num,
    subtract(Members,[X1/Y1,X2/Y2],NewFam),
    append(SouthGrp,[X1/Y1,X2/Y2],NewSouthGrp),
    append(Moves,[X1+X2],NewMoves),
    moveNorth(NewFam,NewSouthGrp,Num,NewMoves,NewTime).

 moveNorth(Member,SouthGrp,Num,Moves,Time) :-
    member(X1/Y1,SouthGrp),
    NewTime is Time+Y1,
    select(X1/Y1,SouthGrp,NewSouthGrp),
    moveSouth([X1/Y1|Member],Num,NewSouthGrp,[Moves|X1],NewTime).

 moveFamily(Name,Num,_Moves,_Time) :-
    family(Name,Members),
    moveSouth(Members,[],Num,[],0).

what I'm trying to do here is to move a List of names with property of Name/Time, to the south side, each time i can only move 2 members to the South, then needs to move one of the South members back to North.

now SWI-Prolog is giving me:

ERROR: =</2: Type error: `character' expected, found `mother/2'"

which I understand it means my types at =< operator is/are wrong. What I don't understand is what caused this to happen.

PS: The fact I used was:

family(original, [father/1, mother/2, child/5, granny/10]).

The query I used was:

moveFamily(original,19,Moves,Time).

Upvotes: 2

Views: 2777

Answers (1)

Wouter Beek
Wouter Beek

Reputation: 3407

1

The following line in your code assumes that X1/Y1 and X2/Y2 always occur next to each other (in that order) in the list of members:

subtract(Members, [X1/Y1,X2/Y2], NewFam),

But the way in which X1/Y1 and X2/Y2 are selected from the list of members shows that this need not be the case:

member(X1/Y1, Members),
member(X2/Y2, Members),

2

Another bug is in [Moves|X1], where X1 need not be a list at all.

3

Then the specific error that you asked your question about: in one call to moveSouth/4 you have swapped the second and third argument which leads to INTEGER =< LIST.

Hope this helps!

Upvotes: 1

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