序言清单差异例程 [英] Prolog list difference routine
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问题描述
我正在尝试在序言中实现列表差异例程. 由于某些原因,以下操作失败:
I am trying to implement a list difference routine in prolog. For some reason the following fails:
difference(Xs,Ys,D) :- difference(Xs,Ys,[],D).
difference([],_,A,D) :- D is A, !.
difference([X|Xs],Ys,A,D) :-
not(member(X,Ys)),
A1 is [X|A],
difference(Xs,Ys,A1,D).
尝试时:
?- difference([1,2],[],D).
我收到此错误:
ERROR: '.'/2: Type error: `[]' expected, found `1' ("x" must hold one character)
^ Exception: (10) _L161 is [2|1] ?
推荐答案
您的用法A1为[X | A]是不正确的.谓词是仅用于算术. 顺便说一句,SWI-Prolog具有内置的减谓词:
Your usage A1 is [X|A] is incorrect. Predicate is is used only for arithmetics. Btw, SWI-Prolog has built-in subtract predicate:
1 ?- subtract([1,2,3,a,b],[2,a],R).
R = [1, 3, b].
2 ?- listing(subtract).
subtract([], _, []) :- !.
subtract([A|C], B, D) :-
memberchk(A, B), !,
subtract(C, B, D).
subtract([A|B], C, [A|D]) :-
subtract(B, C, D).
true.
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