题
我想实现在序言列表差异程序。 出于某种原因,下面的失败:
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).
当尝试:
?- difference([1,2],[],D).
我得到这个错误:
ERROR: '.'/2: Type error: `[]' expected, found `1' ("x" must hold one character)
^ Exception: (10) _L161 is [2|1] ?
解决方案
您使用A1是[X | A]是不正确。谓词的是强>仅用于运算。 顺便说一句,SWI-Prolog的具有内置的减法谓词:
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.
这是你需要什么?
其他提示
minus([H|T1],L2,[H|L3]):-
not(member(H,L2)),
minus(T1,L2,L3).
minus([H|T1],L2,L3):-
member(H,L2),
minus(T1,L2,L3).
minus([],_,[]).
minus([1,2,3,4,3], [1,3], L).
output: L=[2,4]
使用发现所有的溶液变得明显:
difference(Xs,Ys,D) :-
findall(X,(member(X,Xs),not(member(X,Ys))),D).
always (subtructLists(List, [Head|Rest], Result): -
(
delete_element(Head, List, Subtructed)
, !
, subtructLists(Subtructed, Rest, Result)
) ; (
subtructLists(List, Rest, Result)
)
).
always (subtructLists(List, [], List)).
always( delete_element(X, [X|Tail], Tail)).
always( delete_element(X, [Y|Tail1], [Y|Tail2]): -
delete_element(X, Tail1, Tail2)
).
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