Prolog列出差异
问题描述：
我正在尝试编写序言中的程序，该程序将执行以下操作:
I'm trying to make program in prolog that will do something like this:
diffSet([a,b,c,d], [a,b,e,f], X).
X = [c,d,e,f]
我写了这个:
diffSet([], _, []).
diffSet([H|T1],Set,Z):- member(Set, H), !, diffSet(T1,Set,Z).
diffSet([H|T], Set, [H|Set2]):- diffSet(T,Set,Set2).
但是那样，我只能从第一个列表中获取元素.如何从第二个元素中提取元素?
But in that way I can only get elements from the first list. How can I extract the elements from the second one?
@edit: 成员正在检查H是否在Set中
@edit: member is checking if H is in Set
member([H|_], H).
member([_|T], H):- member(T, H).
答
There is a builtin that remove elements from the list:
diffSet([], X, X).
diffSet([H|T1],Set,Z):-
member(H, Set), % NOTE: arguments swapped!
!, delete(T1, H, T2), % avoid duplicates in first list
delete(Set, H, Set2), % remove duplicates in second list
diffSet(T2, Set2, Z).
diffSet([H|T], Set, [H|Set2]) :-
diffSet(T,Set,Set2).