小编Kon*_*ous的帖子

我在Prolog中有两个稍微不同的谓词unique_element/2实现.当给定元素X和列表L时,谓词成功,元素X在列表中仅出现一次.以下是实现和结果:

实施1:

%%% unique_element/2
unique_element(Elem, [Elem|T]) :-
    not(member(Elem, T)). 
unique_element(Elem, [H|T]) :-
    member(Elem, T), 
    H\==Elem, 
    unique_element(Elem, T), 
    !. 

结果:

?- unique_element(X, [a, a, b, c, c, b]). 
false.

?- unique_element(X, [a, b, c, c, b, d]).
X = a ;
X = d.

实施2:

%%% unique_element/2
unique_element(Elem, [Elem|T]) :- 
    not(member(Elem, T)). 
unique_element(Elem, [H|T]) :-
    H\==Elem, 
    member(Elem, T), 
    unique_element(Elem, T), 
    !. 

如果你没有第一眼就注意到:"H\== Elem"和"成员(Elem,T)"在第二个impl,规则2上被翻转.

结果:

?- unique_element(X, [a, a, b, c, c, b]).
X = a.

?- unique_element(X, [a, b, c, c, …