List 在prolog中管理列表

我是Prolog新手，正在寻求帮助。我想做的是得到一个列表，它由在给定列表中至少重复两次的元素组成

范例 L'=[1,2,1,3,4,3,2]=>L=[1,2,3]

到目前为止，我能够计算每个连续变量的发生率

% pack(L1,L2) :- the list L2 is obtained from the list L1 by packing
%    repeated occurrences of elements into separate sublists.
%    (list,list) (+,?)

pack([],[]).
pack([X|Xs],[Z|Zs]) :- transfer(X,Xs,Ys,Z), pack(Ys,Zs).

% transfer(X,Xs,Ys,Z) Ys is the list that remains from the list Xs
%    when all leading copies of X are removed and transfered to Z

transfer(X,[],[],[X]).
transfer(X,[Y|Ys],[Y|Ys],[X]) :- X \= Y.
transfer(X,[X|Xs],Ys,[X|Zs]) :- transfer(X,Xs,Ys,Zs).

% encode(L1,L2) :- the list L2 is obtained from the list L1 by run-length
%    encoding. Consecutive duplicates of elements are encoded as terms [N,E],
%    where N is the number of duplicates of the element E.
%    (list,list) (+,?)

encode(L1,L2) :- pack(L1,L), transform(L,L2).

transform([],[]).
transform([[X|Xs]|Ys],[[N,X]|Zs]) :- length([X|Xs],N), transform(Ys,Zs).

这将返回以下Touple列表

?- encode([a,a,a,a,b,c,c,a,a,d,e,e,e,e],X).
X = [[4,a],[1,b],[2,c],[2,a],[1,d][4,e]]

但构建一个包含重复至少两次的不同元素的列表仍然存在问题

如果有人能帮我，或者给我指出大致的方向，那就太好了

---

提前感谢

您可以编写一个过程：

% E it's the list of are elements from L that repeat at least twice
elements_that_repeat_at_least_twice(L, E) :-
  elements_that_repeat_at_least_twice(L, [], E).

elements_that_repeat_at_least_twice([H|Ls], Dupl, E) :-
  ...

在重复至少两次的元素中，添加的列表Dupl将使您验证的每个元素保持多次。使用[H | Ls]检查L的每个元素。 使用memberchk/2验证H是否在L中：那么它至少是重复的。如果它还没有在Dupl中，添加到它，然后递归。请记住编写递归基本情况（在空列表[]处停止）

现在我看到您添加了一些代码：然后我完成了建议：

elements_that_repeat_at_least_twice([], Dupl, Dupl).
elements_that_repeat_at_least_twice([H|Ls], Dupl, E) :-
  (  memberchk(H, Ls)
  -> ( \+ memberchk(H, Dupl)
     -> Dupl1 = [H|Dupl]
     ;  Dupl1 = Dupl
     )
  ;  Dupl1 = Dupl
  ),
  elements_that_repeat_at_least_twice(Ls, Dupl1, E).

完成后，请记住反转重复列表

an element E of list L should:
   be a member of list L',
   be a member of list L'' where L'' is list L' if we remove element E.

检查，和/或