Perfoming成员检查差异列表,但是如何? [英] Perfoming member check on a difference list, but how?
问题描述
我试图回答另一个问题(虽然是错误的),但这导致了一个关于差异列表"(或列表差异")的问题,这似乎是一个更合适的名称,除非"Escherian Construction"不是首选. >
我们有一个元素obj(X,Y)
的完整列表(X
和Y
都接地).我们只想保留从前到后遍历列表时尚未遇到X
的第一个obj(X,_)
.这些第一个元素"必须按出现顺序出现在结果中.
让我们通过测试用例指定问题:
% Testing
:- begin_tests(collapse_dl).
test(one) :- collapse_dl([],[]).
test(two) :- collapse_dl([obj(a,b)],
[obj(a,b)]).
test(three) :- collapse_dl([obj(a,b),obj(a,c)],
[obj(a,b)]).
test(four) :- collapse_dl([obj(a,b),obj(a,c),obj(b,j)],
[obj(a,b),obj(b,j)]).
test(five) :- collapse_dl([obj(a,b),obj(a,c),obj(b,j),obj(a,x),obj(b,y)],
[obj(a,b),obj(b,j)]).
:- end_tests(collapse_dl).
rt :- run_tests(collapse_dl).
现在,使用过滤,列表前缀和reverse/2
易于实现,但是使用差异列表和列表追加呢?
但是,我无法使seen/2
谓词正常工作.它检查obj(A,_)
是否已在差异列表中.但是该谓词的适当终止是什么?
% This is called
collapse_dl([],[]) :- !.
collapse_dl([X|Xs],Out) :-
Dlist = [X|Back]-Back, % create a difflist for the result; X is surely in there (as not yet seen)
collapse_dl(Xs,Dlist,Out). % call helper predicate
% Helper predicate
collapse_dl([],Ldown,Lup):- % end of recursion; bounce proper list back up
Ldown = Lup-[]. % the "back" of the difflist is unified with [], so "front" becomes a real list, and is also Lup
collapse_dl([obj(A,_)|Objs],Ldown,Out) :-
seen(obj(A,_),Ldown), % guard: already seen in Ldown?
!, % then commit
collapse_dl(Objs,Ldown,Out). % move down chain of induction
collapse_dl([obj(A,B)|Objs],Ldown,Out) :-
\+seen(obj(A,_),Ldown), % guard: not yet seen in Ldown?
!, % then commit
Ldown = Front-Back, % decompose difference list
Back = [obj(A,B)|NewTail], % NewTail is fresh! Append via difflist unification magic
collapse_dl(Objs,Front-NewTail,Out). % move down chain of induction; Front has been refined to a longer list
% Membership check in a difference list
seen(obj(A,_),[obj(A,_)|_Objs]-[]) :- !. % Yup, it's in there. Cut retry.
seen(Obj,[_|Objs]-[]) :- ... % But now???
更新
带有Paulo的代码段:
% Membership check in a difference list
seen(Element, List-Back) :-
List \== Back,
List = [Element|_].
seen(Element, List-Back) :-
List \== Back,
List = [_| Tail],
seen(Element, Tail-Back).
我们现在通过了所有测试.
member(Element, List-Back) :-
List \== Back,
List = [Element|_].
member(Element, List-Back) :-
List \== Back,
List = [_| Tail],
member(Element, Tail-Back).
I tried to answer another question (wrongly though) and this led to a question on "difference lists" (or "list differences", which seems a more appropriate name, unless "Escherian Construction" isn't preferred)
We have a fully ground list of elements obj(X,Y)
(both X
and Y
ground). We want to retain only the first obj(X,_)
where X
hasn't been encountered yet when going through the list front to back. Those "first elements" must appear in order of appearance in the result.
Let's specify the problem through test cases:
% Testing
:- begin_tests(collapse_dl).
test(one) :- collapse_dl([],[]).
test(two) :- collapse_dl([obj(a,b)],
[obj(a,b)]).
test(three) :- collapse_dl([obj(a,b),obj(a,c)],
[obj(a,b)]).
test(four) :- collapse_dl([obj(a,b),obj(a,c),obj(b,j)],
[obj(a,b),obj(b,j)]).
test(five) :- collapse_dl([obj(a,b),obj(a,c),obj(b,j),obj(a,x),obj(b,y)],
[obj(a,b),obj(b,j)]).
:- end_tests(collapse_dl).
rt :- run_tests(collapse_dl).
Now, this is easy to implement using filtering, list prepend and reverse/2
, but what about using difference lists and list append?
however, I'm not able to get the seen/2
predicate to work. It checks whether obj(A,_)
is already in the difference list. But what's a proper termination for this predicate?
% This is called
collapse_dl([],[]) :- !.
collapse_dl([X|Xs],Out) :-
Dlist = [X|Back]-Back, % create a difflist for the result; X is surely in there (as not yet seen)
collapse_dl(Xs,Dlist,Out). % call helper predicate
% Helper predicate
collapse_dl([],Ldown,Lup):- % end of recursion; bounce proper list back up
Ldown = Lup-[]. % the "back" of the difflist is unified with [], so "front" becomes a real list, and is also Lup
collapse_dl([obj(A,_)|Objs],Ldown,Out) :-
seen(obj(A,_),Ldown), % guard: already seen in Ldown?
!, % then commit
collapse_dl(Objs,Ldown,Out). % move down chain of induction
collapse_dl([obj(A,B)|Objs],Ldown,Out) :-
\+seen(obj(A,_),Ldown), % guard: not yet seen in Ldown?
!, % then commit
Ldown = Front-Back, % decompose difference list
Back = [obj(A,B)|NewTail], % NewTail is fresh! Append via difflist unification magic
collapse_dl(Objs,Front-NewTail,Out). % move down chain of induction; Front has been refined to a longer list
% Membership check in a difference list
seen(obj(A,_),[obj(A,_)|_Objs]-[]) :- !. % Yup, it's in there. Cut retry.
seen(Obj,[_|Objs]-[]) :- ... % But now???
Update
With Paulo's code snippet:
% Membership check in a difference list
seen(Element, List-Back) :-
List \== Back,
List = [Element|_].
seen(Element, List-Back) :-
List \== Back,
List = [_| Tail],
seen(Element, Tail-Back).
So, term equivalence, or dis-equivalence in this case, is the solution!
We now pass all the test.
Try (taken from Logtalk difflist
library object):
member(Element, List-Back) :-
List \== Back,
List = [Element|_].
member(Element, List-Back) :-
List \== Back,
List = [_| Tail],
member(Element, Tail-Back).
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