打开列表和成员 [英] Open list and member
问题描述
因为我想避免 append/3
的成本,所以我使用差异/开放列表.
Since I want to avoid cost of append/3
, I use difference/open lists.
然而,开放列表的问题在于 member/2
通过将元素添加到尾部来对开放列表做出反应.例如:
The problem with an open list however is that member/2
reacts with an open list by adding the element to the tail. For example:
?- L=[a|_],member(b,L).
L = [a, b|_G1196] ;
L = [a, _G1195, b|_G1199] ;
L = [a, _G1195, _G1198, b|_G1202] ;
L = [a, _G1195, _G1198, _G1201, b|_G1205] ;
L = [a, _G1195, _G1198, _G1201, _G1204, b|_G1208] ;
L = [a, _G1195, _G1198, _G1201, _G1204, _G1207, b|_G1211]
这是正确的行为,因为一个开放列表有一个无界的尾部",并且 member/2
函数将这个尾部/孔(变量)与成员的第一个参数统一起来.
This is correct behavior since an open list has an unbounded "tail" and the member/2
function unifies this tail/hole ( variable) with the first argument of member.
不过,我正在寻找一种方法,可以检查打开列表中是否存在与给定元素相等的元素.我该怎么做?
I'm looking however for a way I can check if there is an element in the open list that is equal to the given element. How can I do this?
推荐答案
您可以编写自己的 member/2
版本:member_open/2
:
You could write your own version of member/2
: member_open/2
:
member_open(_,X) :-
var(X),
!,
fail.
member_open(X,[X|_]).
member_open(X,[_|T]) :-
member_open(X,T).
或更纯粹的方法:
member_open(X,Y) :-
\+var(Y),
Y = [X|_].
member_open(X,Y) :-
\+var(Y),
Y = [_|T],
member_open(X,T).
谓词假设一个开放列表的尾部是var/1
.如果谓词发现这样的尾部,则执行切割 (!
) 并失败.
The Predicate makes the assumption that an open list has a tail that is var/1
. If the predicate finds such a tail, it performs a cut (!
) and fails.
演示:
?- member_open(a,[]).
false.
?- member_open(a,[a]).
true ;
false.
?- member_open(a,[a,a]).
true ;
true ;
false.
?- member_open(a,[a,a|_]).
true ;
true ;
false.
?- member_open(b,[a,a|_]).
false.
?- member_open(X,[X,a|_]).
true ;
X = a ;
false.
?- member_open(X,[c,a|_]).
X = c ;
X = a ;
false.
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