Prolog:交换列表的两半 [英] Prolog: Swapping two halves of a list

问题描述

我正在 prolog 中编写一个谓词，它将具有偶数个变量的列表分成两半并交换它们.例如 [a,b,c,d] --> [c,d,a,b].

append([], List, List).追加([头|尾]，列表，[头|休息]):-追加(尾部，列表，休息).除(L，X，Y): -追加(X，Y，L)，长度(X，N)，长度(Y，N).交换([]， []).交换([A]，D): -除(A，B，C)，追加(C，B，D).

我希望通过将 [A] 分成两个大小相等的较小列表，然后以相反的顺序将它们附加在一起，然后将变量D"分配给列表来实现这一点.

我得到的是假"，为什么这不起作用?
我对 prolog 很陌生，所以这可能是一个愚蠢/简单的问题，谢谢！

解决方案

你的问题是为什么 swap([a,b,c,d],[c,d,a,b])失败.这是真正的原因:

<预>?- 交换([_/*a*/,_/*b*/|_/*,c,d*/],_/*[c,d,a,b]*/).:- op(950, fy, *).*(_).交换([]，_/*[]*/).交换([A]，D): -* divide(A, B, C),* 追加(C，B，D).

因此，不仅您的原始查询失败，而且即使是这种泛化也失败了.即使你问

?- 交换([_,_|_],_).错误的.

你只会失败.看到了吗?

你也可以反过来问.有了上面的概括，我们可以问:

?- 交换(Xs，Ys).Xs = [];Xs = [_A].

所以你的第一个参数必须是空列表或只有一个元素的列表.您当然还想描述更长的列表.

I am writing a predicate in prolog that will break a list with an even number of variables into two halves and swap them. For example [a,b,c,d] --> [c,d,a,b].

append([], List, List).
append([Head|Tail], List, [Head|Rest]) :- 
   append(Tail, List, Rest).

divide(L, X, Y) :-
   append(X, Y, L),
   length(X, N),
   length(Y, N).

swap([], []).
swap([A], D) :-
   divide(A, B, C),
   append(C, B, D).

I would expect this to work by dividing [A] into two smaller equal sized lists, then appending them together in the reverse order, and then assigning the variable "D" to the list.

What I am getting is "false", why does this not work?
I'm very new to prolog so this might be a silly/simple question, thanks!

解决方案

Your question is why swap([a,b,c,d],[c,d,a,b]) fails. And here is the actual reason:

?- swap([_/*a*/,_/*b*/|_/*,c,d*/],_/*[c,d,a,b]*/).

:- op(950, fy, *).
*(_).

swap([], _/*[]*/).
swap([A], D) :-
   * divide(A, B, C),
   * append(C, B, D).

So, not only does your original query fail, but even this generalization fails as well. Even if you ask

?- swap([_,_|_],_).
false.

you just get failure. See it?

And you can ask it also the other way round. With above generalization, we can ask:

?- swap(Xs, Ys).
   Xs = []
;  Xs = [_A].

So your first argument must be the empty list or a one-element list only. You certainly want to describe also longer lists.

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