list Prolog：检查某项是否是列表中的最后一项？
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Prolog: Checking if something is the last item in the list?
提问by user3295806
I am new to prolog and am basically trying to write a clause that would evaluate as true if a given item is the last item in a given list. Here is what I have:
我是 prolog 的新手，基本上是在尝试编写一个子句，如果给定的项目是给定列表中的最后一个项目，则该子句将评估为真。这是我所拥有的：
last(X,[Y|X]).
last(X,[Y|Z]) :- last(X,Z).
I thought this would do the trick, but when I ask prolog:
我认为这可以解决问题，但是当我问序言时：
?- last(c,[a,b,c]).
Prolog returns false. I tried the following query to see what Prolog thinks should fit my definition of last:
Prolog 返回 false。我尝试了以下查询以查看 Prolog 认为应该符合我对 last 的定义的内容：
?- last(c,X).
X = [_G530|c] ;
X = [_G530, _G533|c] ;
X = [_G530, _G533, _G536|c]
So, what I am not sure about is why the "|" symbol is still in the list?
所以，我不确定的是为什么“|” 符号仍在列表中？
Update: last([c],[a,b,c]) produces the desired behavior. However, I'm not sure why my 1st argument has to be a list?
更新：last([c],[a,b,c]) 产生所需的行为。但是，我不确定为什么我的第一个参数必须是列表？
回答by traitor
You might want this:
你可能想要这个：
last(X,[X]).
last(X,[_|Z]) :- last(X,Z).
The |denotes a 'tail' or 'the rest of the list.'
的|表示一个“尾巴”或“列表的其余部分。”
With ?- last(c,X).Prolog produces lists (according to your first definition) that have c as the last item.
使用?- last(c,X).Prolog 生成列表（根据您的第一个定义），其中 c 作为最后一项。
When you query ?- last(c,[a,b,c])., it returns false because you haven't defined a case for a list of only one item [X]or [X|[]]. So it fails when list is shorter than two items.
当您查询时?- last(c,[a,b,c]).，它返回 false，因为您还没有为只有一个项目[X]或的列表定义案例[X|[]]。所以当列表短于两个项目时它会失败。
However, last([c],[a,b,c])succeeds because you get [b|_29]or whatever denoting that the tail part might be any list. So it '_29' can be '[c]', satisfying the first definition like last([c],[b|[c]]).Remember that a nonempty list in Prolog is actually a pair of the first list item (head) and a list of the rest (tail). Usually written as [head|tail].
但是，last([c],[a,b,c])成功是因为您得到[b|_29]或表明尾部可能是任何列表的任何内容。所以它 '_29' 可以是 '[c]'，满足第一个定义就像last([c],[b|[c]]).记住 Prolog 中的非空列表实际上是一对第一个列表项（头）和一个列表项（尾）。通常写作[头|尾]。
回答by false
Why not view things from a grammatical viewpoint. So what is the last element in a list?
为什么不从语法的角度来看待事物。那么列表中的最后一个元素是什么？
last(X, Xs) :-
phrase( ( ..., [X] ), Xs).
... --> [] | [_], ... . % any sequence
回答by Dylon Dickinson
Why not just use append?
为什么不直接使用 append 呢？
last(X,Y) :-
append(_,[X],Y).
回答by SQB
Because the tail of a list is a list itself.
因为列表的尾部是列表本身。
A list in Prolog can be seen as [H | T], where His the first element (the head), and T(the tail) is a listof all other elements.
在Prolog中的列表可以被视为[H | T]，在这里H是第一个元素（头），和T（尾部）的列表的所有其它元件。
Your list [a, b, c]is actually [a | [b | [c | [] ] ] ]when you decompose it (the last tail is always an empty list):
你的列表[a, b, c]实际上是[a | [b | [c | [] ] ] ]当你分解它时（最后一个尾巴总是一个空列表）：
List: [a, b, c] Head: a Tail: [b, c]
List: [b, c] Head: b Tail: [c]
List: [c] Head: c Tail: []
So when you get to the point where you're asking if last(c, [b, c]), that decomposes to last(c, [b|[c]]), which is why the first clause can't be instantiated, because Xcan't be both cand [c]. That is why it didwork when you asked last([c],[a,b,c]).
因此，当您问到 if 时last(c, [b, c])，它会分解为last(c, [b|[c]])，这就是为什么不能实例化第一个子句的原因，因为X不能同时是c和[c]。这就是为什么当你问它时它确实有效last([c],[a,b,c])。
The version proposed by alphaworks, because it takes this into account:
% A list ends with an element if contains exactly that element.
last(X,[X]).
% A list ends with an element if its tail ends with that element.
% (Since we don't care about the head, we use an underscore).
last(X,[_|T]) :- last(X,T).
