Prolog中简单递归任务的问题

Question

我目前正在解决 prolog 中的一些问题，但我似乎无法解决一个关于递归的问题，我收到了这张表：Gryffindor Table。

鉴于这些信息，我用这些内容制作了自己的知识库：

/*
 This order represents how they are sit from left to right
 parvati_patil is left to lavender_brown,
 lavender_brown is left to neville_longbottom
 and so on, until we reach parvati again at the end.
*/

seatedTogether(parvati_patil, lavender_brown).
seatedTogether(lavender_brown, neville_longbottom).
seatedTogether(neville_longbottom, alicia_spinnet).
seatedTogether(alicia_spinnet, fred_weasley).
seatedTogether(fred_weasley, george_weasley).
seatedTogether(george_weasley, lee_jordan).
seatedTogether(lee_jordan, dennis_creevey).
seatedTogether(dennis_creevey, dean_thomas).
seatedTogether(dean_thomas, ginny_weasley).
seatedTogether(ginny_weasley, angelina_johnson).
seatedTogether(angelina_johnson, seamus_finnigan).
seatedTogether(seamus_finnigan, colin_creevey).
seatedTogether(colin_creevey, harry_potter).
seatedTogether(harry_potter, hermione_granger).
seatedTogether(hermione_granger, ron_weasley).
seatedTogether(ron_weasley, natalie_mcdonald).
seatedTogether(natalie_mcdonald, katie_bell).
seatedTogether(katie_bell, parvati_patil).

% X is left to Y if they are seatedTogether(X,Y)
isAtLeft(X,Y):-seatedTogether(X,Y).
% X is right to Y if they are seatedTogether(Y,X)
isAtRight(X,Y):-seatedTogether(Y,X).

/*
 This rule just tells us who X is two places away from Y,
 X is two places away from Y if 
 X is seatedTogether(X,Z)
 and that Z is seatedTogether(Z,Y).
*/
twoPlacesAway(X,Y):-seatedTogether(X, Z), seatedTogether(Z,Y).

/*
  This rule just tells us whos sitting @ the table
  by just unifying X with the values of seatedTogether(X,Y)
  without regarding Y.
*/
atTable(X):-seatedTogether(X,_).


/*
  Between two:
  Its supposed to tell us whos Z is between X and Y
  The perfect case (for me) would be that X and Y are sitting
  together, so they have no one in the middle.
  The other way around would be that
  X is not equal to Y
  X1 is sitting left to X,
  and then we call it again with
  X1, Y and Z1 as args,
  after each call, we equal
  Z to X1 value.
*/
betweenTwo(X,Y,Z):-isAtLeft(X,Y),isAtRight(Y,X).
betweenTwo(X,Y,Z):-
    X \= Y,
    isAtLeft(X, X1),
    betweenTwo(X1, Y, Z1),
    Z = X1.

问题在于最后一个规则定义，如果我这样称呼它：

betweenTwo(ron_weasley, alicia_spinnet, Z).

Z的值应该是：

• natalie_mcdonald,
• 凯蒂贝尔，
• parvati_patil，
• lavender_brown,
• neville_longbottom.

但Z只与值统一

• natalie_mcdonald.

我相信我非常接近它，但我真的不知道该规则出了什么问题。我定义了它，所以 X 逐步等于 Y 的值，但是对于 Y 之前的值，它应该落在完美的情况下并停止移动，并统一它之前的其余元素。有什么想法吗？

Answer 1

首先：你的基本情况（你称之为“完美情况”）说“如果X在Y的左边，而Y在X的右边，那么每个Z都在他们”，而不是“他们之间没有人”（条件也是多余的）。基本情况应该说明谓词何时成立（即，当某些Z 介于X 和Y 之间时），而不是不成立时。另请参阅此答案：https://stackoverflow.com/a/3001941/9204

第二：在非基本情况下，你有

isAtLeft(X, X1),
betweenTwo(X1, Y, Z1),
Z = X1.

当 Prolog 在isAtLeft(ron_weasley, X1) 中搜索X1 时，唯一的答案是natalie_mcdonald，后来Z = X1 强制Z 也是natalie_mcdonald。

所以这部分代码相当于

isAtLeft(X, Z),
betweenTwo(Z, Y, Z1).

或者用文字“Z在X和Y之间如果：X不等于Y，X直接在Z的左边，还有一些Z1介于Z 和Y"之间，这没有意义。

您也应该注意关于单例变量Z1 的警告。