Prolog-如何使尾巴不为空 [英] Prolog - How do I get the tail to not be null

问题描述

我有以下问题:

I have the following problem:

定义一个谓词sorted(LL)，当列表LL时满足 包含其他列表，这些列表按长度递增的顺序排序. 例如:

Define a predicate sorted(LL), that is satisfied when the list LL contains other lists that are sorted in order of increasing length. For example:

?- sorted([[],[1],[1,1],[1,1,1]]) -> yes.
?- sorted([[],[1],[1,1]]) -> yes.
?- sorted([[1],[],[1,1],[1,1,1]]) -> no.

到目前为止，我有这段代码:

And I have this code so far:

% shorter/2

shorter([],_).
shorter([_|T1], [_|T2]) :- shorter(T1,T2).

% sorted/1

sorted([]).
sorted([_]).
sorted([L1,L2 | T]) :- shorter2(L1, L2), sorted([L2,T]).

问题包含在上面的行:sorted([L2,T]).当列表列表中仅剩一个元素时，该调用将追加一个空列表[]，因为short/2将失败.在下面的SWIPL跟踪中对此进行了描述.

The problem is contained in the above line: sorted([L2,T]). When only one element is left in the list of lists, that call will append an empty list [] because of which shorter/2 will fail. It is depicted in the following SWIPL trace.

[trace]  ?- sorted([[1],[2,3]]).
   Call: (6) sorted([[1], [2, 3]]) ? creep
   Call: (7) shorter2([1], [2, 3]) ? creep
   Call: (8) shorter2([], [3]) ? creep
   Exit: (8) shorter2([], [3]) ? creep
   Exit: (7) shorter2([1], [2, 3]) ? creep
   Call: (7) sorted([[2, 3], []]) ? creep <-- empty list appended
   Call: (8) shorter2([2, 3], []) ? creep
   Fail: (8) shorter2([2, 3], []) ? creep
   Fail: (7) sorted([[2, 3], []]) ? creep
   Fail: (6) sorted([[1], [2, 3]]) ? creep

推荐答案

您在sorted/1谓词的最后一个子句中有两个错别字，应该是:

You have two typos in the last clause of the sorted/1 predicate, which should be:

sorted([L1,L2| T]) :- shorter(L1, L2), sorted([L2| T]).

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