Prolog Quicksort 使用第二个元素作为枢轴 [英] Prolog Quicksort using second element as a pivot

问题描述

我一直在尝试学习 prolog，我想使用列表的第二个元素作为快速排序的枢轴.

I've been trying to learn prolog and I want to use the second element of a list as the pivot of a quicksort.

我想用 [Head |[枢轴|Tail] ] 作为方法中的输入会起作用，但是我不确定我可以在哪里放置Head"，即第一个元素.

I thought using [Head | [Pivot | Tail] ] as the input in the method would work, but then I was not sure where I could place "Head", the first element.

像这样:

qsort([],[]):- !.
qsort([Head|[Pivot|Tail]],Sorted):-
        split(Pivot,[Head|Tail],Less,Greater),
        qsort(Less,SortedLess),
        qsort(Greater,SortedGreater),
        append(SortedLess,[Pivot|SortedGreater],Sorted).
split(_,[],[],[]).
split(Pivot,[X|T],[X|Less],Greater):-
        X=<Pivot,split(Pivot,T,Less,Greater).
split(Pivot,[X|T],Less,[X|Greater]):-
        X>Pivot,split(Pivot,T,Less,Greater).

但是，当我尝试使用 qsort([8, 3, 4, 12, 25, 4, 6, 1, 9, 22, 6], Sorted) 对列表进行排序时. 它只是返回假.我做错了什么?

However, when I try to sort the list with qsort([8, 3, 4, 12, 25, 4, 6, 1, 9, 22, 6], Sorted). It simply returns false. What am I doing wrong?

推荐答案

但是，当我尝试使用 qsort([8, 3, 4, 12, 25, 4, 6, 1, 9, 22, 6], Sorted) 对列表进行排序时.它只是返回 false.我做错了什么?

However, when I try to sort the list with qsort([8, 3, 4, 12, 25, 4, 6, 1, 9, 22, 6], Sorted). It simply returns false. What am I doing wrong?

最终，该算法将调用带有 exactly one 元素的列表，而您没有定义与该列表匹配的 qsort/2 子句.

Eventually this algorithm will make calls with a list with exactly one element, and you did not define a qsort/2 clause that will match with that list.

您可以通过添加规则来解决此问题:

You can resolve this by adding a rule:

qsort([],[]).
qsort([X], [X]).
qsort([Head, Pivot|Tail],Sorted):-
        split(Pivot,[Head|Tail],Less,Greater),
        qsort(Less,SortedLess),
        qsort(Greater,SortedGreater),
        append(SortedLess,[Pivot|SortedGreater],Sorted).

这给了我们:

?- qsort([8, 3, 4, 12, 25, 4, 6, 1, 9, 22, 6], Sorted).
Sorted = [1, 3, 4, 4, 6, 6, 8, 9, 12|...] ;
false.

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