Lisp循环列表 [英] Lisp cyclic lists

问题描述

我们从Lisp那里得到了作业，我需要使用循环"列表(我不知道什么是正确的命名方式). 循环"列表是指列表，其中最后一个cons的cdr指向同一列表的第一个.

We have been given homework from lisp where I need to use "cyclic" list (I don't know what is the right naming for this). By "cyclic" list, I mean list, where cdr of the last one cons points to the very first one of the same list.

(Value1 . PointerValue2) (Value2 . PointerValue3) (Value3 . PointerValue1)

我们被教导要使用以下方法创建此类列表:

We have been taught to create such a lists with:

(defun cykl (l)
  (setf (cdr (last l)) l)
)

我使用的Lisp软件( Lispbox )不支持这种列表.我也曾在Debian上尝试过 clisp ，但是创建了这样的列表后它崩溃了.

The Lisp software (Lispbox) I use does not support this kind of lists. I have also tried clisp on Debian but it has crashed after creating such a list.

您知道哪些支持Lisp的实现(免费软件，操作系统无关)吗?

What lisp implementations do you know that does support this (freeware, os independent)?

推荐答案

所有lisp实现，包括clisp，都支持

All lisp implementations, including clisp, support circular lists.

当您说崩溃"时，您可能是指堆栈溢出错误(或内存不足错误)，当您尝试打印(记住"read-eval-PRINT"循环?)时，您总是会得到此错误；当 *print-circle* 是nil.将其设置为t会强制Lisp使用#n#表示法:

When you say "crashed", you probably mean a stack overflow error (or an out-of-memory error), which you will always get when you try to print (remember the 'read-eval-PRINT' loop?) a circular structure when *print-circle* is nil. Setting it to t forces Lisp to use the #n# notation:

[1]> (defparameter l (list 1 2 3))
L
[2]> l
(1 2 3)
[3]> (setq *print-circle* t)
T
[4]> (setf (cdr (last l)) l)
#1=(1 2 3 . #1#)

另请参见函数列表长度

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