我想编写一个从列表中删除尾随nil的函数。我首先尝试用递归优雅地编写它,但结果是这样的:
(defun strip-tail (lst)
(let ((last-item-pos (position-if-not #'null lst :from-end t)))
(if last-item-pos
(subseq lst 0 (1+ last-item-pos)))))
; Test cases.
(assert (eq nil (strip-tail nil)))
(assert (eq nil (strip-tail '(nil))))
(assert (equal '(a b) (strip-tail '(a b nil nil))))
(assert (equal '(a nil b) (strip-tail '(a nil b nil))))
(assert (equal '(a b) (strip-tail '(a b))))
可以说很清楚,但我不相信。是否有一种更为疯狂的方式呢?
答案 0 :(得分:3)
嗯,版本是:
代码:
(defun list-right-trim (list &optional item)
(setf list (reverse list))
(loop for e in list while (eq item e) do (pop list))
(reverse list))
这是另一种变体:
代码:
(defun list-right-trim (list &aux (p nil))
(loop for i from 0 and e in list
when (and (null p) (null e))
do (setf p i)
else when (and p e) do (setf p nil))
(if p (subseq list 0 p) list))
答案 1 :(得分:3)
(defun strip-tail (ls)
(labels ((strip-car (l)
(cond ((null l) nil)
((null (car l)) (strip-car (cdr l)))
(t l))))
(reverse (strip-car (reverse ls)))))
示例运行(针对您的测试用例):
[1]> (assert (eq nil (strip-tail nil)))
NIL
[2]> (assert (eq nil (strip-tail '(nil)))) ;'
NIL
[3]> (assert (equal '(a b) (strip-tail '(a b nil nil))))
NIL
[4]> (assert (equal '(a nil b) (strip-tail '(a nil b nil))))
NIL
[5]> (assert (equal '(a b) (strip-tail '(a b))))
NIL
[6]>
答案 2 :(得分:2)
这个怎么样?
(defun strip-tail (lst)
(if lst
(let ((lst (cons (car lst) (strip-tail (cdr lst)))))
(if (not (equal '(nil) lst)) lst))))
...想知道如何使其尾递归,但这个版本会耗尽大型列表的堆栈。
答案 3 :(得分:1)
这是我想出的,假设你不介意这是破坏性的:
(defvar foo (list 'a 'b 'c nil 'd 'e 'nil 'nil 'f nil nil))
(defun get-last-non-nil (list &optional last-seen)
(if list
(if (car list)
(get-last-non-nil (cdr list) list)
(get-last-non-nil (cdr list) last-seen))
last-seen))
(defun strip-tail (list)
(let ((end (get-last-non-nil list)))
(if (consp end)
(when (car end) (setf (cdr end) nil) list))))
(strip-tail foo) -> (A B C NIL D E NIL NIL F)
答案 4 :(得分:0)
我尝试使用递归但它在GNU CL上不起作用:
(defun strip(lst)
(if (null (last lst))
(strip (butlast lst))
lst))
这个想法是:
答案 5 :(得分:-1)
嗯,这不是一个真正的答案,但我想我也把它放在这里,所以它有更好的可见性。
在您的原始实现中,您认为应该处理非列表项吗?
* (strip-tail "abcde")
"abcde"
* (strip-tail 42)
debugger invoked on a TYPE-ERROR in thread #<THREAD "initial thread" {A69E781}>:
The value 42 is not of type SEQUENCE.