Question

我四处搜索，但没有找到简单的答案。

如何在不使用step的情况下使用给定的loop在Common Lisp中对列表进行切片？ subseq不会采用第三个参数，即(subseq lst start end step)。

，这似乎很奇怪

Python等价物：

 lst[start:end:step]

Answer 1

标准CL中没有类似的内容。可以使用REDUCE或DO来实现它。我只想使用LOOP：

辅助功能：

(defun %take (it what)
  (cond ((eq what :all) it)
        ((eq what :none) nil)
        ((and (numberp what) (plusp what))
         (subseq it 0 what))
        ((and (numberp what) (minusp what))
         (last it (- what)))
        ((and (consp what)
              (= (length what) 1)
              (numberp (first what)))
         (nth (first what) it))
        ((and (consp what)
              (= (length what) 2)
              (numberp (first what))              
              (numberp (second what)))
         (let ((end (if (minusp (second what))
                        (+ (length it) (second what))
                      (second what))))
           (subseq it (first what) end)))
        ((and (consp what)
              (= (length what) 3)
              (numberp (first what))
              (numberp (second what))
              (numberp (third what)))
         (let ((start (first what))
               (end (if (minusp (second what))
                        (+ (length it) (second what))
                      (second what)))
               (by-step (third what)))
           (loop for e = (subseq it start) then (nthcdr by-step e)
                 for i from start below end by by-step
                 collect (first e))))))

TAKE：

(defun take (thing &rest description)
  "Taking things from lists like in Mathematica
Description is one or more of:
   :all | :none | [sign]number | ( start [end [step]])"
  (cond ((null description) nil)
        ((and (consp description)
              (= (length description) 1))
         (%take thing (first description)))
        (t (loop for e in (%take thing (first description))
                 collect (apply #'take e (rest description))))))

示例：

CL-USER 27 > (take '(0 1 2 3 4 5 6 7 8 9 10 11) '(2 7 2))
(2 4 6)

CL-USER 28 > (defun sublist (list start end step)
               (take list (list start end step)))
SUBLIST

CL-USER 29 > (sublist '(0 1 2 3 4 5 6 7 8 9 10 11) 2 7 2)
(2 4 6)

用stepize切片列表

1 个答案: