0
我已經遍尋四周,但沒有找到一個簡單的答案。按步驟劃分列表
如何在Common Lisp中對給定的step
進行切片,而不是使用loop
? 看起來很奇怪,subseq
將不會採取第三個參數,即(subseq lst start end step)
。
的Python相當於:
lst[start:end:step]
我已經遍尋四周,但沒有找到一個簡單的答案。按步驟劃分列表
如何在Common Lisp中對給定的step
進行切片,而不是使用loop
? 看起來很奇怪,subseq
將不會採取第三個參數,即(subseq lst start end step)
。
的Python相當於:
lst[start:end:step]
沒有什麼標準CL相似。人們可以使用REDUCE
或DO
來實現它。我只想用LOOP
:
輔助功能:
(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))))))
例子:
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)