Lisp-循环遍历列表并替换值

Question

在这个问题中，我有三个（结构相同的）列表。两个包含所有数字，另一个包含nil。我试图通过添加两个列表中的相应值来替换空列表中的相应值。到目前为止我使用循环并使用setf来替换值。

(defun add-two-lists (list1 list2 list3)
   (loop for a in list1
        for b in list2
        for c in list3 do
        (setf c (+ a b))))

问题是这个功能没有破坏性。如何使这个功能具有破坏性？

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好的，我知道我可以使用apply来做这件事，但是为了将来或相切的目的，有没有办法使用循环来做同样的事情？

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我决定求助于我的倒数第二个解决方案;使用list-length来横向列表。

(defun add-two-lists (list1 list2 list3)
       (loop for x from 0 to (- (list-length list1) 1) do
            (setf (nth x list3) (+ (nth x list1) (nth x list2))))
       (values list3))

Answer 1

这是一种方式：

(defun add-two-lists (list1 list2 list3)
   (loop for a in list1
        for b in list2
        for c on list3 do
        (rplaca c (+ a b)))

附录

这是使用map而不是loop的另一种方式：

(defun add-two-lists (list1 list2 list3)
  (mapl #'(lambda (cl al bl) (rplaca cl (+ (car al) (car bl))))
    list3 list1 list2))

Answer 2

另一种在不使用循环的情况下做同样事情的方法（虽然它在概念上相似）

(defun add-two-lists (a b c &optional (d c))
  (if a
    (add-two-lists
     (cdr a) (cdr b)
     (cdr (rplaca c (+ (car a) (car b)))) d) d))

(add-two-lists '(1 2 3 4 5) '(1 2 3 4 5) '(nil nil nil nil nil))

修改

(defun add-two-lists (a b c &optional (d c))
  (if a
    (add-two-lists
     (cdr a) (cdr b)
     (cdr (rplaca c (+ (car a) (car b)))) d) d))

(time
 (dotimes (i 1e6)
   (add-two-lists '(1 2 3 4 5)
          '(1 2 3 4 5)
          '(nil nil nil nil nil))))

;; Evaluation took:
;;   0.077 seconds of real time
;;   0.076004 seconds of total run time (0.076004 user, 0.000000 system)
;;   98.70% CPU
;;   214,723,476 processor cycles
;;   0 bytes consed

(defun add-two-lists-1 (list1 list2 list3)
  (loop for a in list1
     for b in list2
     for c on list3 do
       (rplaca c (+ a b))))

(time
 (dotimes (i 1e6)
   (add-two-lists-1 '(1 2 3 4 5)
          '(1 2 3 4 5)
          '(nil nil nil nil nil))))

;; Evaluation took:
;;   0.060 seconds of real time
;;   0.060004 seconds of total run time (0.060004 user, 0.000000 system)
;;   100.00% CPU
;;   169,395,444 processor cycles
;;   0 bytes consed

编辑2

但请注意优化的版本行为。可能，再次，YMMV，但这是我用SBCL获得的64位Debian。

(defun add-two-lists (a b c &optional (d c))
  (declare (optimize (speed 3) (safety 0)))
  (declare (type list a b c d))
  (if a
    (add-two-lists
     (cdr a) (cdr b)
     (cdr (rplaca
       c 
       (the fixnum
         (+ (the fixnum (car a))
        (the fixnum (car b)))))) d) d))

(time
 (dotimes (i 1e6)
   (add-two-lists '(1 2 3 4 5)
          '(1 2 3 4 5)
          '(nil nil nil nil nil))))

;; Evaluation took:
;;   0.041 seconds of real time
;;   0.040002 seconds of total run time (0.040002 user, 0.000000 system)
;;   97.56% CPU
;;   114,176,175 processor cycles
;;   0 bytes consed

(defun add-two-lists-1 (list1 list2 list3)
  (declare (optimize (speed 3) (safety 0)))
  (loop for a fixnum in list1
     for b fixnum in list2
     for c cons on list3 do
       (rplaca c (the fixnum (+ a b)))))

(time
 (dotimes (i 1e6)
   (add-two-lists-1 '(1 2 3 4 5)
          '(1 2 3 4 5)
          '(nil nil nil nil nil))))

;; Evaluation took:
;;   0.040 seconds of real time
;;   0.040003 seconds of total run time (0.040003 user, 0.000000 system)
;;   100.00% CPU
;;   112,032,123 processor cycles
;;   0 bytes consed

Answer 3

Common Lisp提供了一个函数：MAP-INTO。