Question

我想从数字列表

计算每个子列表/级别/表面级别的最大值

Ex: (1 2 5 (4 2 7 (4 6) 9) 7 8) => (8 9 6)

我现在拥有的是：

maximum (l) ;;function to compute the maximum number for a simple list, it works

(defun max-superficial (lista acc acc2) ;;main function: lista - my list, acc - my final list 
                                        ;;of results, acc2 - accumulation list for a sublist 
    (typecase lista 
        (null 
            (typecase acc2

;; if my list is empty and I have nothing accumulated, just return the final list
                (null acc)

;;if my list is empty but I have something in my accumulation list, just add the maximum 
;;of acc2 to my final list
                (t (nconc acc (list (maximum acc2))))))

        (cons (destructuring-bind (head . tail) lista
            (typecase head
                (list

;;if my list isn't empty and the head of the list is a list itself, call 
;;the function again for the head with an empty accumulation list and then call it again 
;;for the tail
                            (nconc acc 
                                (list (max-superficial head acc nil))
                                (max-superficial tail acc acc2)))

;; otherwise just accumulate the head and call the function for the tail 
---problem here             (t (nconc acc2 (list head))
                             (print '(wtf))
                             (print acc)
                             (print acc2)
                             (print head)
                             (max-superficial tail acc acc2)))))))

问题在于我只编写了这个程序，我想测试它并在列表中排除＆＃34; ---问题在这里＆＃34;它不会把我的头脑添加到累积列表中。

For: (max-superficial '(1 2) nil nil) --result should be ==> wtf nil (1) 1 wtf nil (1 2) 2 2
                                                   My result: wtf nil nil 1 wtf nil nil 2 nil

我单独检查了(nconc some-list (list 3))完全按照它应该做的...将数字3添加到某个列表的后面。我不知道为什么nconc acc2 (list head)无法正常工作

尝试用追加替换nconc，但它也没有用。显然，您无法使用append / nconc将元素添加到空列表中。怎么样？

Answer 1

更简单的实施：

(defun max-superficial/sublists (list)
  (loop for num in list
       if (listp num) append (max-superficial/sublists num) into sublists
       else if (numberp num) maximize num into max
       else do (error "Not a number or list: ~a" num)
       finally (return (cons max sublists))))

;; If you want the max of each "level" or depth in a tree,
;; then you need to be able to operate on levels. Here are some
;; functions that are analogous to FIRST, REST, and POP:

(defun top-level (tree)
  (remove-if-not #'numberp tree))

(defun rest-levels (tree)
  (apply #'append (remove-if-not #'listp tree)))

(defmacro pop-level (tree)
  `(let ((top (top-level ,tree)))
     (setf ,tree (rest-levels ,tree))
     top))

(defun max-superficial (tree &key use-sublists)
  "It wasn't clear if you wanted the max in each sublist or the max
at each depth, so both are implemented. Use the :use-sublists key
to get the max in each sublist, otherwise the max at each depth
will be computed."
  (if use-sublists
      (max-superficial/sublists tree)
      (loop for top-level = (pop-level tree)
         collect (if top-level (reduce #'max top-level)) into result
         unless tree do (return result))))

Answer 2

这是一个（不是特别有效）的解决方案：

(defun max-avoiding-nil (a b)
  (cond ((null a) b)
    ((null b) a)
    (t (max a b))))

(defun depth-maximum (a b)
  (cond ((null a) b)
    ((null b) a)
    (t
     (cons (max-avoiding-nil (car a) (car b))
           (depth-maximum (cdr a) (cdr b))))))

(defun tree-max-list (list depth)
  (reduce #'depth-maximum tree
          :key (lambda (elt) (tree-max elt depth))
          :initial-value '()))

(defun tree-max (tree depth)
  (if (listp tree)
      (tree-max-list tree (1+ depth))
    (append (make-list depth 'nil) (list tree))))

(defun tree-maximums (tree)
  (tree-max-list tree 0))

(tree-maximums '(1 2 5 (4 2 7 (4 6) 9) 7 8)) => (8 9 6)
(tree-maximums '()) => nil
(tree-maximums '(1)) => (1)
(tree-maximums '((2))) => (nil 2)
(tree-maximums '((2) (3))) => (nil 3)

每个级别（表面级别）LISP的最大数量

2 个答案: