Question

我在这里遇到一个小问题，我一直收到错误，我似乎无法弄清楚为什么，代码应该改变列表的范围，所以，如果我们给它一个值为(1 2 3 4)的列表，并且我们想要将范围从11改为14，那么结果将是(11 12 13 14) 问题是最后一个名为scale-list的函数会回复错误说：

调试器输入 - Lisp错误:(错误的类型参数number-or-marker-p nil）

任何人都知道为什么？我使用aquamacs作为编辑提前谢谢

;;finds minimum in a list
(defun minimum (list)
  (car (sort list #'<)))

;;finds maximum in a list
(defun maximum (list)
  (car (sort list #'>)))

;;calculates the range of a list
(defun range (list)
  (- (maximum list) (minimum list)))

;;scales one value to another range
(defun scale-value (list low high n)
   (+ (/ (* (- (nth (- n 1) list)
               (minimum list))
            (- high low))
         (range list))
      low))


;;is supposed to scale the whole list to another range
(defun scale-list (list low high n)
  (unless (= n 0)
   (cons (scale-value list low high n)
         (scale-list list low high (- n 1)))))

(scale-list '(1 2 3 4) 21 24 4)

Answer 1

需要改进最大值和最小值的定义。 SORT是破坏性的。用类似'（1 2 3 4）的字面常量调用SORT也是错误的 - 再次，SORT是破坏性的。

更好的定义：

(defun minimum (list)
  (reduce #'min list))

(defun maximum (list)
  (reduce #'max list))

更有效的范围定义：

(defun range (list)
  (loop for e in list
        maximize e into max
        minimize e into min
        finally (return (- max min))))

SCALE-LIST和SCALE-VALUE也不像Lisp一样。如果你在递归函数中像这样调用NTH那么就会出错。你应该对列表进行递归，而不是索引。 SCALE-VALUE为每个呼叫调用RANGE和MINIMUM。为什么呢？

检查此变体：

;;scales one value to another range
(defun scale-value (item low high min range)
   (+ (/ (* (- item min)
            (- high low))
         range)
      low))

;;is supposed to scale the whole list to another range
(defun scale-list (list low high)
  (let ((min (minimum list))
        (range (range list)))
    (labels ((scale-list-aux (list)
               (when list
                 (cons (scale-value (first list) low high min range)
                       (scale-list-aux (rest list))))))
      (scale-list-aux list))))

(scale-list '(1 2 3 4) 21 24)

你还能提高多少？例如，我将摆脱递归并用MAPCAR替换它。

Answer 2

我发布代码是因为出了问题......

;;finds minimum in a list
(defun minimum(list)
  (car  (sort list #'<)))
;;finds maximum in a list
(defun maximum(list)
  (car (sort list #'>)))
;;calculates the range of a list
(defun range(list)
  (- (maximum list) (minimum list)))

;;scales one value to another range
(defun scale-value(list low high n)
     (+ (/ (* (- (nth (- n 1) list) (minimum list)) (- high low)) (range list)) low))


;;is supposed to scale the whole list to another range
(defun scale-list(list low high n)
  (unless (= n 0)
   (cons (scale-value list low high n) (scale-list list low high (- n 1)))))

(scale-list '(1 2 3 4) 21 24 4)

Answer 3

您的实际堆栈跟踪类似于：

-(nil 0.1)
  (* (- (nth ... list) (minimum list)) (- high low))
  (/ (* (- ... ...) (- high low)) (range list))
  (+ (/ (* ... ...) (range list)) low)
  scale-value((0.1) 20 30 3)

我猜你确定了一个错误的第n个元素，这会返回nil，这会使减法变得混乱。

常见的口齿不清的问题

3 个答案: