Question

我是初学者，我正在尝试自学Common Lisp，在我的自学期间，我编写了一个函数，我相信它应该适用于递归添加两个参数。但是，该功能总是失败。那是为什么？

(defun sum (n m)
  ;;;Returns the sum of n and m using recursion
  (cond ((eq m 0) n))
  (sum (1+ n) (1- m)))

据我了解，它应该连续加1到n，同时递减m直到m为0，此时递归加法完成

Answer 1

这种添加真的很奇怪的用例，但我会解释你的错误在哪里：

(defun sum (n m)
  ;;;Returns the sum of n and m using recursion
  (cond ((eq m 0) n)) ;; <= This line is ignored, you not returnin N.
  (sum (1+ n) (1- m))) ;; <= this will be called forever

你应该写：

(defun sum (n m)
  "Recursively increment N and decrement M untill M = 0"
  (if (= m 0) ;; don't use EQ for numbers, use EQL or =.
    n
    (sum (1+ n) (1- m)))) ;; Otherwise recursive call

让我们跟踪它以了解它是如何工作的：

CL-USER> (sum 0 10) 
  0: (SUM 0 10)
    1: (SUM 1 9)
      2: (SUM 2 8)
        3: (SUM 3 7)
          4: (SUM 4 6)
            5: (SUM 5 5)
              6: (SUM 6 4)
                7: (SUM 7 3)
                  8: (SUM 8 2)
                    9: (SUM 9 1)
                      10: (SUM 10 0)
                      10: SUM returned 10
                    9: SUM returned 10
                  8: SUM returned 10
                7: SUM returned 10
              6: SUM returned 10
            5: SUM returned 10
          4: SUM returned 10
        3: SUM returned 10
      2: SUM returned 10
    1: SUM returned 10
  0: SUM returned 10
10

如果你会接受建议 - 不要尝试用递归来做这些奇怪的事情，如果你想学习如何使用它，试试一些自然递归的情况，如阶乘，斐波纳契，树处理等

Answer 2

我认为你有两个简单的拼写错误：

一个括号太多且
在您的t子句中缺少cond。

你可能意味着：

(defun sum (n m)
  (cond 
   ((eq m 0) n)               ; here you had one parenthesis too many
   (t (sum (1+ n) (1- m)))))  ; here you were missing the `t` symbol

Lisp中的递归加法

2 个答案: