忽略列表中的非数字值并找到求和递归方法 [英] Ignore non-number values in a list and find the sum recursive method

问题描述

我需要在LISP中创建一个递归方法，该方法采用列表中的数字并求和.列表中所有非数字的内容都将被跳过(例如，如果列表中包含"Cheese 12 Dog 8 Shoe 5"，则输出为25).

I need to create a recursive method in LISP that takes the numbers in a list and finds the sum. Anything in the list that is not a number is skipped (For example, if the list contains "Cheese 12 Dog 8 Shoe 5", the output would be 25).

现在，我的代码找到了总和，但是如果列表中没有数字，则抛出错误.什么可以更改以解决此问题?

Right now my code finds the sum, but throws an error if there is anything in the list that is not a number. What can be changed to fix that?

(defun adder (lis)
   (cond
    ((null lis) 0)
    (t (eval (cons '+ lis)) )
  )
)

推荐答案

这可以做到:

(defun adder (lis)
  (if (null lis)
    0
    (let ((c (car lis)))
      (if (numberp c)
        (+ c (adder (cdr lis)))
        (adder (cdr lis))))))

您的版本不是递归的(您不会在加法器内部调用加法器)，也许您的意思是这样的(非递归的)?

Your version is not recursive (you don't call adder inside of adder), maybe you meant something like this (which is non-recursive)?

(defun adder (lis)
  (apply '+ (remove-if-not 'numberp lis)))

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