为什么我的lisp代码给我...应该是lambda表达式? [英] Why does my lisp code give me ...should be a lambda expression?

问题描述

(defun helper-2 (list) 
  (if (null (first (rest list)))
      0)
  (+ (distance ((car list) (first (rest list)))) 
     (helper-2 (rest list))))

我是Lisp的新手，我正在编写一个程序来计算任何多边形的周长，并按顺时针方向输入.我的逻辑是，我使用辅助方法来计算彼此相邻的两个点的长度并求和.递归完成后，我将做一个单独的调用来计算从起点到终点的长度，并对所有内容求和.我已经完成了距离方法，该方法需要2点并返回长度.

I'm new to lisp and I'm writing a program to compute the perimeter of any polygon with input following clockwise order. My logic is that I use a helper method to compute the length of two points next to each other and do the sum. After recursion is done, I will do a separate call to calculate the length from the beginning point to its end and sum everything up. I've finished the distance method which takes 2 points and return the length.

(distance '(2 0) '(4 0)) ;this will output 2

helper-2逻辑: 假设我们有3个点a(2 0)b(3 3)c(4 0)这种方法可以总结ab和bc之间的距离.但是，我不断收到(车头)应该是lambda表达式"错误.有人可以帮忙吗?谢谢你.还是有人可以给我一种更好的方法来计算多边形的周长?

helper-2 logic: assume we have 3 points a (2 0) b (3 3) c (4 0) This method is expected to sum up the distance between ab and bc. However, I keep getting "(car head) should be a lambda expression" error. Can anyone help? Thank you. Or could anyone give me a better way to compute the perimeter of a polygon?

(defun square (n) (* n n))

(defun distance (a b)
  (let ((h (- (second b) (second a)))
        (w (- (first b) (first a))))
    (sqrt (+ (square h) (square w)))))

推荐答案

您的helper-2函数在两个地方是错误的:

Your helper-2 function is wrong in two places:

1. 您应该使用两臂式if，这样它就可以充当if/else.
2. (car list)周围的括号太多.

1. You should be using a two-armed if, so that it functions as an if/else.
2. You have too many parentheses around the (car list).

这是固定版本:

(defun helper-2 (list) 
  (if (null (first (rest list)))
      0
      (+ (distance (car list) (first (rest list))) 
         (helper-2 (rest list)))))

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