Question

我尝试创建一个sqrt+函数，它将获取一个数字列表并返回一个数字列表。谁能告诉我这个功能有什么问题？

#lang pl 03

(: sqrt+ : (Listof Number) -> (Listof Number))
;; a version of `sqrt' that takes a list of numbers, and return a list

;; with twice the elements, holding the two roots of each of the inputs;

;; throws an error if any input is negative.
(define (sqrt+ ns)
  (cond [(null? ns) 0]
        [(< (first ns) 0) (error 'ns "`sqrt' requires a nonnegative input ~s")]
        [else ((sqrt ns) (* (sqrt ns) -1))]))

Type Checker: type mismatch  
  expected: (Listof Number)
  given: Zero
  in: 0
Type Checker: type mismatch
  expected: Nonnegative-Real
  given: (Pairof Nonnegative-Real (Listof Number))
  in: ns
Type Checker: type mismatch
  expected: Nonnegative-Real
  given: (Pairof Nonnegative-Real (Listof Number))
  in: ns
Type Checker: Summary: 3 errors encountered
  in:
   0
   ns
   ns

Answer 1

在else案例中，您应该使用：

(let ([s (sqrt (first ns))])
  (cons s
        (cons (* s -1)
              (sqrt+ (rest ns)))))`

Answer 2

其他原因需要不同，当你有一个空列表时，返回类型也应该是一个空列表，而不是一个数字。所以，你的cond的第一个条款的返回应该是＆＃34; null＆＃34;或者＆＃34; nil＆＃34;。请尝试使用以下代码（语法可能需要稍微调整一下）：

(define (sqrt+ ns)
  (cond [(null? ns) ns]
        [(< (first ns) 0) (error 'ns "`sqrt' requires a nonnegative input ~s")]
        [else (cons (sqrt (first ns))
                    (cons (* -1 (sqrt (first ns)))
                          (sqrt+ (rest ns))))]))

在球拍中创建sqrt功能

2 个答案: