function - Break loop and return in Racket Scheme

Question

I'm trying to find the first missing key in a hash table, that should contain keys [1 ... N], using Scheme.

So far, I have the following code:

(define (find-missing n ht)
  (define c 1)

  (let forVertices ()
    (cond (not (hash-has-key? ht c))
      (c)
    )

    (set! c (+ c 1))
    (when (>= n c) (forVertices))
  )
)

When I execute the function to test it, nothing is returned. What am I doing wrong?

Answer 1

括号有几个问题，并且else案例丢失了。这应该可以解决错误：

(define (find-missing n ht)
  (define c 1)
  (let forVertices ()
    (cond ((not (hash-has-key? ht c))
           c)
          (else
           (set! c (+ c 1))
           (when (>= n c)
             (forVertices))))))

……但是您应该知道，您编写程序的方式根本不是惯用的。一般来说，在Scheme中你应该避免改变状态（set!操作），你可以通过正确设置递归和传递正确的参数来编写一个等效的过程。此外，最好在递归退出时返回一些东西（例如：） ，否则如果没有缺少键#f，您的过程将返回。#<void>在这里，这就是我的意思：

(define (find-missing n ht)
  (let forVertices ((c 1))
    (cond ((> c n) #f)
          ((not (hash-has-key? ht c)) c)
          (else (forVertices (+ c 1))))))