Question

编写一个名为direct-num-occurs的程序？检查数字列表中是否出现数字。例子：

> (direct-num-occurs? 1 '(2 3 1 4))
;=> #t

> (direct-num-occurs? 1 '(2 3 5 4))
;=> #f

这就是我的尝试：

(define direct-num-occurs?
  (lambda (num ws)
    (cond
      [(null? ws) #f]
      [(equal? num (car ws)) #t]
      [else (direct-num-occurs? (cdr ws) num)])))

但是我收到一个错误，上面写着预期的一对。

Answer 1

为了将来参考，您将重新实施member程序：

(define (direct-num-occurs? n lst)
  (if (member n lst) #t #f))

...但我想你是想从头开始写它。显示的实现基本上是正确的，除了你在最后一行以错误的顺序传递了参数。这应该解决它：

(define direct-num-occurs?
  (lambda (num ws)
    (cond
      [(null? ws) #f]
      [(equal? num (car ws)) #t]
      [else (direct-num-occurs? num (cdr ws))])))

当然，它按预期工作：

(direct-num-occurs? 1 '(2 3 1 4))
=> #t

(direct-num-occurs? 1 '(2 3 5 4))
=> #f

检查数字列表中是否出现数字

1 个答案: