Question

我的else语句行给我一个错误。我的其他任何代码行是否会影响else表达式？

(define (sumAdd list)
  (cond
    ((null? list) '())
    ((null? (cdr list)) list)
    ((symbol? list) sumAdd(cdr list))
    (else (+ (car list)(sumAdd (cdr list))))
    )
  )

Answer 1

如果我理解正确，您想将元素混合类型的列表中的所有数字相加。在这种情况下，您的代码中有几个错误：

(define (sumAdd list)                 ; `list` clashes with built-in procedure
  (cond
    ((null? list) '())                ; base case must be zero for addition
    ((null? (cdr list)) list)         ; why discard the last element?
    ((symbol? list) sumAdd(cdr list)) ; that's not how procedures are called
    (else (+ (car list) (sumAdd (cdr list)))))) ; this line is fine :)

这是实现该过程的正确方法：

(define (sumAdd lst)
  (cond
    ((null? lst) 0)                           ; base case is zero
    ((symbol? (car lst)) (sumAdd (cdr lst)))  ; skip current element
    (else (+ (car lst) (sumAdd (cdr lst)))))) ; add current element

它按预期工作：

(sumAdd '(1 a 2 b 3 c))
=> 6

方案中的“列表总数”代码有什么问题吗？

1 个答案: