error-handling - 方案:Why am i getting this error -assertion-violation: argument of wrong type [car] (car '())

Question

我在计划中的功能是尝试获取省的详细信息:

(define (getProvincesHelper sales provincesList)

  (if (null? sales) 

      provincesList

      ( getProvincesHelper (cdr sales) (append  (list (cadr(cadddr sales))) provincesList))

  ) 
)

(define (getProvinces sales)

 (getProvincesHelper sales '())

)

数据列表是这种格式

;(orderNum (orderDate shipDate) (grossSale discount profit unitPrice) (deliveryMethod province) product)

(3 ("10/13/2010" "10/20/2010") (261.54 0.04 -213.25 38.94) ("Regular Air" "Nunavut") "Eldon Base for stackable storage shelf, platinum")

我得到错误:

assertion-violation: argument of wrong type [car]
                     (car '())

Answer 1

这段代码:

(cadr (cadddr sales))

要求 sales具有足够的结构，以便您不会从非对值中删除 car或 cdr。这对于名称中的每个a和d都是这样的:

(cadr '(1))

产生相同的错误， cdr有效，但 car上的 '()无效。