Question

我在启动程序时始终收到错误first: expects a non-empty list：(make-subject (make-person 22 'm 'MW17K) (list 220 301 189 272 311))并且无法找到原因。

;;equals 25 but my code keeps erroring. please help
(person-age (subject-person (first sub)))     

(define-struct person (age sex code))

(define-struct subject (person times))

(define VP01 (make-subject (make-person 22 'm 'MW17K) (list 220 301 189 272 311)))
(define VP02 (make-subject (make-person 25 'f 'MP25G) (list 234 197 253 257 206)))
(define VP03 (make-subject (make-person 23 'f 'CT03R) (list 197 202 214 222 233)))
(define VP04 (make-subject (make-person 20 'm 'MM09R) (list 273 314 257 264 217)))
(define VP05 (make-subject (make-person 19 'm 'KR22I) (list 198 197 228 253 199)))
(define VP06 (make-subject (make-person 26 'm 'FR01B) (list 212 204 289 294 223)))
(define VP07 (make-subject (make-person 28 'f 'RA15R) (list 258 323 189 247 303)))
(define VP08 (make-subject (make-person 22 'm 'RP18R) (list 221 307 182 271 316)))
(define VP09 (make-subject (make-person 24 'f 'GH31W) (list 230 295 304 264 237)))
(define VP10 (make-subject (make-person 19 'f 'OM29Q) (list 299 194 242 303 243)))

(define subjects (list VP01 VP02 VP03 VP04 VP05 VP06 VP07 VP08 VP09 VP10))

(define (idk sub)
  (cond
    [(empty? sub) empty]        
    [(< (person-age (subject-person (first sub)))
        (person-age (subject-person (first (rest sub)))))
      (idk (first sub))]
    [else (idk (first (rest sub)))]))

(idk subjects)

Answer 1

您的类型不匹配：

(define (idk sub)   
  (cond
    [(empty? sub) empty]        
    [(< (person-age (subject-person (first sub)))   ; here2 -------- NB
        (person-age (subject-person (first (rest sub)))))
      (idk (first sub))]    ; here1 ------------------------ NB
    [else (idk (first (rest sub)))]))  ; here3

(idk subjects)

使用idk列表调用

subject，因此其定义中的sub是subject的列表。这意味着 here1 的(first sub)为subject。

然后你在 here1 （或 here3 ）上用idk调用它，然后在 here2 <{}调用(first sub) < / em>的。此时sub是subject，而不是列表。因此错误，

first：预计非空列表给定：(make-subject (make-person 22 'm 'MW17K) (list 220 301 189 272 311))

Answer 2

我认为使用let和car，cdr使代码更清晰：

(let loop ([loop_list subjects])
   (if (>= (length loop_list) 2)
       (if (>= (person-age (subject-person (car loop_list)))
               (person-age (subject-person (cadr loop_list))))
           (person-age (subject-person (car loop_list)))
           (loop (cdr loop_list)))
       empty))

第一：期望一个非空列表

2 个答案: