Dracket：在没有内置功能的情况下在列表中找到最高值

Question

我所在的 class 这个作业让我很困惑。我能够使大部分功能正常工作，但这一部分使我无法完成它。我有两个函数，这是我需要的，一个是在列表中找到最高数值 (largest-num)，另一个是查找列表并引用它 (max-lon)。我收到以下错误 largest-num: expects only 1 argument, but found 2 我不太确定如何修复它。列出代码：

(define (max-lon a-lon)
  (local [
          (define ACUMM (void))
          (define lst   (void))
          ;; lon number → number
          ;; Purpose: Return max(max of given list, given number)
          ;; Accumulator Invariant: accum = maximum in L - lst
          (define (max-helper)
            (if(empty? lst)
                ACUMM
                (begin
                  (set! ACUMM (largest-num (first lst) ACUMM))
                  (set! lst (rest lst)) 
                  (max-helper))))]
     
    (if (empty? a-lon)
        (error 'max-lon "An empty lon does not have a maximum.")

        (begin
          (set! ACUMM(first a-lon))
          (set! lst (rest a-lon))
          (max-helper)))))

(define (largest-num lst)
  (cond 
    [(empty? (rest lst)) (first lst)]
    [(< (first lst) (first (rest lst))) (largest-num (rest lst))] 
    [else (largest-num (cons (first lst) (rest (rest lst))))]))

任何建议都很好，谢谢

Answer 1

largest-num 可以简化很多，因为它所要做的就是比较两个数字，即 ACUMM，即列表中的 accumulator/place，第一个 lst 是拥有最高数字的数字，而不是而不是处理我最初假设的整个列表，这里是固定代码：

(define (max-lon a-lon)
  (local [
          (define ACUMM (void))
          (define lst   (void))

          (define (max-helper)
            (if(empty? lst)
                ACUMM
                (begin
                  (set! ACUMM (largest? ACUMM (first lst))) 
                  (set! lst (rest lst)) 
                  (max-helper))))]
     
    (if (empty? a-lon)
        (error 'max-lon "An empty lon does not have a maximum.") 
        
        (begin
          (set! ACUMM(first a-lon))
          (set! lst (rest a-lon))
          (max-helper))))) 

(define (largest? a b)
  (if (> a b) a b))

Answer 2

(define (largest-num lst (acc -inf.0))
  (cond ((empty? lst) acc)
        ((and acc (> (car lst) acc)) (largest-num (cdr lst) (car lst)))
        (else (largest-num (cdr lst) acc))))

Dracket：在没有内置功能的情况下在列表中找到最高值

Drracket: Finding highest value in a list without in built function

list

arguments

racket