案例陈述未赋值 [英] Case Statement Not Assigning Value

问题描述

我在调试 case 语句时遇到了一些问题.我希望该语句将数值分配给 note-val，但到目前为止它正在分配 #<void>.我知道 case 语句有问题，因为如果我添加 else 子句，就会应用该值.给定 '(((#\3 #\A) (#\4 #\B)) ((#\4 #\C))) 的示例输入，我在这里做错了什么?(关于 case 语句.我确定还有其他错误，但如果我能解决这个问题，我想尝试自己解决.)

I'm having some trouble debugging a case statement. I was hoping that the statement would assign numeric values to note-val, but so far it is assigning #<void>. I know it's something wrong with the case statement, because if I add an else clause, that value gets applied. Given a sample input of '(((#\3 #\A) (#\4 #\B)) ((#\4 #\C))), what am I doing wrong here? (In regards to the case statement. I'm sure there are other errors, but I'd like to try to work those out myself if I can get this fixed.)

(define (calc-freqs chord)
  (let ((octave (char->int (caaar chord)))
        (note-val (case (cdaar chord)
                    [((#\B #\#) (#\C))      0]
                    [((#\C #\#) (#\D #\b))  1]
                    [((#\D))                2]
                    [((#\D #\#) (#\E #\b))  3]
                    [((#\E) (#\F #\b))      4]
                    [((#\E #\#) (#\F))      5]
                    [((#\F #\#) (#\G #\b))  6]
                    [((#\G))                7]
                    [((#\G #\#) (#\A #\b))  8]
                    [((#\A))                9]
                    [((#\A #\#) (#\B #\b)) 10]
                    [((#\B) (#\C #\b))     11])))
    (cons (* a4 (expt 2 (+ (- octave 4) (/ (- note-val 9) 12))))
          (if (pair? (cdr chord))
              (calc-freqs (cdr chord))
              '()))))

哦，还有 char->int 是我写的一个很小的实用函数，它几乎完全符合它的名称 (#\1 => 1，依此类推).

Oh, and char->int is a tiny utility function I wrote that pretty much does what it says in the name (#\1 => 1, and so on).

推荐答案

case 使用 eqv? 进行匹配.这意味着除了符号、数字、字符、布尔值或空列表之外的任何东西都不会匹配.

case does matching using eqv?. That means that anything other than symbols, numbers, characters, booleans, or the empty list will never match.

就您而言，您试图匹配(非空)列表.那永远行不通.:-((匹配字符串或向量都不起作用.)

In your case, you were trying to match (non-empty) lists. That will never work. :-( (Neither will matching strings or vectors work.)

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