scheme - 这是设计此功能的最佳方式吗？

Question

我叫迈克尔·布特罗斯，目前是一名高中生。明年我将参加滑铁卢大学，作为他们 CS 类(class)的入门，我一直在学习并完成一些作业。他们使用的语言是 Scheme，我一直在学习它。我来自 PHP 和 Ruby 背景，所以我担心我从这些语言中学到的一些习惯会被错误地应用到我正在使用的方案中。这是作业中的问题(顺便说一下，在线提供):

Write a function called sub-time that consumes a string start-time representing a time, and a natural number mins which represents the number of minutes before. The function produces a string representing mins minutes before the given time start-time.

The string consumed will be formatted as follows: a two digit number, followed by “ hours ”, followed by a two digit number, followed by “ minutes”. The time consumed is in 24-hour format. If the number of hours or minutes is less than 10, then the number will include a leading 0. The string produced must be in exactly the following format: a number, followed by “ hours ”, followed by a number, followed by “ minutes”. Note that the string produced does not have a leading 0 before numbers less than 10. It is possible that the time produced could represent a time on a previous day.

For example,
• (sub-time “03 hours 15 minutes” 0) produces
“3 hours 15 minutes”,
• (sub-time “13 hours 05 minutes” 845) produces
“23 hours 0 minutes”, and
• (sub-time “13 hours 05 minutes” 2881) produced
“13 hours 4 minutes”

The built-in Scheme functions string->number and number->string may be useful.

It is very important that you produce the string exactly as described, otherwise the autotests will fail. In particular, all characters must be in lower case and there must be single spaces between all parts of the string.

Do not use any cond expressions in your solution.

(老实说，我直到现在才注意到最后一行，我的解决方案确实使用了条件表达式。有什么办法可以避免它们？)

这是我的解决方案:

;; Assignment 1 Question 4
(define (convert-to-string hours minutes)
  (string-append (number->string hours) " hours " (number->string minutes) " minutes"))

(define (subtract_hours start_hours sub_hours minutes)
  (let ((hours (- start_hours (modulo sub_hours 24))))
    (if (< hours 0)
        (convert-to-string (+ 24 hours) minutes)
        (convert-to-string hours minutes))))

(define (subtract_minutes start_hours start_minutes sub_hours sub_minutes)
  (let ((minutes (- start_minutes sub_minutes)))
    (if (< minutes 0)
        (subtract_hours start_hours (+ 1 sub_hours) (+ 60 minutes))
        (subtract_hours start_hours sub_hours minutes))))

(define (sub-time start-time mins)
  (let ((start_hours (string->number (substring start-time 0 2))) 
        (start_minutes (string->number (substring start-time 9 11)))
        (sub_hours (quotient mins 60))
        (sub_minutes (modulo mins 60)))        
    (subtract_minutes start_hours start_minutes sub_hours sub_minutes)))

我对 Scheme 非常陌生，尽管我的解决方案有效(通过我有限的测试)，但我想知道一些经验丰富的老手会怎么说。谢谢你的时间!

Answer 1

是的，您可以在没有任何条件的情况下编写此代码。考虑将小时+分钟转换为分钟数，然后进行减法，然后将结果转换回小时+分钟。