如何在RACKET中做所有乘法运算 [英] how to do multiply-all function in RACKET

问题描述

练习22.5.11 开发一个函数multiple-all，它接受一个数字列表，并返回将所有数字相乘的结果.
例如: (check-expect (multiply-all (cons 3 (cons 5 (cons 4 empty)))) 60)
提示:空列表的正确答案"是什么?一开始可能不是您想的那样！

Exercise 22.5.11 Develop a function multiply-all that takes in a list of numbers and returns the result of multiplying them all together.
For example: (check-expect (multiply-all (cons 3 (cons 5 (cons 4 empty)))) 60)
Hint: What is the "right answer" for the empty list? It may not be what you think at first!

解决方案:数据定义类似于字符串列表:

Solution: The data definition is similar to that for list-of-strings:

; A list-of-numbers is either
; empty or
; a nelon (non-empty list of numbers).
#|
(define (function-on-lon L)
; L a list of numbers
(cond [ (empty? L) ...]
[ (cons? L) (function-on-nelon L)]
))
|#
; A nelon looks like
; (cons number lon )
#|
(define (function-on-nelon L)
; L a cons
; (first L) a number
; (rest L) a lon
; (function-on-lon (rest L)) whatever this returns
...)
|#

有什么建议吗?

推荐答案

对于最简单的解决方案，请为此使用apply:

For the simplest solution, use apply for this:

(define (multiply-all lst)
  (apply * lst))

如果您需要从头开始构建过程，只需记住基本情况(一个空列表)应返回1，并且递归步骤应使用标准解决方案模板乘以当前值，如下所示:

If you need to build the procedure from scratch, just remember that the base case (an empty list) should return 1, and the recursive step should multiply the current value using the standard solution template, like this:

(define (multiply-all lst)
  (if (empty? lst)
      1
      (* (first lst)
         (multiply-all (rest lst)))))

要获得更好的答案，您可以尝试使用尾部递归:

For a nicer answer, you can try using tail recursion:

(define (multiply-all lst)
  (let loop ([lst lst] [acc 1])
    (if (empty? lst)
        acc
        (loop (rest lst) (* (first lst) acc)))))

无论如何，这些程序都能按预期工作:

Anyway the procedures work as expected:

(multiply-all '())
=> 1
(multiply-all '(3 5 4))
=> 60

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