tonumber函数(tonumber'(一二三)-->123 [英] Tonumber function (tonumber ‘(one two three) --> 123

问题描述

解决后如何在球拍中拼写一个数字?(spellNum) ，现在我正在尝试编写一个与此函数相反的函数.即

After the solution of how to spell a number in racket? (spellNum) ,now I am trying to write a function which is opposite of this function. i.e

(tonumber ‘(one two three) --> 123

到目前为止我已经编写了这个工作代码

so far I have written this working code

(define (symbol->digit n)
  (case n
    ('zero 0)
    ('one 1)
    ('two 2)
    ('three 3)
    ('four 4)
    ('five 5)
    ('six 6)
    ('seven 7)
    ('eight 8)
    ('nine 9)
    (else (error "unknown symbol:" n))))



(define (numlist n)
  (map symbol->digit  n))



(numlist '(one two three))

从 numlist 中，我得到了 '(1 2 3).但是下面的函数存在一些问题，我想将列表转换为数字

From numlist, I got '(1 2 3). But to there is some problem in the function below in which I want to convert list to number

(define (list->number l)
  (set! multiplier (* 10 (lenght l)))
  (for/list [(c l)] 
    (* multiplier c))
  (set! multiplier (/ multiplier 10)))

(list->number '(1 2 3))

任何帮助将不胜感激.我在网上找不到所有类型的循环的文档.在http:///docs.racket-lang.org/ts-reference/special-forms.html?q=loop#%28part._.Loops%29

any help will be appreciated. I can't find documentation of all kind of loops online. at http://docs.racket-lang.org/ts-reference/special-forms.html?q=loop#%28part._.Loops%29

我想熟悉 Racket，所以我想避免内置转换函数.在列表-> 数字中，我试图从列表中一个一个地取数字，然后我想根据列表的长度将它们乘以 10,100,1000 等等.以便它可以返回一个数字.例如 '(1 2 3) = 1*100+2*10+3*1

I want to become familiar with Racket so I want to avoid builtin conversion functions. In list->number,I am trying to take digits one by one from list and then i want to multiply them with 10,100,1000 so on depending on the length of list. so that it can return a number. For example '(1 2 3) = 1*100+2*10+3*1

推荐答案

这里与我的之前的解决方案完全相反，再次对 list->number 过程使用尾递归:

Here's the exact opposite of my previous solution, once again using tail recursion for the list->number procedure:

(define (symbol->digit n)
  (case n
    ('zero 0)
    ('one 1)
    ('two 2)
    ('three 3)
    ('four 4)
    ('five 5)
    ('six 6)
    ('seven 7)
    ('eight 8)
    ('nine 9)
    (else (error "unknown symbol:" n))))

(define (list->number lst)
  (let loop ((acc 0) (lst lst))
    (if (null? lst)
        acc
        (loop (+ (car lst) (* 10 acc)) (cdr lst)))))

(define (toNumber lst)
  (list->number (map symbol->digit lst)))

它按预期工作:

(toNumber '(four six seven))
=> 467

只是为了好玩，在 Racket 中，我们可以使用 迭代和理解.即便如此，请注意我们不会在任何地方使用 set!，改变状态是 Python 等语言的规范，但在一般的 Scheme 和 Racket 中，我们尽量避免修改循环内的变量 -有更优雅的方式来表达解决方案:

Just for fun, in Racket we can write a function like list->number using iteration and comprehensions. Even so, notice that we don't use set! anywhere, mutating state is the norm in a language like Python but in Scheme in general and Racket in particular we try to avoid modifying variables inside a loop - there are more elegant ways to express a solution:

(define (list->number lst)
  (for/fold ([acc 0]) ([e lst])
    (+ e (* 10 acc))))

这篇关于tonumber函数(tonumber'(一二三)-->123的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！