在球拍中，如何仅使用string-> list或list-> string函数替换字符串中的单词？

Question

So I was practicing racket beginner language when I came along this question.

Write a function str-replace which consumes a string, a target character, and a replacement character. The function produces a new string, which is identical to the consumed string with all occurrences of the target character (if any) replaced with the replacement character. For example, (string-replace "word" #\\o #\\y) ⇒ "wyrd" .

Note: I may not use any built-in string functions other than string->list and list->string .

So I started with the code now I got stuck, how do I use wrapper function for this code as far now I have only this

;; los is list of string      
(define(str-replace los)
   (+(first los)
   (first (rest los))
   (first (rest (rest los)))
   (first (rest (rest (rest los))))))

Answer 1

Define a conversion function which operates on lists:

(define (replace-in-list input-list from-char to-char)
  (if (null? input-list)
      ...
      (cons ... 
            (replace-in-list ... from-char to-char))))

^{(You have to fill the blank ... )}

And call it from another one:

(define (str-replace input-string from-char to-char)
  (list->string 
    (replace-in-list 
      (string->list input-string) from-char to-char)))