CODEWITHSUNDEEP
racket
Darren Tan
Darren Tan

Reputation: 1

Racket function recursion

hello I'm a student currently trying to make a function in Dr racket that's purpose is when a letter is selected it turns it into underscore

string(word) string(letter) -> string(answer/underscore)

I've only been able to make this happen with one letter as _ which makes the second check true and I can't figure out how to do multiple letters 

(check-expect(underscore "william""li")"_illi__")) 
(check-expect(underscore "william" "l")"__ll___))

My code:

(define (anti-omit word letter)
  (cond[(string=? word letter)letter]
    [(= 1 (string-length word))"_"]
     [else 
      (string-append 
(anti-omit (substring word 0 1)letter)
(anti-omit (substring word 1)letter))]))

Upvotes: 0

Views: 396

Answers (1)

Atharva Shukla
Atharva Shukla

Reputation: 2137

Here's a stub with a purpose statement, signature, and your tests:

;; String String -> String
;; keeps all letters  that occur in l, replaces with "_" otherwise

(check-expect (anti-omit "william" "li") "_illi__")
(check-expect (anti-omit "william" "l") "__ll___")

(define (anti-omit w l)
  "")

Since you're using a student language, and as you suggest in the title of the question that you want to recur on the data... you'll need a recursive data-definition:

;; String is one of:
;; - ""
;; - (string-append 1String String)

Complete the definition of this function:

;; String -> String
;; is str equal to "" ?
(define (string-empty? str)
  ...)

a more restrictive data-definition for other two functions (which correspond to rest and first of a list):

;; NEString is one of:
;; - (string-append 1String "")
;; - (string-append 1String NEString)

Some other functions for you to complete:

;; NEString -> String
;; remove the first 1String from str
(define (string-rest str)
  ...)

;; NEString -> String
;; the first 1String form str
(define (string-first str)
  ...)

Now you can make functions like this on Strings:

;; [1String -> 1String] String -> String
;; Applies the function f on each 1String in s
(define (map-str f s)
  (cond  [(string-empty? s) ""]
         [else (string-append (f (string-first s)) (map-str f (string-rest s)))]))

Similarly, complete this definition:

;; 1String String -> Boolean
;; is does 1String occur in String?
(define (member-str e l)
  ...)

Our "wish list" is complete, and we can compose our helpers to finish the final function:

(define (anti-omit w l)
  (map-str (λ (x) (if (member-str x l) x "_")) w))

A similar version that uses explode and implode and doesn't need any of the functions we defined (use this as a reference implementation):

(define (anti-omit-ref w l)
  (implode (map (λ (x) (if (member? x (explode l)) x "_")) (explode w))))

Upvotes: 1

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