CODEWITHSUNDEEP
schemelispracket
Mahmud Adam
Mahmud Adam

Reputation: 3579

string-append inside recursive function

Given a structure layer defined:

(define-struct layer [color doll])

I am trying to write a function that will produce a string of colors of all the dolls.

; colors: Russian doll (RD) -> String
; consumes an RD and produces a string of all colors
(define (colors an-rd)
   (cond
       [(string? an-rd) (string-append an-rd ",")]
       [else
          (colors (layer-doll an-rd))]))

(colors
  (make-layer
   "yellow"
    (make-layer
      "green" "red")))

I would like for colors function, given the above input, to produce yellow, green, red; however, it only returns red. What am I doing wrong here?

Upvotes: 0

Views: 216

Answers (1)

Óscar López
Óscar López

Reputation: 236004

The base case is in correct (it should simply return a string), and consequently the call to string-append is in the wrong place; the idea is to append a piece of the string at each recursive call until there are no more pieces. Try this instead:

(define (colors an-rd)
  (cond
    [(string? an-rd) an-rd]
    [else
     (string-append (layer-color an-rd)
                    ", "
                    (colors (layer-doll an-rd)))]))

For example:

(colors
 (make-layer
  "yellow"
  (make-layer
   "green" "red")))

=> "yellow, green, red"

Upvotes: 2

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