Remove multiple characters from a list if they are next to each other in Scheme

Question

I have to make a Dr. Racket program that removes letters from a list if they are following the same letter as itself. For example: (z z f a b b d d) would become (z f a b d

Answer 1

This problem will be simpler if you introduce a helper function.

I recommend something like this (where angle brackets mean you need to fill out the details):

(define (remove-duplicates x)
  (cond
    [                              '()]  ; no duplicates in empty list
    [                       x]    ; no duplicates in a list with one element
    [  (cons (car x) (remove-from-front (car x) (cdr x)))]
    [else                                      (cons (car x) (remove-duplicates (cdr x)))]))

(define (remove-from-front e x)
  (cond
    [                   '()]                 ; e is not the first element of x
    [  (remove-from-front e (cdr x))] ; skip duplicate
    [else                           (remove-duplicates x)]))       ; no more es to remove