Question

I have this iterative process in Scheme. (In fact I don’t really know what kind of process it really is)

(define (contains-double? lst)
 (cond
  ((or (null? lst) (null? (cdr lst))) #f)
  ((eq? (car lst) (cadr lst)) #t)
  (else (contains-double? (cdr lst)))))

It checks If there are 2 of the same numbers next to each other.
For example:

(contains-double? '(1 2 3 3 3 5 2))    => #t
(contains-double? '(1 2 3 5 3 2))      => #f
(contains-double? '(1 2 3 2 2))        => #t

Can I make this process recursive?

Thanks in advance

Answer 1

The procedure in the question is recursive (contains-double? is calling itself), but the process it generates is iterative because the procedure is written in a tail-recursive style – meaning that there is nothing to do after the procedure returns from the recursive call, except return its value.

The process generated by this procedure can be made recursive by removing the tail recursion:

(define (contains-double? lst)
  (if (or (empty? lst) (empty? (rest lst)))
      #f
      (or (contains-double? (rest lst))
          (= (first lst) (second lst)))))