Question

I need compare every second element in the list but I don’t know how. Here is an example:

(compare? '(1 x 2 x 3 x 4)) -> #t
(compare? '(1 x 2 x 3 o)) -> #f

I can only compare second and fourth element:

(define compare?
  (lambda (list)
    (equal? (cadr list) (cadddr list))))

I need 6th, 8th, 10th etc…. I don’t know length of the list. Please, help me.

Answer 1

Try this answer, filling-in the blanks:

(define (compare? lst)
  (if <???> ; if the list has at most two elements
      #t    ; then return true
      (let ((elt (cadr lst)))        ; grab the first element to be compared
        (let loop ((lst (cddr lst))) ; step on the second group of elements
          (cond (<???> #t)           ; if there's only one or zero elements left
                (<???> #f)           ; if the second element is not equal to `elt`
                (else (loop (cddr lst)))))))) ; otherwise continue iterating