Getting every nth atom using scheme does not pick up the last atom

Question

The program is suppose to pick out every third atom in a list. Notice that the last atom \'p\' should be picked up, but its not. Any suggestions as to why the last atom is n

Answer 1

You're missing a couple of base cases:

(define (every3rd lst)
  (cond ((or (null? lst) (null? (cdr lst))) lst)
        ((null? (cdr (cdr lst))) (list (car lst)))
        (else (cons (car lst)
                    (every3rd (cdr (cdr (cdr lst))))))))

See how the following cases should be handled:

(every3rd '())
=> '()

(every3rd '(a))
=> '(a)

(every3rd '(a b))
=> '(a)

(every3rd '(a b c))
=> '(a)

(every3rd '(a b c d))
=> '(a d)

(every3rd '(a b c d e f g h i j k l m n o p))
=> '(a d g j m p)