everythird

Every third, `et':

(defun et(in)
  (COND
    ( (EQUAL NIL in) NIL)
    ( (< (LIST-LENGTH in) 3) NIL )
    ( T (CONS (caddr in) (et (cdddr in))) )
  )
)


Break 13 [17]> (et `(1 2 3 4 ))
(3)
Break 13 [17]> (et `(1 2))
NIL
Break 13 [17]> (et `(1 2 3 4 5 6 7 8 9 0))
(3 6 9)
Break 13 [17]> (et `(1 2 3 4 5 6 7 8 9 0 2))
(3 6 9)
Break 13 [17]> (et `(1 2 3))
(3)


/////////////////////// PRACTICE /////////////////////////

def et(in):
  return (CONS caddr(in) et(cdddr(in)))

(defun et(in)(COND ((EQUAL NIL in) NIL) (T (CONS (caddr in) (et (cdddr in)) ) ) ))

(defun et(in)
  (COND 
    (  ((< (LIST-LENGTH in) 3) NIL)
       (T 
         (CONS (caddr in) (et (cdddr in)))
       )
      )
    )