An iterative version of f(n).

gistfile1.scm

(define (f' n)
  (define (f'-iter a b c count)
	; f(n) = a + b + c = f(n-1) + 2f(n-2) + 3f(n-3).
	; Use f(n) to calculate f(n+1) = f(n) + 2f(n-1) + 3f(n-2).
	(if (= count (+ n 1))
		a
		(f'-iter (+ a b c) (* 2 a) (* 3 (/ b 2)) (+ count 1))))
  (if (< n 3)
	  n
	  ; f(3) = f(2) + 2f(1) + 3f(0).
	  (f'-iter 2 2 0 3)))