1.11 of sicp

gistfile1.el

;A function f is defined by the rule that f(n) = n if n<3 and
; f(n) = f(n - 1) + 2f(n - 2) + 3f(n - 3) if n> 3. 
; Write a procedure that computes f by means of a recursive process.
; Write a procedure that computes f by means of an iterative process.
(defun func_f (n)
  (if (< n 4) n
    (+ (func_f (- n 1))
       (* 2 (func_f (- n 2)))
       (* 3 (func_f (- n 3))))))

(print (func_f 6))

(defun iter_f (n)
  (if (< n 4) n
    (iter_f_iter 1 2 3 n)))

(defun iter_f_iter (n1 n2 n3 n)
  (if (< n 4)
      n3
    (iter_f_iter n2
                 n3
                 (+ n3 (* 2 n2) (* 3 n1))
                 (- n 1))
    ))

(print (iter_f 6))