SICP Exercise 1.11

SICP 1.11

 ; recursive version  
 (define (f-rec n)  
  (cond ((<= n 3) n)  
     (else (+ (f-rec (- n 1))   
          (* 2 (f-rec (- n 2)))   
          (* 3 (f-rec (- n 3)))))))  
   
 ; iterative version  
 (define (f n)  
  (cond ((<= n 3) n)  
     (else (f-iter 1 2 3 3 n))))  
   
 (define (f-iter fn-1 fn-2 fn-3 last-calc end)  
  (cond ((= last-calc end) fn-3)  
     (else (f-iter fn-2 fn-3 (+ fn-3 (* 2 fn-2) (* 3 fn-1)) (+ last-calc 1) end))))