SICP exercise 1.11

sicp_exercise_1_11.scm

(define (f n)
  (cond ((= n 1) 1)
        ((= n 2) 2)
        ((= n 3) 3)
        (else (+ (f (- n 1))
                  (* 2 (f (- n 2)))
                  (* 3 (f (- n 3))))))
  )

(define (g n)
  (define (iter a b c count max)
    (if (> count max)
      c
      (iter b c (+ (* 3 a) (* 2 b) c) (+ count 1) max))
    )
  (cond ((= n 1) 1)
        ((= n 2) 2)
        ((iter 1 2 3 4 n)))
  )
  
(define (expect actual expectation expected)
  (assert (= actual expected))
  (display ".")
  )
  
(define to-equal 1)

(expect (f 1) to-equal 1)
(expect (f 2) to-equal 2)
(expect (f 3) to-equal 3)
(expect (f 4) to-equal 10)
(expect (f 5) to-equal 22)

(expect (g 1) to-equal 1)
(expect (g 2) to-equal 2)
(expect (g 3) to-equal 3)
(expect (g 4) to-equal 10)
(expect (g 5) to-equal 22)

Could you replace lines 2-4 with (cond ((< n 4) n)

?

Yes :-)

I realised this code was not the best but once I'd finally got it working, I wasn't in the mood to fix it. Future exercise solutions are getting better...