# weima learns to program

my attempt to do the exercises in sicp.

## Friday, June 27, 2008

### sicp exercise 1.19

;  Exercise 1.19. There is a clever algorithm for computing the Fibonacci numbers in a logarithmic number
;  of steps. Recall the transformation of the state variables a and b in the fib-iter process of section
;  1.2.2: a <-- a + b and b <-- a. Call this transformation T, and observe that applying T over and over
;  again n times, starting with 1 and 0, produces the pair Fib(n + 1) and Fib(n). In other words, the
;  Fibonacci numbers are produced by applying Tn, the nth power of the transformation T, starting with
;  the pair (1,0). Now consider T to be the special case of p = 0 and q = 1 in a family of transformations
;  Tpq, where Tpq transforms the pair (a,b) according to a <-- bq + aq + ap and b <-- bp + aq.
;  Show that if we apply   such a transformation Tpq twice, the effect is the same as using a single
;  transformation Tp'q' of the same form, and compute p' and q' in terms of p and q. This gives us an
;  explicit way to square these transformations, and thus we can compute Tn using successive squaring,
;  as in the fast-expt procedure.  Put this all together to complete the following procedure, which runs
;  in a logarithmic number of steps
;
;
; (define (fib n)
;   (fib-iter 1 0 0 1 n))
; (define (fib-iter a b p q count)
;   (cond ((= count 0) b)
;     ((even? count)
;         (fib-iter a
;                   b
;                 <??> ; compute p'
;                 <??> ; compute q'
;                 (/ count 2)))
;     (else (fib-iter (+ (* b q) (* a q) (* a p))
;                     (+ (* b p) (* a q))
;                     p
;                     q
;                     (- count 1)))))

;
;  p' = p^2 + q^2
;  q' = q^2 + 2pq
;

(define (fib n)
(fib-iter 1 0 0 1 n))
(define (fib-iter a b p q count)
(cond ((= count 0) b)
((even? count)
(fib-iter a
b
(+ (* p p) (* q q))
(+ (* q q) (* 2 p q))
(/ count 2)))
(else (fib-iter (+ (* b q) (* a q) (* a p))
(+ (* b p) (* a q))
p
q
(- count 1)))))

(display (fib 1)) (newline)
(display (fib 2)) (newline)
(display (fib 3)) (newline)
(display (fib 4)) (newline)
(display (fib 5)) (newline)
(display (fib 6)) (newline)
(display (fib 7)) (newline)
(display (fib 8)) (newline)
(display (fib 9)) (newline)
(display (fib 10)) (newline)
(display (fib 11)) (newline)
(display (fib 12)) (newline)
(display (fib 5000)) (newline)