sicp/chapter1.scm

#lang sicp
(define (average x y) (/ (+ x y) 2))
(define (square x) (* x x))
(define (cube x) (* x x x))
(define (% a b)
  (remainder a b))

;(define (sqrt x)
;  (define (good-enough? guess)
;    (< (abs (- (square guess) x)) 0.00001))
;  (define (improve guess)
;    (average guess (/ x guess)))
;  (define (sqrt-iter guess)
;    (if (good-enough? guess)
;        guess
;        (sqrt-iter (improve guess))))
;  (sqrt-iter 1.0))

;(define (factorial n)
;  (if (= n 1) 1 (* n (factorial (- n 1)))))

(define (factorial n)
  (define (fact-iter product counter max-count)
    (if (> counter max-count)
        product
        (fact-iter (* counter product)
                   (+ counter 1)
                   max-count)))
  (fact-iter 1 1 n))


(define (A x y)
  (cond ((= y 0) 0)
        ((= x 0) (* 2 y))
        ((= y 1) 2)
        (else (A (- x 1)
                 (A x (- y 1))))))

;(define (fib n)
;  (cond ((= n 0) 0)
;        ((= n 1) 1)
;        (else (+ (fib(- n 1))
;                 (fib(- n 2))))))

(define (fib n)
  (define (fib-iter a b count)
    (if (= count 0)
        b
        (fib-iter (+ a b) a (- count 1))))
  
  (fib-iter 1 0 n))


(define (count-change amount)
  (define (first-denomination kinds-of-coins)
    (cond ((= kinds-of-coins 1) 1)
          ((= kinds-of-coins 2) 5)
          ((= kinds-of-coins 3) 10)
          ((= kinds-of-coins 4) 25)
          ((= kinds-of-coins 5) 50)))

  (define (cc amount kinds-of-coins)
    (cond ((= amount 0) 1)
          ((or (< amount 0)
               (= kinds-of-coins 0))
           0)
          (else
           (+ (cc amount (- kinds-of-coins 1))
              (cc (- amount (first-denomination kinds-of-coins))
                  kinds-of-coins)))))
  (cc amount 5))


(define (ptri n)
  (if (< n 3) n
      (+
       (ptri (- n 1))
       (+ (* (ptri (- n 2)) 2)
          (* (ptri (- n 3)) 3)))))


(define (p x) (- (* 3 x) (* 4 (cube x))))
(define (sine angle)
  (if (not (> (abs angle) 0.1))
      angle
      (p (sine (/ angle 3.0)))))

;(define (expt b n)
;  (if (= n 0) 1 (* b (expt b (- n 1)))))

(define (expt b n) (expt-iter b n 1))
(define (expt-iter b counter product)
  (if (= counter 0)
      product
      (expt-iter b
                 (- counter 1)
                 (* b product))))

(define (fast-exp b n)
  (cond ((= n 0)
         1)
        ((even? n)
         (square (fast-exp b (/ n 2))))
        (else
         (* b (fast-exp b (- n 1))))))

(define (gcd a b)
  (if (= b 0)
      a
      (gcd b (% a b))))

(define (smallest-divisor n)
  (find-divisor n 2))

(define (find-divisor n test-divisor)
  (cond ((> (square test-divisor) n)
         n)
        ((divides? test-divisor n)
         test-divisor)
        (else (find-divisor
               n
               (+ test-divisor 1)))))

(define (divides? a b)
  (= (% b a) 0))

(define (prime? n)
  (= n (smallest-divisor n)))

(define (expmod base exp m)
  (cond ((= exp 0) 1)
        ((even? exp)
         (remainder
          (square (expmod base (/ exp 2) m))
          m))
        (else
         (remainder
          (* base (expmod base (- exp 1) m))
          m))))

(define (fermat-test n)
  (define (try-it a)
    (= (expmod a n n) a))
  (try-it (+ 1 (random (- n 1)))))

(define (fast-prime? n times)
  (cond ((= times 0) true)
        ((fermat-test n)
         (fast-prime? n (- times 1)))
        (else false)))

(define (timed-prime-test n)
  (newline)
  (display n)
  (start-prime-test n (runtime)))

(define (start-prime-test n start-time)
  (if (prime? n)
      (report-prime ( - (runtime) start-time))))

(define (report-prime elapsed-time)
  (display " *** ")
  (display elapsed-time))

;(define (sum-integers a b)
;  (if (> a b)
;      0
;      (+ a (sum-integers (+ a 1) b))))

;(define (sum-cubes a b)
;  (if (> a b)
;      0
;      (+ (cube a) (sum-cubes (+ a 1) b))))

;(define (pi-sum a b)
;  (if (> a b)
;      0
;      (+ (/ 1.0 (* a (+ a 2)))
;         (pi-sum (+ a 4) b))))

(define (sum term a next b)
  (if (> a b)
      0
      (+ (term a)
         (sum term (next a) next b))))

(define (inc n) (+ n 1))
(define (sum-cubes a b)
  (sum cube a inc b))

(define (identity x) x)
(define (sum-integers a b)
  (sum identity a inc b))

(define (pi-sum a b)
  (sum (lambda (x) (/ 1.0 (* x (+ x 2))))
       a
       (lambda (x) (+ x 4))
       b))

(define (integral f a b dx)
  (* (sum f (+ a (/ dx 2.0))
          (lambda (x) (+ x dx))
          b)
     dx))

(define (f x y)
  (let ((a (+ 1 (* x y)))
        (b (- 1 y)))
    (+ (* x (square a))
       (* y b)
       (* a b))))

(define tolerance 0.00001)
(define (fixed-point f first-guess)
  (define (close-enough? v1 v2)
    (< (abs (- v1 v2))
       tolerance))
  (define (try guess)
    (let ((next (f guess)))
      (if (close-enough? guess next)
          next
          (try next))))
   (try first-guess))

(define (average-damp f)
  (lambda (x)
    (average x (f x))))

(define dx 0.00001)
(define (deriv g)
  (lambda (x)
    (/ (- (g (+ x dx)) (g x))
       dx)))

(define (newton-transform g)
  (lambda (x)
    (- x (/ (g x)
            ((deriv g) x)))))

(define (newtons-method g guess)
  (fixed-point (newton-transform g) guess))

(define (sqrt x)
  (newtons-method
   (lambda (y)
     (- (square y) x))
   1.0))