sicp/zapiski/sicp_sec_1_2.scm

#lang scheme


;; 1.9

;; 1. primer: rekurziven
;; (+ 4 5) = inc((inc(inc(inc(5)))) = inc((inc(inc(6))) = inc((inc(7)) = inc(8) = 9
;; 2. primer: iterativen
;; (+ 4 5):
;; (+ 3 6)
;; (+ 2 7)
;; (+ 1 8)
;; (+ 0 9)
;; 9


;; 1.10
;; Ackermannova funkcija
;; https://en.wikipedia.org/wiki/Ackermann_function - tukaj malo drugacna definicija, mislim, da se indeksiranje premakne za 3

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

(A 1 10 )
(A 2 4)
(A 3 3)

;; (f n) = 2*n
;; (g n) = 2^n
;; (h n) = 

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

(fib 10)
;;;;
;;;;

;; 1.11

;; rekurzivno
(define (f-rec n)
( cond (( < n 3 ) n)
( else ( + (f-rec ( - n 1 )) (* 2 (f-rec (- n 2))) (* 3 (f-rec (- n 3)))))))

(f-rec 6)


;;;; iz knjige
;;;;
( define (fib2 n)
( fib-iter 1 0 n) )

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

(fib2 10)
;;;;
;;;;

;; iterativno
( define (f2 n)
( f-iter n (if (< n 2) n 2)) )

( define ( f-iter max-count sum)
( if (< max-count 3 )
max-count
(+ (f-iter (- max-count 1) sum) (* 2 (f-iter (- max-count 2) sum )) (* 3 (f-iter (- max-count 3) sum )))))

(f2 6)


;; 1.12
(define (pascal r c)
  (if (or (= r 0) (= c 1) (= c r))
      1
      (+ (pascal (- r 1) c) (pascal (- r 1) (- c 1)))))

(pascal 5 3)

;; 1.13 na papirju

;; 1.14 na papirju


;;;; iz knjige
;;;;
( define ( count-change amount )
( cc amount 5 ) )

( 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 ) ) ) ) )
(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) ) )

( count-change 11)
;;;;
;;;;


;;;; iz knjige
;;;;
(define (square x) (* x x))

( define ( even? n)
(= (remainder n 2 ) 0 ) )

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

(fast-expt 2 5)
;;;;
;;;;

;; 1.15
;; a) 5
;; b) O(log n) (?)

;; 1.16
( define ( fast-exp-iter n b a )
( if (= n 0 )
a
(if (even? n) (fast-exp-iter ( / n 2 ) (square b) a  ) ( * b ( fast-exp-iter ( - n 1 ) (* a b) a ) ) ) ) )

(fast-exp-iter 10 2 1 )

;; 1.17

;;;; iz knjige
;;;;
(define (mult a b)
(if (= b 0)
0
(+ a (mult a (- b 1 ) ) ) ) )

(mult 3 4)
;;;; 
;;;;

;; naloga
(define (double x) (+ x x))
(define (halve x) (/ x 2)) ;; a bi to moralo biti kako drugace; zakaj uporabljam deljenje pri implementaciji mnozenja s sestevanjem

( define ( fast-mult a b)
( cond ( (= b 0 ) 0 )
( ( even? b ) ( double ( fast-mult a (halve b) ) ) )
( else ( + a ( fast-mult a ( - b 1 ) ) ) ) ) )

(fast-mult 8 7)

;; 1.18

( define ( fast-mult-iter b a c)
( if (= b 0 )
0
(if (even? b) (fast-mult-iter (halve b) (double a) c ) ( + a ( fast-mult-iter ( - b 1 ) a c ) ) ) ) )

(fast-mult-iter 18 10 0 )

;; 1.21

;;;; iz knjige
;;;;
( 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 )
(= (remainder b a) 0 ) )

(smallest-divisor 19999)

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



;; 1.22

(define (runtime) (current-milliseconds)) ;; brez tega error: runtime: unbound identifier in: runtime

;;;; iz knjige
;;;;
(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)) -1))
(define (report-prime elapsed-time)
(display  "***" )
(display elapsed-time))
;;;; 
;;;;

(timed-prime-test 87178291199) ;; 35742549198872617291353508656626642567

;; https://en.wikipedia.org/wiki/List_of_prime_numbers


;; 1.26
;; ker se s klicanjem (/ exp 2) v navadnem mnozenju parameter exp ne razpolovi v naslednjem koraku ?