[SOLVED] CS (define (prime? n)

(define (prime? n)
(define (iter i)
(cond ((>= i (/ n 2)) #t)
((= 0 (modulo n i)) #f)
(else (iter (+ i 1)))))
(iter 2))

(define (filter pred lis);iterative version
(define (iter L result)
(cond ((null? L) result)
((pred (car L)) (iter (cdr L) (append result (list (car L)))))
(else (iter (cdr L) result))))
(iter lis ()))

(define (range a b)
(define (iter i result)
(cond ((< i a) result)(else (iter (- i 1) (cons i result)))))(iter b ‘()))#|Lists: range: (10000 10001 10002 … 999999 1000000); ~1 million steps, O(n) filter: (10007 10009 … +~75k values); ~1million steps x prime? = O(n^2) cdr: (10009 … +~75k values) ; 1 step, but still storing ~75k values car: 10009; 1 step|##|Streams:range: (10000 . # ); 1 step
filter: (10007 . # ) ;~8 steps, but O(n) b/c prime?
cdr: (10009 . # ) ;2 steps
car: 10009;1 step
|#

;(define (stream-cons a b) ;error! b/c applicative order
;(cons a (delay b)))

;(stream-cons 1 (+ 1 1))
;(stream-cons 1 2)
;(cons 1 (delay 2))
;(1 . # )

(define-syntax stream-cons
(syntax-rules ()
((stream-cons a b) (cons a (delay b)))))
(define (stream-car strm)
(car strm))
(define (stream-cdr strm)
(force (cdr strm)))

(define (stream-range a b)
(if (> a b) ()
(stream-cons a (stream-range (+ a 1) b))))

(define (range a b)
(if (> a b) ()
(cons a (range (+ a 1) b))))

;(stream-range 1 100))
;(stream-cons 1 (stream-range (+ 1 1) 100))
;(cons 1 (delay (stream-range (+ 1 1) 100)))
;(1 . [stream-range (+ 1 1) 100])

;(stream-cdr (stream-range 1 100))
;(stream-cdr (1 . [stream-range (+ 1 1) 100]))
;(force (cdr (1 . [stream-range (+ 1 1) 100])))
;(force [stream-range (+ 1 1) 100])
;(stream-range (+ 1 1) 100)
;(stream-range 2 100)
;(stream-cons 2 (stream-range (+ 2 1) 100))
;

(define (stream-ref n strm)
(cond ((eq? strm ()) #f)
((= n 0) (stream-car strm))
(else (stream-ref (- n 1) (stream-cdr strm)))))

;(stream-ref 3 (stream-range 5 10))
;(stream-ref 3 (5 . [stream-range 6 10]))
;(stream-ref (- 3 1) (stream-cdr (5 . [stream-range 6 10])))
;(stream-ref 2 (force (cdr (5 . [stream-range 6 10]))))
;(stream-ref 2 (force [stream-range 6 10]))
;(stream-ref 2 (stream-range 6 10))
;(stream-ref 1 (stream-range 7 10))
;(stream-ref 0 (stream-range 8 10))
;(stream-car (8 . [stream-range 9 10]))
;8

(define (stream-filter pred strm)
(cond ((null? strm) ())
((pred (stream-car strm))
(stream-cons (stream-car strm)
(stream-filter pred (stream-cdr strm))))
(else (stream-filter pred (stream-cdr strm)))))

;(stream-filter even? (stream-range 1 10))
;(stream-filter even? (1 . [stream-range 2 10]))
;(stream-filter even? (stream-cdr (1 . [stream-range 2 10])))
;(stream-filter even? (stream-range 2 10))
;(stream-filter even? (2 . [stream-range 3 10]))
;(stream-cons (stream-car (2 . [stream-range 3 10])) (stream-filter even? (stream-cdr (2 . [stream-range 3 10])))
;(cons (stream-car (2 . [stream-range 3 10])) (delay (stream-filter even? (stream-cdr (2 . [stream-range 3 10])))))
;(cons 2 [stream-filter even? (stream-cdr (2 . [stream-range 3 10]))])
;(2 . # )

(define (ints-starting-from i)
(stream-cons i (ints-starting-from (+ i 1))))

(define sevens
(stream-filter (lambda(x)(= (modulo x 7) 0)) (ints-starting-from 1)))