SICP Exercise 1.22

A procedure that checks a prime number.

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

A procedure that measures the time for "prime?".
I rewrite a bit.

(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))
      #f))
(define (report-prime elapsed-time)
  (display " *** ")
  (display elapsed-time)
  #t)

DrScheme doesn't have "runtime".

(define (runtime)
  (current-milliseconds))

My answer.

(define (search-for-primes min max cnt)
  (cond ((> min max))
        ((= cnt 3))
        ((even? min) (search-for-primes (+ min 1) max cnt))
        (else
         (if (timed-prime-test min)
             (search-for-primes (+ min 2) max (+ cnt 1))
             (search-for-primes (+ min 2) max cnt)))))

(search-for-primes 1000 9999 0)
(search-for-primes 10000 99999 0)
(search-for-primes 100000 999999 0)
(search-for-primes 1000000 9999999 0)

It's impossible to measure the time.