## Segmented Sieve Of Eratosthenes

### February 5, 2010

We use `primes`

from the previous Sieve of Eratosthenes exercise and `isqrt`

from the Standard Prelude:

`(define (primes-range l r b)`

(let* ((ps (cdr (primes (+ (isqrt r) 1))))

(qs (map (lambda (p) (modulo (* -1/2 (+ l 1 p)) p)) ps))

(zs '()) (z (lambda (p) (set! zs (cons p zs)))))

(do ((t l (+ t b b))

(qs qs (map (lambda (p q) (modulo (- q b) p)) ps qs)))

((= t r) (reverse zs))

(let ((bs (make-vector b #t)))

(do ((qs qs (cdr qs)) (ps ps (cdr ps))) ((null? qs))

(do ((j (car qs) (+ j (car ps)))) ((<= b j))

(vector-set! bs j #f)))

(do ((j 0 (+ j 1))) ((= j b))

(if (vector-ref bs j) (z (+ t j j 1))))))))

`Ps`

and `qs`

are as in the problem description, `zs`

accumulates the resulting primes, and `t`

is the beginning of each sieving block, initially `l`

. The outer loop, on `t`

, establishes the sieving blocks and updates `qs`

from one block to the next. The inner loop on `qs`

and its nested loop on `j`

perform the sieving. The final loop on `j`

collects the primes from the current block. Here is a sample use:

`> (length (primes #e1e6))`

78498

> (length (primes #e2e6))

148933

> (- 148933 78498)

70435

> (length (primes-range #e1e6 #e2e6 #e1e5))

70435

You can run the example at http://programmingpraxis.codepad.org/ZQA6ldK8.

Pages: 1 2

My first try

[…] Of Eratosthenes February 5, 2010 Mithrandir Leave a comment Go to comments Today’s Programming Praxis problem describes an algorithm which may be used to find the prime numbers in a large interval, so large […]

Won’t doing “(primes (+ (isqrt r) 1))” sometimes add an extra prime that you won’t need?

For instance, try it with l = 200, and r = 325 (basically, any time r is at least the value of

an even square, but less than an odd square).

I have worked on this problm http://www.spoj.pl/problems/PRIME1/ about this,but this semms too hard for me!!

[…] I would probably choose the first alternative given your problem size. Implementations of both the segmented Sieve of Eratosthenes and Lehmer’s prime-counting function are available at my […]