Question

I’m trying to implement sieve of Eratosthenes in Clojure. One approach I would like to test is this:

1. Get range (2 3 4 5 6 … N)
2. For 2 <= i <= N
   • Pass my range through filter that removes multiplies of i
   • For i+1th iteration, use result of the previous filtering

I know I could do it with loop/recur, but this is causing stack overflow errors (for some reason tail call optimization is not applied).

How can I do it iteratively? I mean invoking N calls to the same routine, passing result of ith iteration to i+1th.

Answer 1

(defn sieve [beg end]
  (letfn [(siever [to-sieve sieved]
                  (if (empty? to-sieve) sieved
                  (let [[f & r] to-sieve]
                    (if (> f (Math/sqrt end)) (into sieved to-sieve)
                        (recur (remove #(zero? (mod % f)) r) 
                           (conj sieved f))))))]
    (siever (range beg (inc end)) [])))

This one seems to work just fine. Tested it with (sieve 2 1000000) and returns within seconds without blowing.