Everybody loves the Sieve of Eratosthenes

unsorted — cgrand, 30 July 2009 @ 11 h 27 min

If I judge by traffic logs for this blog, many newcomers want to compute prime numbers in Clojure.

A recent thread on the mailing list prompted me to test an idea I had for a while: to use a map to implement the Sieve of Eratosthenes.

The first implementation was this one:

(defn primes [max]
  (let [enqueue (fn [sieve n factor]
                  (let [m (+ n factor)]
                    (assoc sieve m
                      (conj (sieve m) factor))))
        next-sieve (fn [sieve candidate]
                     (if-let [factors (sieve candidate)]
                       (reduce #(enqueue %1 candidate %2)
                         (dissoc sieve candidate)
                       (enqueue sieve candidate candidate)))]
    (apply concat (vals (reduce next-sieve {} (range 2 max))))))

where the sieve is a map from the next non-prime numbers to their factors. It’s so naive that even numbers are tested for primality but it doesn’t perform that bad: on my box, it takes 3s to compute all primes below 1,000,000 (it’s roughly as fast as clojure.contrib.lazy-seqs/primes when the seq isn’t yet memoized).

I wasn’t happy with the way I handled the case where a non-prime was already checked off (ie was a key of the map): I was conjing onto the list of prime factors for this number. If instead I tried to check off n+p (where n is the already known non-prime and p the current prime) or n+2p or n+3p… until I found a yet unknown non-prime I wouldn’t need to maintain a list of factors, nor to conj or reduce over it. And I was hoping that less allocations would yield better perfs.

Here is the second iteration:

(defn primes2 [max]
  (let [enqueue (fn [sieve n factor]
                  (let [m (+ n factor)]
                    (if (sieve m)
                      (recur sieve m factor)
                      (assoc sieve m factor))))
        next-sieve (fn [sieve candidate]
                     (if-let [factor (sieve candidate)]
                       (-> sieve
                         (dissoc candidate)
                         (enqueue candidate factor))
                       (enqueue sieve candidate candidate)))]
    (vals (reduce next-sieve {} (range 2 max)))))

and it computes all the primes below 1,000,000 in 1.8s instead of 3s but it still tests even numbers for primality.

primes3 is primes2 modified to only test odd numbers:

(defn primes3 [max]
  (let [enqueue (fn [sieve n factor]
                  (let [m (+ n (+ factor factor))]
                    (if (sieve m)
                      (recur sieve m factor)
                      (assoc sieve m factor))))
        next-sieve (fn [sieve candidate]
                     (if-let [factor (sieve candidate)]
                       (-> sieve
                         (dissoc candidate)
                         (enqueue candidate factor))
                       (enqueue sieve candidate candidate)))]
    (cons 2 (vals (reduce next-sieve {} (range 3 max 2))))))

and it computes the same list of primes in 1.5s.

Out of curiosity, I wrote a lazy version of primes3:

(defn lazy-primes3 []
  (letfn [(enqueue [sieve n step]
            (let [m (+ n step)]
              (if (sieve m)
                (recur sieve m step)
                (assoc sieve m step))))
          (next-sieve [sieve candidate]
            (if-let [step (sieve candidate)]
              (-> sieve
                (dissoc candidate)
                (enqueue candidate step))
              (enqueue sieve candidate (+ candidate candidate))))
          (next-primes [sieve candidate]
            (if (sieve candidate)
              (recur (next-sieve sieve candidate) (+ candidate 2))
              (cons candidate 
                (lazy-seq (next-primes (next-sieve sieve candidate) 
                            (+ candidate 2))))))]
    (cons 2 (lazy-seq (next-primes {} 3)))))

and, surprisingly (better locality?), it computes the primes below 1,000,000 in 1s.


  1. Here's my naive, ugly and untested take on the problem:
    (defn not-divisible-by?[num denum]
    (not (= (mod num denum) 0)))

    (defn primes
    ([pos] (primes (vec (range 2 pos)) 0))
    ([clist pos]
    (>= pos (count clist)) clist
    (let [val (nth clist pos)
    starting (inc pos)]
    (recur (into (subvec clist 0 starting) (vec (filter #(not-divisible-by? % val) (subvec clist starting (count clist))))) starting)))))

    Comment by zemariamm — 15 September 2009 @ 4 h 36 min
  2. [...] reading this post [...]

  3. [...] favorite solution was by Cristophe Grande, not just because he followed a similar approach to me, but also because he was very clever in his [...]

  4. [...] of SoE. I would recommend checking out Christophe Grand’s treatise on the subject titled Everybody loves the Sieve of Eratosthenes for a great discussion on writing real world prime sieves in [...]

  5. [...] Clojure is very big on laziness… and I’m feeling to lazy to re-implement the Sieve in a one-liner. I’m sure it could be done! In the meantime, I’ll point you to cgrand’s great article about the Sieve of Eratosthenes in clojure. [...]

  6. thanks for this good article..

    Comment by heiji — 27 August 2011 @ 16 h 15 min
  7. [...] I wasn’t happy with this performance, especially after seeing Christophe Grand’s solution and timing on his blog post Everybody loves the Sieve of Eratosthenes. [...]

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