Greetings,
I'm new to Clojure and working my way through Project Euler problems
for learning. I stuck with problem 7, not with solution, but with
performance. My code needs about 90sec to find solution.
(ns problem7)
(defn no-dividers?
"checks if number can be divided with any of dividers"
[number dividers]
(let [divs (take-while #(<= % (. Math sqrt number)) dividers)]
(reduce #(and %1 (pos? (rem number %2)))
true
divs)))
(defn find-primes [n]
"finds first n primes"
(loop [primes [2 3] x 1 f -]
(if (= (count primes) n)
primes
(let [k (f (* 6 x) 1)]
(recur
(if (no-dividers? k primes) (concat
primes [k]) primes)
(if (= f -) x (inc x))
(if (= f -) + -))))))
(find-primes 10001)
And, for example, this code is much faster:
(defn div? [n d]
(= 0 (rem n d)))
(defn smallest-prime-factor [number]
(loop [n number d 2]
(cond (> d (int (Math/sqrt number))) n
(= n d) n
(div? n d) d
true (recur n (inc d)))))
(def primes (lazy-cat '(2 3)
(filter #(= %1 (smallest-prime-factor %1))
(take-nth 2 (iterate inc 5)))))
(nth primes 10001)
Now, since i have advanced a bit in reading about Clojure, I know that
preferred solution would be by using lazy sequences, but my concern is
about performance of my solution, no mater how bad is with style.
I should probably say that my background is pure imperative (mostly
Java, C/C++...).
Thanks in advance.
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