Since we're having a shuffle-a-thon, here's a version I wrote that I kind of
like for its simplicity, even though it uses a recursive call that Clojure
apparently can't optimize. (FWIW I wrote a version that used loop/recur
instead, but it was actually slower for some reason):
(defn shuffle
[lst]
(if (empty? (rest lst))
lst
(let [index (rand-int (count lst))
item (nth lst index)
remainder (concat (subvec (into [] lst) 0 index)
(subvec (into [] lst) (inc index)))]
(cons item (shuffle remainder)))))
-Lee
On Apr 4, 2010, at 11:01 PM, Per Vognsen wrote:
> The seq-utils library has a shuffle function that calls out to
> java.util.Collections.
>
> If you want to do it yourself, the easiest would be
>
> (defn naive-shuffle [xs]
> (let [v (vec xs)]
> (->> (count v) range lex-permutations rand-elt (map v))))
>
> This uses the seq-utils and combinatorics libraries from clojure.contrib.
>
> However, generating all permutations obviously doesn't scale beyond a
> couple of elements. For that you will need a real algorithm like the
> Knuth shuffle:
>
> (defn swap-elts [x i j]
> (assoc x i (x j) j (x i)))
>
> (defn rand-range
> ([n] (rand-int n))
> ([m n] (+ m (rand-int (- n m)))))
>
> (defn knuth-shuffle [xs]
> (let [v (vec xs)]
> (reduce #(swap-elts %1 %2 (rand-range %2 (count %1))) v (range (count
> v)))))
>
> -Per
>
> On Mon, Apr 5, 2010 at 3:14 AM, Linus Ericsson
> <oscarlinuserics...@gmail.com> wrote:
>> Hello Clojure!
>>
>> Is there any straight-forward way to randomly reorder a list?
>>
>> ie:
>>
>> (randomize-list (list 1 2 3 4))
>> -> (3 2 1 4)
>>
>> Regards,
>> Linus Ericsson
>>
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--
Lee Spector, Professor of Computer Science
School of Cognitive Science, Hampshire College
893 West Street, Amherst, MA 01002-3359
lspec...@hampshire.edu, http://hampshire.edu/lspector/
Phone: 413-559-5352, Fax: 413-559-5438
Check out Genetic Programming and Evolvable Machines:
http://www.springer.com/10710 - http://gpemjournal.blogspot.com/
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