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
> <[email protected]> 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
>>
>> --
>> You received this message because you are subscribed to the Google
>> Groups "Clojure" group.
>> To post to this group, send email to [email protected]
>> Note that posts from new members are moderated - please be patient with your
>> first post.
>> To unsubscribe from this group, send email to
>> [email protected]
>> For more options, visit this group at
>> http://groups.google.com/group/clojure?hl=en
>
> --
> You received this message because you are subscribed to the Google
> Groups "Clojure" group.
> To post to this group, send email to [email protected]
> Note that posts from new members are moderated - please be patient with your
> first post.
> To unsubscribe from this group, send email to
> [email protected]
> For more options, visit this group at
> http://groups.google.com/group/clojure?hl=en
>
> To unsubscribe, reply using "remove me" as the subject.
--
Lee Spector, Professor of Computer Science
School of Cognitive Science, Hampshire College
893 West Street, Amherst, MA 01002-3359
[email protected], 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/
--
You received this message because you are subscribed to the Google
Groups "Clojure" group.
To post to this group, send email to [email protected]
Note that posts from new members are moderated - please be patient with your
first post.
To unsubscribe from this group, send email to
[email protected]
For more options, visit this group at
http://groups.google.com/group/clojure?hl=en
