Please search through previous messages/tickets before posting new
issues. This issue has been fixed as of commit da412909d36551a526ed and
will be included in the -beta2 release. It was originally ticketed here:
http://dev.clojure.org/jira/browse/CLJ-816
(let [m (into {} (for [x (range 100000)] [(rand) (rand)]))]
(println (count (distinct (map hash (keys m)))))
((juxt count identity) (persistent!
(reduce dissoc! (transient m) (keys m)))))
100000
[0 {}]
Cheers,
Aaron Bedra
--
Clojure/core
http://clojure.com
On 08/24/2011 04:32 AM, Alan Malloy wrote:
On Aug 24, 12:27 am, Alan Malloy<a...@malloys.org> wrote:
On Aug 23, 11:38 pm, Ken Wesson<kwess...@gmail.com> wrote:
What does zipmap do if the key seq contains duplications?
That was my instinct too, but (a) a few thousand numbers won't collide
very often at all given the problem space, and (b) some experimenting
indicates that the key-seq is always 100k elements large - no
duplicates.
HOWEVER, I did find the problem: the keys themselves don't collide,
but their hashes do. The number of elements that get messed up is
related to the number of hash collisions, but I still don't quite
understand how they interact.
Here's a modified code snippet that demonstrates the issue:
user> (let [m (apply zipmap (repeatedly 2 #(repeatedly 100000 rand)))]
(println (count (distinct (map hash (keys m)))))
((juxt count identity) (persistent!
(reduce dissoc! (transient m) (keys m)))))
100000 ;; no collisions
[0 {}] ;; map is empty at the end
user> (let [m (apply zipmap (repeatedly 2 #(repeatedly 100000 rand)))]
(println (count (distinct (map hash (keys m)))))
((juxt count identity) (persistent!
(reduce dissoc! (transient m) (keys m)))))
99996 ;; four collisions
[8 {0.30426231137219917 0.8531183785687654, 0.8893047006425385
0.4788315896128895, 0.47854633997540674 0.45133768991797785,
0.5265638224227486 0.7724779126227945}] ;; a four-element map that
reports its count as eight!!!
That last comment seems to indicate a very serious error somewhere:
not only is the transient map broken, but it creates a broken
persistent object. I'll file a JIRA issue for this, and see if I can
find out any more about the cause.
FWIW, I'm using 1.2.1 for the above output.
Ticket is at http://dev.clojure.org/jira/browse/CLJ-829
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