Thanks to everyone who replied in this thread. I am impressed by the
signal-to-noise ration on this list and by the quality of replies. I
will try to reply to everyone in a single post, trying to summarize.
> On Jul 13, 11:15 am, Jan Rychter <j...@rychter.com> wrote:
>> I've been trying to implement stacks using vectors in Clojure. In case
>> you wonder why, it's because I need stacks with a fast item count
>> operation.
Rob <rob.nikan...@gmail.com> writes:
> If that's the only reason, you don't have to use vectors. The
> following page says that 'count' for lists is O(1)
Indeed -- I missed that, and my experiments seemed to indicate that
calling (count) really slows things down. It turns out it was because my
lists were turning into seqs behind my back.
Mark Engelberg <mark.engelb...@gmail.com> writes:
> Using conj and pop on vectors for stack operations should work just
> fine. Don't use subvec though; nested subvecs will really start to
> slow things down.
I went ahead and implemented my stacks using both lists and vectors,
then did some careful benchmarking. It turns out there is no discernible
performance difference.
I am not sure what you mean about subvecs, though -- I currently use
them for dropn and rot operations, and I don't know how to avoid using
them:
(defmacro stack-dropn [stack n]
`(let [stack# ~stack]
(subvec stack# 0 (- (stack-count ~stack#) ~n))))
(defmacro stack-rot [stack]
`(let [stack# ~stack]
(if (> (stack-count stack#) 1)
(apply conj (vector (peek stack#)) (subvec stack# 0 (dec (count
stack#))))
stack#)))
(incidentally, if anyone can suggest a better rot implementation, I
would be very grateful -- rot "rotates" the stack, putting the last
element of the vector first)
So, to summarize -- my real performance problem is that my lists were
becoming lazy seqs without me knowing. It turns out you REALLY REALLY
have to know the difference between a list and a seq.
And, since performance of lists and vectors in my case is identical, I
don't know which one to choose now for my stacks?
--J.
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