harto wrote:
> Hello,
>
> I've just started learning Clojure, so I'm trying to figure out the
> correct way of doing things. I've been trying to create and 'modify' a
> large vector for an online programming exercise, but I'm running into
> some performance issues.
>
> Any general tips would be appreciated!
>
Well, I'm a beginner too, but I'll take a stab at answering your questions.
> Firstly, I'm creating a vector of booleans like this:
>
> (defn vector-of [n value]
> (vec (for [_ (range n)] value)))
>
> It takes quite a long time for large values of n, though:
>
> user=> (time (dorun (vector-of 1e7 true)))
> "Elapsed time: 6734.509528 msecs"
>
I suspect you're being bitten by the repeated creation of the underlying
data structures. 'range' is going to create a lazy sequence, from which
'for' is going to create its own, lazy structure, which 'vec' then
iterates over to create its own. Just running '(time (dorun (range
1e7)))' shows that it's taking up a non-trivial amount of time just on
its own.
This modified version does the same thing a little more quickly:
(defn vector-of [n value]
(loop [v []
c 0]
(if (< c n) (recur (assoc v c value) (inc c)))))
user> (time (dorun (vector-of 1e7 true)))
"Elapsed time: 1000.409039 msecs"
> Secondly, I'm iterating across one of these large vectors using
> something like the following (contrived) function:
>
> (defn set-flags [v]
> (loop [i 0
> v v]
> (if (< i (count v))
> (recur (inc i) (assoc v i false))
> v)))
>
> user=> (let [v (vector-of 1e7 true)] (time (dorun (set-flags v))))
> "Elapsed time: 15563.916114 msecs"
>
> Am I missing anything obvious here? That seems like a really long time
> to me.
>
With the modified vector-of, above, this problem seems to go away for me:
user> (time (dorun (set-flags (vector-of 1e7 true))))
"Elapsed time: 1009.362723 msecs"
Cheers,
Danny.
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