Hi,
how about simply doing this:
(defn foo-accessor
([] foo-vector)
([n] (subvec foo-vector 0 (min n (count foo-vector)))))
I wouldn't worry too much about subvectors. Unless you identify them as a
bottleneck with the profiler.
Kind regards
Meikel
Durch MOTOBLUR™ verbunden
-----Ursprüngliche Nachricht-----
Von: Don Jackson <cloj...@clark-communications.com>
An: "clojure@googlegroups.com" <clojure@googlegroups.com>
Gesendet: Fr, 01 Feb 2013, 01:23:36 MEZ
Betreff: (into [] (take n v)) vs (subvec v 0 n)
In the app I am working on, I have a number of pre-computed,cached
collections,
and for each collection, I have an accessor function that returns either the
entire collection or the first n elements.
Currently the underlying collection is a vector, so I had something like this:
(defn foo-accessor
([] foo-vector)
([n] (take n foo-vector)))
It occurred to me that take returns a list, and so the type returned by my
accessor was dependent on how it was called,
I thought I would change that, and remembered subvec, so I substituted in
subvec for take, like this:
([n] (subvec foo-vector 0 n))
That worked great until (< (count foo-vector) n), and I got an
IndexOutOfBoundsException
So, then without thinking much, I wrote
(defn subvec-safe
"subvec fails if you specify an end that is greater than count. This version
checks that, and DoesTheRightThing!"
([v start]
(subvec v start))
([v start end]
(if (> (count v)
end)
(subvec v
start
end)
(subvec v
start
(count v)))))
(Yes, it is safe ONLY for the end value…)
And
(defn takev
"take for a vector, returns a subvec or the vector itself. Uses subvec-safe
so specifying a n longer than (count v) works"
[n vec]
(subvec-safe vec
0
n))
Even before finishing takev, it occurred to me that
(defn- takev
"take for a vector"
[n vec]
(into []
(take n
vec)))
Might be easier/faster.
subvec returns a clojure.lang.APersistentVector$SubVector
whereas (into [] (take …)) returns a vector.
Any thoughts on which of the above is "better"?
Thanks,
Don
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