On Mar 29, 10:18 am, David Cabana <drcab...@gmail.com> wrote:
> On Thu, Mar 29, 2012 at 12:18 AM, simon.T <simon.j....@gmail.com> wrote:
> > The obvious way is like the following, which traverse the sequence 2 times.
> > ...
>
> The obvious way does not necessarily traverse the sequence twice. If
> a sequence S satisfies the 'counted?' predicate, (count S) takes
> constant time. In particular
> user=> (counted? [:a :b :c])
> true
>
> user=> (counted? '(:a :b :c))
> true
>
> user=> (counted? {:a 1 :b 2 :c 3})
> true
>
> user=> (counted? #{:a :b :c})
> true
>
> The examples are stolen
> from:http://clojuredocs.org/clojure_core/clojure.core/counted_q
>
> So it is very likely that (/ (reduce + coll) (count coll)) will not
> traverse 'coll' twice, and the natural way is the preferred way.
> <Insert standard warning about premature optimization./>
"Very likely" strikes me as a huge overstatement here. Most sequences
that you want to average won't be source-code literals, they'll be
lazy sequences, and those aren't counted.
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