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lispclojurescheme

Which is better?: (reduce + ...) or (apply + ...)?

Should I use

(apply + (filter prime? (range 1 20)))

or 

(reduce + (filter prime? (range 1 20)))

Edit: This is the source for prime in clojure from optimizing toolkit.

(defn prime?  [n]  
  (cond
    (or (= n 2) (= n 3))          true
    (or (divisible? n 2) (< n 2)) false
    :else                        
     (let [sqrt-n (Math/sqrt n)]
       (loop [i 3]
           (cond
              (divisible? n i) false
              (< sqrt-n i)     true
              :else            (recur (+ i 2)))))))
Solution

  • If you are asking in terms of performance, the reduce is better by a little:

    (time (dotimes [_ 1e6] (apply + (filter even? (range 1 20)))))
"Elapsed time: 9059.251 msecs"
nil

(time (dotimes [_ 1e6] (reduce + (filter even? (range 1 20)))))
"Elapsed time: 8420.323 msecs"
nil

    About 7% difference in this case, but YMMV depending on the machine.

    You haven't provided your source for the prime? function, so I have substituted even? as the predicate. Keep in mind that your runtime may be dominated by prime?, in which case the choice between reduce and apply matters even less.

    If you are asking which is more "lispy" then I would say that the reduce implementation is preferrable, as what you are doing is a reduce/fold in the functional programming sense.