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POWhy is my Scala tail-recursion faster than the while loop?
 

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  1. POWhy is my Scala tail-recursion faster than the while loop?
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    <p>Here are two solutions to exercise 4.9 in Cay Horstmann's Scala for the Impatient: "Write a function lteqgt(values: Array[Int], v: Int) that returns a triple containing the counts of values less than v, equal to v, and greater than v." One uses tail recursion, the other uses a while loop. I thought that both would compile to similar bytecode but the while loop is slower than the tail recursion by a factor of almost 2. This suggests to me that my while method is badly written.</p> <pre><code>import scala.annotation.tailrec import scala.util.Random object PerformanceTest { def main(args: Array[String]): Unit = { val bigArray:Array[Int] = fillArray(new Array[Int](100000000)) println(time(lteqgt(bigArray, 25))) println(time(lteqgt2(bigArray, 25))) } def time[T](block : =&gt; T):T = { val start = System.nanoTime : Double val result = block val end = System.nanoTime : Double println("Time = " + (end - start) / 1000000.0 + " millis") result } @tailrec def fillArray(a:Array[Int], pos:Int=0):Array[Int] = { if (pos == a.length) a else { a(pos) = Random.nextInt(50) fillArray(a, pos+1) } } @tailrec def lteqgt(values: Array[Int], v:Int, lt:Int=0, eq:Int=0, gt:Int=0, pos:Int=0):(Int, Int, Int) = { if (pos == values.length) (lt, eq, gt) else lteqgt(values, v, lt + (if (values(pos) &lt; v) 1 else 0), eq + (if (values(pos) == v) 1 else 0), gt + (if (values(pos) &gt; v) 1 else 0), pos+1) } def lteqgt2(values:Array[Int], v:Int):(Int, Int, Int) = { var lt = 0 var eq = 0 var gt = 0 var pos = 0 val limit = values.length while (pos &lt; limit) { if (values(pos) &gt; v) gt += 1 else if (values(pos) &lt; v) lt += 1 else eq += 1 pos += 1 } (lt, eq, gt) } } </code></pre> <p>Adjust the size of bigArray according to your heap size. Here is some sample output:</p> <pre><code>Time = 245.110899 millis (50004367,2003090,47992543) Time = 465.836894 millis (50004367,2003090,47992543) </code></pre> <p>Why is the while method so much slower than the tailrec? Naively the tailrec version looks to be at a slight disadvantage, as it must always perform 3 "if" checks for every iteration, whereas the while version will often only perform 1 or 2 tests due to the else construct. (NB reversing the order I perform the two methods does not affect the outcome).</p>
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    1. USwaifnstray
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    1. COI have often wondered about that myself. The answer surely lies in JIT. It would be interesting to repeat the benchmark while disabling JIT entirely.
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      1. USDaniel C. Sobral
 

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