How to measure time of a statement in scala console?

Question

I'm using Scala to measure performance of regex engine of java. The regexp below executes in around 3 seconds but yet I'm not able to measure it using System.currentTimeMillis. (the last expression returns 0)

scala> val b = System.currentTimeMillis; val v = new Regex("(x+)+y").findAllIn("x"*25); b-System.currentTimeMillis b: Long = 1330787275629 v: scala.util.matching.Regex.MatchIterator = empty iterator res18: Long = 0 

Do you now why the last returned value is 0, and not the amount of ms that scala spend on executing the regexp?

Answer 1

The unexplained duration comes from the REPL calling toString on the iterator returned from findAllIn. This in turn calls Regex.MatchIterator#hasNext, which triggers the search.

scala> def time[A](a: => A) = {      |   val now = System.nanoTime      |   val result = a      |   val micros = (System.nanoTime - now) / 1000      |   println("%d microseconds".format(micros))      |   result      | } time: [A](a: => A)A  scala> :power ** Power User mode enabled - BEEP WHIR GYVE ** ** :phase has been set to 'typer'.          ** ** scala.tools.nsc._ has been imported      ** ** global._, definitions._ also imported    ** ** Try  :help, :vals, power.<tab>           **  scala> :wrap time Set wrapper to 'time'  scala> new Regex("(x+)+y").findAllIn("x"*25).toString 3000737 microseconds res19: String = empty iterator  scala> {new Regex("(x+)+y").findAllIn("x"*25); 0} 582 microseconds res20: Int = 0