4 Oct 20:25
A suggestion to optimize the for loops
From: Eastsun <flushtime <at> 126.com>
Subject: A suggestion to optimize the for loops
Newsgroups: gmane.comp.lang.scala
Date: 2008-10-04 18:27:06 GMT
Subject: A suggestion to optimize the for loops
Newsgroups: gmane.comp.lang.scala
Date: 2008-10-04 18:27:06 GMT
By the scala reference,the expression
for(x <- e;if y) z
would translate to
e.filter((x1,x2..,xn) => y).foreach{case x => z}
It would call the filter method in this way,and sometimes the filter method
is very expensive.
Because the filter method may create a new object of type e.
And we can avoid this by translate the expression to:
e.foreach{ case x if y =>z; case _ => }
For example:
/////////////////////////////////////////////////////////////////////////////
Welcome to Scala version 2.7.2.RC1 (Java HotSpot(TM) Client VM, Java
1.6.0_10-rc).
Type in expressions to have them evaluated.
Type :help for more information.
scala> def test(call : => Unit){
| val start = System.currentTimeMillis
| var cnt = 0
| while(cnt < 10000){
| call
| cnt += 1
| }
| println((System.currentTimeMillis - start)+" ms")
| }
test: (=> Unit)Unit
(Continue reading)
> While your suggestion may work for List and Seq, for comprehensions work
> with *any* classes that have map, flatMap, filter, and foreach. If the
> meaning of "filter" was different for my custom class, removing the filter
> call from the generated code would change the meaning of my program.
Dare I say that if you've written code such that the meaning of for
(...; if (stuff)) { ... } is different from for(...) { if(stuff) {
...} } then you've done something stupid and your code deserves to be
broken. 
> This seems like premature optimization to me. I've been doing Scala coding
> for nearly 2 years. I've rarely had to focus on the performance of a given
> method. When I have had to focus on performance, I manually recode things
> as while loops which gives markedly better performance. And I've got a
Um. So let me get this straight:
When you want performance, for loops are not fast enough. Therefore
for loop performance should not be improved?
That seems... odd logic.
It's also just wrong. 
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