From: Alex Young <alex@...>
Date: 2007-07-30T17:01:26+09:00
Subject: Re: Performance diffrence between ifs and case

John Carter wrote:
> On Sun, 29 Jul 2007, Sean Surname wrote:
> 
>> Phlip wrote:
>>> forgottenwizard wrote:
>>>
>>>> What kind of diffrences are there, effectivly, between case and a 
>>>> series
>>>> of if statements, like in this:
>>>
>>> Google for "premature optimization is the root of all evil".
>>
>> Classify as "nostrum-emitting moron."
> 
> a) Phlip emits very valid and important nostrums.
> b) Definitely not a moron, you'll miss some Good Stuff if you make that 
> mistake.
> c) Once anyone actually benchmarks the difference and hence contributes 
> real data to the
>    discussion....
logic_benchmark.rb:

require 'benchmark'

def nop
end

def if_equiv_method(str)
   if "hi" === str
     nop
   elsif "bye" === str
     nop
   else
     nop
   end
end

def if_method(str)
   if str == "hi"
     nop
   elsif str == "bye"
     nop
   else
     nop
   end
end

def case_method(str)
   case str
   when "hi"
     nop
   when "bye"
     nop
   else
     nop
   end
end

strings = %w{hi bye foo}

Benchmark.bmbm(20) do |x|
   x.report("if"){for i in 0..1000000 do 
if_method(strings[rand(strings.length)]) end }
   x.report("if_equiv"){for i in 0..1000000 do 
if_equiv_method(strings[rand(strings.length)]) end }
   x.report("case"){for i in 0..1000000 do 
case_method(strings[rand(strings.length)]) end }
end

Results:

C:\WINDOWS\system32\cmd.exe /c ruby logic_benchmark.rb
Rehearsal -------------------------------------------------------
if                    2.578000   0.000000   2.578000 (  2.610000)
if_equiv              2.797000   0.000000   2.797000 (  2.953000)
case                  2.625000   0.000000   2.625000 (  2.812000)
---------------------------------------------- total: 8.000000sec

                           user     system      total        real
if                    2.578000   0.000000   2.578000 (  2.625000)
if_equiv              2.781000   0.000000   2.781000 (  2.828000)
case                  2.625000   0.000000   2.625000 (  2.672000)

It's pretty marginal, and I did have them come out much closer on some runs.

> 
>   Write it up here...
>      http://rubygarden.org/ruby/page/show/RubyOptimization
> ...the wiki page where I keep breadcrumbs of info for when desperation
> requires me to shovel perfectly correct nostrums to one side and put in 
> a twist of
> speed.
> 
> Speaking of which... Rubygarden seems to be slow..
Too slow for me to write this up, as a matter of fact.

-- 
Alex