From: Gavin Kistner <gavin@...>
Date: 2005-06-20T22:14:32+09:00
Subject: x==1 vs 1==x

--Apple-Mail-1--767860423
Content-Transfer-Encoding: 7bit
Content-Type: text/plain;
	charset=US-ASCII;
	delsp=yes;
	format=flowed

I'm against _premature_ optimization in theory, but believe that a  
certain innate knowledge of what is and what is not efficient is  
important for making reasonable choices in the first place.

To that end, when I had to write code to handle exactly one case out  
of many, where I knew that the operator would be #==, I thought I'd  
test case versus if/elsif. As I expected, the case statement  
performed worse (thought not as much as I expected).

On a whim, I also tested comparing x == 1 separate from 1 == x. Even  
when x was a Fixnum, the latter was surprisingly and significantly  
slower. Can anyone explain the following results?

8
Fixnum
Rehearsal ---------------------------------------------
case        1.500000   0.020000   1.520000 (  2.249505)
if x==n     0.930000   0.020000   0.950000 (  1.332420)
if n==x     1.330000   0.020000   1.350000 (  1.914666)
------------------------------------ total: 3.820000sec

                 user     system      total        real
case        1.510000   0.020000   1.530000 (  2.343995)
if x==n     0.920000   0.020000   0.940000 (  1.277303)
if n==x     1.340000   0.020000   1.360000 (  1.937022)


require 'benchmark'

n = 100_000
x = rand( 10 )
p x, x.class
Benchmark.bmbm(10){ |x|
   x.report('case'){
     n.times{
       case x
         when 0 : 0
         when 1 : 1
         when 2 : 2
         when 3 : 3
         when 4 : 4
         when 5 : 5
         when 6 : 6
         when 7 : 7
         when 8 : 8
         else 9
       end
     }
   }
   x.report('if x==n'){
     n.times{
       if x==0
         0
       elsif x==1
         1
       elsif x==2
         2
       elsif x==3
         3
       elsif x==4
         4
       elsif x==5
         5
       elsif x==6
         6
       elsif x==7
         7
       elsif x==8
         8
      else
         9
       end
     }
   }

   x.report('if n==x'){
     n.times{
       if 0==x
         0
       elsif 1==x
         1
       elsif 2==x
         2
       elsif 3==x
         3
       elsif 4==x
         4
       elsif 5==x
         5
       elsif 6==x
         6
       elsif 7==x
         7
       elsif 8==x
         8
      else
         9
       end
     }
   }
}

--
(-, /\ \/ / /\/


--Apple-Mail-1--767860423--