From: Robert Klemme <bob.news@...>
Date: 2005-06-20T22:25:35+09:00
Subject: Re: x==1 vs 1==x

Gavin Kistner wrote:
> 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?

Yes, probably: you have something else in x than you expect.  x is really
the benchmark instance and not some Fixnum. :-)

Kind regards

    robert

>
> 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
>      }
>    }
> }