From: Timothy Goddard <interfecus@...>
Date: 2007-03-08T20:15:05+09:00
Subject: Re: Sorting objects...

On Mar 8, 9:19 pm, "rretzbach" <rretzb...@googlemail.com> wrote:
> On 8 Mrz., 08:09, "Farrel Lifson" <farrel.lif...@gmail.com> wrote:
>
>
>
> > On 08/03/07, Justin Collins <colli...@seattleu.edu> wrote:
>
> > > Farrel Lifson wrote:
> > > > On 08/03/07, Jason Burgett <jas...@gmail.com> wrote:
> > > >> I have a series of objects in an array. Each object corresponds to an
> > > >> application and has several attributes. For instance I can call:
>
> > > >> @application.name
> > > >> @application.icon
> > > >> @application.ranking
>
> > > >> "ranking" is always an integer. So if I have an array of these objects,
> > > >> say, @allApplications how do I sort using each application's ranking
> > > >> (@application.ranking)?
>
> > > > applications.sort_by{|object| object.ranking}
>
> > > Another way would be to add a <=> method
>
> > > class Application
> > >   def <=> other
> > >      self.ranking <=> other.ranking
> > >   end
> > > end
>
> > > -Justin
>
> > Using only <=> does have some speed consequences:
>
> > require 'benchmark'
> > srand
>
> > class Application
> >   attr_reader :ranking
> >         def initialize
> >           @ranking = rand
> >         end
> >         def <=>(other)
> >                 @ranking <=> other.ranking
> >         end
> > end
>
> > applications = Array.new(1000000){Application.new}
>
> > Benchmark.bm do |bmark|
> >         bmark.report("Sort")  { applications.sort }
> >         bmark.report("Sort By") {applications.sort_by {|o| o.ranking}}
> > end
>
> > C:\Documents and Settings\flifson\Desktop>ruby sort_test.rb
> >                 user           system      total          real
> > Sort        20.782000   0.031000   20.813000 ( 20.844000)
> > Sort By  6.687000     0.000000   6.687000   (  6.687000)
>
> > Farrel- Zitierten Text ausblenden -
>
> > - Zitierten Text anzeigen -
>
> I don't understand, can someone please explain this loss of speed?

Sorting compares pairs of objects many more times than the number of
objects actually being sorted. The <=> method extracts the ranking
from the objects each time a pair is compared. The sort_by method only
extracts the ranking from each object once then compares these. The
time difference represents the overhead of the <=> method call and the
time required to retrieve instance variables for each of the pair.