From: Phrogz <phrogz@...>
Date: 2009-10-31T08:00:12+09:00
Subject: Re: two stage array sorting

On Oct 30, 11:17 am, John W Higgins <wish...@gmail.com> wrote:
> [Note:  parts of this message were removed to make it a legal post.]
>
> Morning Tim,
>
>
>
>
>
> On Fri, Oct 30, 2009 at 9:35 AM, Tim Ferrell <s0nsp...@gmail.com> wrote:
>
> > I have an array of strings like so:
>
> >  Tuesday 12:00 AM
> >  Sunday 9:00 AM
> >  Tuesday 3:00 PM
> >  Friday 7:30 AM
> >  Wednesday 5:00 PM
>
> > and I want the array sorted by day name and then time. Is it possible to
> > do this without resorting to a two stage process? Currently I am
> > breaking the array out into arrays by day and sorting those by time and
> > then reassembling... it just feels dirty :-)
>
> require 'date'
>
> def sort_datetime_array(ary)
>   fmt = '%A %I:%M %p'
>   ary.sort{ |a,b|
>     DateTime.strptime(a, fmt) <=> DateTime.strptime(b, fmt)
>   }
> end

Note that using sort is much slower than using sort_by (though perhaps
slightly more memory-efficient). This is possibly only significant if
you're sorting large amount of items, however. I mostly prefer sort_by
because it is DRYer (and hence less typing :).

require 'date'
require 'benchmark'

class Array; def rand; self[ Kernel.rand(length) ]; end; end
DAYS = %w[ Monday Tuesday Wednesday Thursday Friday Saturday Sunday ]
AMPM = %w[ AM PM ]
FORMAT = '%A %I:%M %p'

Benchmark.bmbm do |x|
  [10,100,1_000,10_000].each do |n|
    a = (0..n).map{ "#{DAYS.rand} #{rand(12)+1}:#{rand(60)} #
{AMPM.rand}" }
    x.report( "sort #{n}") do
      a.sort{ |y,z| Date.strptime(y,FORMAT) <=> Date.strptime
(z,FORMAT) }
    end
    x.report( "sort_by #{n}") do
      a.sort_by{ |y| Date.strptime(y,FORMAT) }
    end
  end
end

#=> Rehearsal -------------------------------------------------
#=> sort 10         0.020000   0.000000   0.020000 (  0.020270)
#=> sort_by 10      0.000000   0.000000   0.000000 (  0.003464)
#=> sort 100        0.200000   0.000000   0.200000 (  0.204563)
#=> sort_by 100     0.030000   0.000000   0.030000 (  0.032382)
#=> sort 1000       2.390000   0.040000   2.430000 (  2.455060)
#=> sort_by 1000    0.300000   0.010000   0.310000 (  0.316943)
#=> sort 10000     19.700000   0.280000  19.980000 ( 20.197492)
#=> sort_by 10000   3.270000   0.060000   3.330000 (  3.361712)
#=> --------------------------------------- total: 26.300000sec
#=>                     user     system      total        real
#=> sort 10         0.020000   0.000000   0.020000 (  0.016442)
#=> sort_by 10      0.000000   0.000000   0.000000 (  0.003143)
#=> sort 100        0.190000   0.000000   0.190000 (  0.190505)
#=> sort_by 100     0.030000   0.000000   0.030000 (  0.026150)
#=> sort 1000       2.300000   0.030000   2.330000 (  2.351752)
#=> sort_by 1000    0.290000   0.010000   0.300000 (  0.298472)
#=> sort 10000     19.020000   0.240000  19.260000 ( 19.437129)
#=> sort_by 10000   3.310000   0.050000   3.360000 (  3.389367)