From: William James <w_a_x_man@...>
Date: 2006-12-27T07:05:07+09:00
Subject: Re: Hash with two identical keys?

Trans wrote:
> William James wrote:
> > Trans wrote:
> > > thanks Ara and Jon,
> > >
> > > I see what your saying. I was using #== not #eql? in comparing the
> > > keys. So I see why it's faling now. How do I get aorund this? I'm
> > > caching object based on therr initialization paramaters, which has to
> > > be an array. Is there a simpler way or do I have to do something like:
> > >
> > >   class Parameters < Array
> > >     alias :eql? :==
> > >   end
> > >
> > > T.
> >
> > How about association lists?
>
> Thanks William! That's what I did and worked (desipite inefficency).
>
> T.

Here's a speed comparison for various numbers of keys:

require 'benchmark'

$iterations = 40_000

def rand_sym
  letters = ('a'..'z').to_a
  sym = ""
  8.times{ sym << letters[ rand(letters.size) ] }
  sym.to_sym
end

def test_assoc n
  alist = []
  keys = []
  while alist.size < n do
    key = rand_sym
    unless keys.include?( key )
      alist << [ key, true ]
      keys << key
    end
  end
  $iterations.times{
    keys.each{|key|  fail if alist.assoc(key)[1] != true }
  }
end

def test_hash n
  hash = {}
  keys = []
  while hash.size < n
    key = rand_sym
    unless hash.include?( key )
      hash[key] = true
      keys << key
    end
  end
  $iterations.times{
    keys.each{|key|  fail if hash[key] != true }
  }
end

Benchmark.bm(8) do |x|
  [1,2,3,7,20].each{ |n|
    x.report("assoc %2d" % n) { test_assoc n }
    x.report("hash  %2d" % n)  { test_hash n  }
  }
end

              user     system      total        real
assoc  1  0.150000   0.000000   0.150000 (  0.171000)
hash   1  0.130000   0.000000   0.130000 (  0.140000)
assoc  2  0.291000   0.000000   0.291000 (  0.310000)
hash   2  0.200000   0.000000   0.200000 (  0.231000)
assoc  3  0.440000   0.000000   0.440000 (  0.460000)
hash   3  0.291000   0.000000   0.291000 (  0.311000)
assoc  7  1.172000   0.000000   1.172000 (  1.252000)
hash   7  0.590000   0.000000   0.590000 (  0.640000)
assoc 20  5.188000   0.000000   5.188000 (  5.578000)
hash  20  1.652000   0.000000   1.652000 (  1.813000)