Hash with two identical keys?

Ruby
1

From Facets’ multiton.rb (which is primarily Floran Franks’ work), I’m
getting somthing that doesn’t make any sense:

    POOLS[self] ||= {}
    p POOLS[self].class
    p POOLS[self].keys

Is outputing:

Hash
[[{:strip_comments=>false}], [{:strip_comments=>false}]]

How can two identical keys be in a hash?

T.

2

On 12/26/06, Trans [email protected] wrote:

[[{:strip_comments=>false}], [{:strip_comments=>false}]]

How can two identical keys be in a hash?

T.

irb(main):005:0> bob = [{:whee => false}]
=> [{:whee=>false}]
irb(main):006:0> bob.hash
=> 23244868
irb(main):007:0> roger = [{:whee => false}]
=> [{:whee=>false}]
irb(main):008:0> roger.hash
=> 23235888
irb(main):009:0> bob.eql? roger
=> false
irb(main):010:0> bob[0].eql? roger[0]
=> false
irb(main):011:0> bob[0].hash
=> 23244870
irb(main):012:0> roger[0].hash
=> 23235890

I think it boils down to: there is no Hash#hash which inspects the
contents - Hash inherits Object#hash which is Object#object_id, and
the two hashes, while they appear to have the same contents, are not
the same object. So the keys are not “identical”.

-A

3

Looks to me that your keys are arrays.

My guess is that

p POOLS[self].keys[0].class

will return Array, instead of Symbol, which is what you’re probably
expecting. So, the hash doesn’t have identical keys. It has two
different keys, each of which is a different array with identical
members. Try

p POOLS[self].keys[0].object_id
p POOLS[self].keys[1].object_id

and you’ll see the keys really are different.

4

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.

5

Trans wrote:

T.

How about association lists?

irb(main):029:0> a1=[[:strip,false]]
=> [[:strip, false]]
irb(main):030:0> a2=[[:strip,false]]
=> [[:strip, false]]
irb(main):031:0> h={}
=> {}
irb(main):032:0> h[a1] = ‘foo’
=> “foo”
irb(main):033:0> h[a2] = ‘bar’
=> “bar”
irb(main):034:0> h
=> {[[:strip, false]]=>“bar”}

6

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

Ugh. Nothing like that works either. It’s not using eql? or equal?, but
rather #hash (I guess that’s actually what you were trying to say Ara).
This doen’t make any sense to me. Why do identical strings and arrays
have the same #hash value but not hashes?

T.

7

Trans wrote:

end

Ugh. Nothing like that works either. It’s not using eql? or equal?, but
rather #hash (I guess that’s actually what you were trying to say Ara).
This doen’t make any sense to me. Why do identical strings and arrays
have the same #hash value but not hashes?

Nope. It doesn’t even use #hash. So it must be using #object_id with an
exception for Strings and Arrays. Dissapointing to say the least.

I had to resort to recursively converting all hashes to arrays.

T.

8

Eric H. wrote:

class Parameters < Array
not implemented that way.
I see. So it’s not using object_id but

a.hash == b.hash && a.eql?(b)

Is that right?

a = []
a << a
a.hash

are easier to compute than:

h = {}
h[h] = h
h.hash

Hmm… the expection gums up the works.

T.

9

William J. wrote:

end

T.

How about association lists?

Thanks William! That’s what I did and worked (desipite inefficency).

T.

10

Trans wrote:

thanks Ara and Jon,

Oops. Just saw that was Alex, not Ara, sorry about that Alex! And
thanks for the help.

T.

11

On Dec 26, 2006, at 11:22, Trans wrote:

alias :eql? :==

end

Ugh. Nothing like that works either. It’s not using eql? or equal?,
but
rather #hash (I guess that’s actually what you were trying to say
Ara).

A Hash only asks if one object is #eql? to another when they have the
same #hash. You can’t use a Hash as a Hash key because Hash#hash is
not implemented that way.

This doen’t make any sense to me. Why do identical strings and arrays
have the same #hash value but not hashes?

Likely because:

s = ‘’
s.hash

and:

a = []
a << a
a.hash

are easier to compute than:

h = {}
h[h] = h
h.hash

–
Eric H. - [email protected] - http://blog.segment7.net

I LIT YOUR GEM ON FIRE!

12

On Wed, 27 Dec 2006, Trans wrote:

[[{:strip_comments=>false}], [{:strip_comments=>false}]]

How can two identical keys be in a hash?

T.

hi trans-

afaik multiton.rb is mine

http://codeforpeople.com/lib/ruby/multiton/multiton-1.0.2/lib/multiton.rb

the logic behind POOLS is that objects are cached this way

POOLS[ class_of_object ][ args_given_to_new ] = obj

in otherwords, contructing two objects with the same argument lists will
contruct only one object. it’s the argument lists which are used to
determine
uniqueness. alternatively uniqueness will be determined via the method
‘multiton_id’ if you’re class has implimented that instance method or
the
object in question responds to it otherwise. so, in your case, you
might use
something like

hash.to_a.sort.hash

or something unique like that. eg

class MyClass
include Multiton

 attr :multiton_id

 def initialize h = {}
   @multiton_id = h.to_a.sort.hash
   super
 end

end

kind regards.

-a

13

William J. wrote:

sym.to_sym
end
key = rand_sym
Benchmark.bm(8) do |x|
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)

Nice. Doesn’t matter a whole lot a few keys but there is a clear slow
down.

I came up with another possibility however. I won;t work for all cases,
but using Marshal.dup on the args instead of converting to assoc gives
the proper result too. Wonder how that would benchmark?

T.

14

Trans wrote:

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)

15

[email protected] wrote:

http://codeforpeople.com/lib/ruby/multiton/multiton-1.0.2/lib/multiton.rb

Somehow I got Florian F.s name attached to that. Well, unless
Florian’s got something to say about it, I’ll reattribute to you. Sorry
about that!

T.