From: John Joyce <dangerwillrobinsondanger@...>
Date: 2007-05-09T09:58:46+09:00
Subject: Re: Equality question


On May 9, 2007, at 1:55 AM, Brian Candler wrote:

> On Wed, May 09, 2007 at 12:58:01AM +0900, Sebastian Hungerecker wrote:
>> Brian Candler wrote:
>>> o1 = [:defuzz_configs, {"conf"=>"foo.txt"}]
>>> o2 = [:defuzz_configs, {"conf"=>"foo.txt"}]
>>> p o1 == o2              # prints true
>>> p o1.hash == o2.hash    # prints false ??             <<<<<<<<<<<
>>
>> That's because the Hashs in o1 and o2 are two different objects.
>
> So hashes and arrays don't behave the same in this regard?
>
> o1 = ["hello", ["world"]]
> o2 = ["hello", ["world"]]
> p o1 == o2
> p o1.hash == o2.hash    # prints true
>
> o1 = {"key"=>"hello"}
> o2 = {"key"=>"hello"}
> p o1 == o2
> p o1.hash == o2.hash    # prints false
>
> I wonder why this is?
>

it's worth exploring further! (the ruby cookbook has a good section  
on ruby's hashes)
I made two hashes like yours.

irb(main):011:0> o1.__id__
=> 1611054
irb(main):012:0> o2.__id__
=> 1586104
irb(main):013:0> o1.hash
=> 1611054
irb(main):014:0> o2.hash
=> 1586104

As you can see the two hashes don't share the same object id, .hash  
method of objects creates a hash value for the object (confusing  
terminology, I know) but with hashes , that hash value just happens  
to be the same as the object id.
They may contain (references to) the same objects, so that makes  
these two hashes look a lot alike, but they're as different as you  
and your clone.

What's more confusing? Try messing with symbols in your hashes. Then  
wrap your head around it.
I finally got a handle on symbols. They're like hard links in unix.  
little wormholes if you will.