From: "David A. Black" <dblack@...>
Date: 2009-02-14T02:14:18+09:00
Subject: Re: If you are unhappy with the direction of Ruby 1.8.7+, respond

Hi --

On Sat, 14 Feb 2009, Rick DeNatale wrote:

> On Fri, Feb 13, 2009 at 11:07 AM, David A. Black <dblack@rubypal.com> wrote:
>
>> On Sat, 14 Feb 2009, James Coglan wrote:
>> Regardless of whether they're *supposed* to, the Rubies I've used before
>>
>>> 1.8.7 actually do iterate in insertion order. Could be machine/OS
>>> specific,
>>> a fluke, whatever. The point is that 1.8.7 definitely does not iterate in
>>> the order that keys appear in a literal, at least on my machine. If this
>>> is
>>> supposed to now be defined behaviour I'll investigate further and file a
>>> bug
>>> if necessary.
>>>
>>
>> I think it probably is machine-dependent, or something like that. I've
>> never heard of hash order being anything but "not guaranteed" before
>> 1.9 -- except that if you iterate twice, I believe it definitely
>> happens in the same order both times (though I'm not 100% sure that
>> that's actually specified or guaranteed either).
>>
>
> No, it's in the absence of key collisions, it's really determined by the
> hash values of the keys,  The iteration is over the hash buckets and hash
> buckets marked as empty are skipped.
>
> Consider your examples which I've excerpted:
>
> % all-ruby -e 'h = {2=>"b", 1=>"a"}; p h'
> ...
> ruby-1.8.6head(2009-02-13p330) {1=>"a", 2=>"b"}
> ...
> ruby-1.8.7head(2009-02-13p116) {1=>"a", 2=>"b"}
> ...
> ruby-1.9.1p0(2009-01-30r21907) {2=>"b", 1=>"a"}
>
> Note that for Ruby 1.8.6 and 1.8.7, the iteration order is by the sort order
> of the keys.  But that's just a side-effect of the hash of a fixnum being
> just the internal immediate representation which happens to be n*2 + 1 for a
> Fixnum n:
>
> irb(main):001:0> 1.hash
> => 3
> irb(main):002:0> 2.hash
> => 5
>
> Had you used what is arguably the more natural order in the literal, i.e. {1
> => "a", 2 => "b"} it would appear as if iteration was by insertion order,
> but that's just an accidental artifact.

That's why I didn't :-)


David

-- 
David A. Black / Ruby Power and Light, LLC
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