From: William James <w_a_x_man@...>
Date: 2006-07-30T02:40:06+09:00
Subject: Re: Hash order bug?

Charles Hoffman wrote:
> I think this is just a common mistake among newbie-ish programmers.
> Which isn't to say anything about Javier specifically, I don't know him,
> but in general, a review of one's Data Structures course may be in order
> when this happens.  But I think it happens to just about every
> programmer at some point.  I remember the first time I tried to do some
> ASP scripting for some web pages and I decided to use the Dictionary
> object.  At one point I wanted to loop through the Dictionary and
> display a listing of its contents.  I got all pissed off when I realized
> that the loop didn't return the items in alphabetical order.  Only later
> did I realize that the underlying data structure is a hash, and that the
> whole point of a hash is to maximize speed of access by sacrificing
> things like, being able to iterate it in any particular order.  Of
> course, the word "Dictionary" does, for most of us, bring to mind
> something that's in alphabetical order, so maybe it was Microsoft's
> fault for givig it a dumb name.  At any rate, it would be a bad idea to
> allow what is essentially a mistake made by programmers to force an
> abnormal, slower implementation onto a data structure whose definition
> is largely agreed upon in computing.  In other words, I'm with the "we
> don't need an 'ordered hash'" crowd.  I think that, besides slowing it
> down, it goes against the very definition of what a hash is.

True.

>
> If you need to iterate through a hash in order by key, it's a simple
> matter to grab an array of the keys first, sort it, then iterate from
> that.  I'm probably going to embarrass myself with my Ruby nuby-ness
> here, but off the top of my head:
>
> my_hash.keys.sort.each{|key| doSomethingWith(my_hash[key])}

h = { 'zebra', 2,  'horse', 0,  'sheep', 1 }
    ==>{"horse"=>0, "sheep"=>1, "zebra"=>2}
h.sort.each{|x| p x}
["horse", 0]
["sheep", 1]
["zebra", 2]
    ==>[["horse", 0], ["sheep", 1], ["zebra", 2]]