From: Pit Capitain <pit@...>
Date: 2001-08-24T18:57:48+09:00
Subject: [ruby-talk:20266] Re: ordered hash

On 24 Aug 2001, at 9:31, Joseph McDonald wrote:

> 
>   Has anyone implemented an OrderedHash in ruby?  That is:
>   Hash.each { } will return items in the order that they were
>   inserted.  Just want to avoid re-inventing the wheel.
> 
> thanks,
> -joe
> 

Hi Joe,

I once implemented an OrderedHash, but it was ordered according 
to the keys. Not quite what you need, sorry.

But if you use an Array in your OrderedHash holding the keys in 
the order you want it should not be too much work. I would start 
with the methods you know you'll need and not worry about all the 
other Hash methods, something like

class OrderedHash < Hash
  
  def []=( key, value )
    ( @ordered_keys ||= [] ) << key unless has_key?( key )
    super
  end
  
  def each
    @ordered_keys.each do | key |
      yield key, fetch( key )
    end if @ordered_keys
  end
  
end

Test:

h = OrderedHash.new
h[ 3 ] = 1
h[ 4 ] = 1
h[ 5 ] = 9
h[ 2 ] = 6
h[ 5 ] = 3
h.each { | k, v | puts "#{k} => #{v}" }

gives

3 => 1
4 => 1
5 => 3
2 => 6

This simple implementation lacks a lot of Hash's features, but if 
Hash.new, Hash#[], Hash#[]=, Hash#each are all you need it 
should be enough.

Pit