From: Brian Candler <b.candler@...>
Date: 2008-09-18T21:42:40+09:00
Subject: Re: Preserve insert order in a Hash

> well, basically I just need a Hash to use String as indexes like
> 
> sourceInfo = Hash.new
> sourceInfo["var1"]=123
> sourceInfo["var2"]=2
> sourceInfo["var3"]=3
> sourceInfo["var4"]=23

What do you want to happen if sourceInfo["var2"] is assigned a second 
time? Do you want to replace it where it originally was in the sequence, 
or do you want to delete it and add the new value to the end? Or do you 
want both elements to appear at the same time? Or doesn't it matter?

I am just wondering because perhaps all you need is

sourceInfo = []
sourceInfo << ["var1",123]
sourceInfo << ["var2",2]
sourceInfo << ["var3",3]
sourceInfo << ["var4",4]
sourceInfo.each { |k,v| puts "#{k}=>#{v}" }

At least, that's fine if all you want to do is iterate over the 
collection and add new elements. Finding or deleting elements by key 
requires a linear search:

dummy,value = sourceInfo.find { |k,v| k == "var2" }

However you can optimise this by building a hash as you go which points 
to the same elements; or you could build an array containing just the 
keys.

  class HashAndArray
    def initialize
      @h, @a = {}, []
    end
    def [](k)
      @h[k]
    end
    def []=(k,v)
      @a << k unless @h.has_key?(k)
      @h[k] = v
    end
    def each
      @a.each { |k| yield k,@h[k] }
    end
  end

  sourceInfo = HashAndArray.new
  sourceInfo["var1"]=123
  sourceInfo["var2"]=2
  sourceInfo["var3"]=3
  sourceInfo["var4"]=23
  sourceInfo.each { |k,v| puts "#{k}=>#{v}" }
  puts sourceInfo["var2"]

Adding a delete() method is left as an exercise.
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