How to get integer index from named order

Question

This might be really obvious but I can't find the answer.

How do you get the integer index from named order, like:

{ :first => 0, :second => 1, :third => 2, :fourth => 3 }

Is there something built in to Ruby or Rails that does this?

Thanks.

Update

Thanks for all your responses. Here's the solution I went with:

def index_for(position) 
  (0..4).to_a.send(position)
end

but arrays only support up to fifth, so it'll be limited to that.

Answer 1

I usually keep an array of hash keys to maintain order.

Answer 2

What version of Ruby are you using? For Ruby 1.8, you can't because in this version a hash is an unordered colletion. This means that when you insert keys, the order is not preserved. When you iterate over the hash, the keys might be returned in a different order from the order you inserted them.

It has changed in Ruby 1.9 though.

Answer 3

Look into Hash which mixes in Enumerable.
I think each_with_index is what you are searching for:

# Calls block with two arguments, the item and its index, for each item in enum. 

hash = Hash.new
%w(cat dog wombat).each_with_index {|item, index|
  hash[item] = index
}
hash   #=> {"cat"=>0, "wombat"=>2, "dog"=>1}

Answer 4

If you need the ordering index, you might need to combine the array and has

keys = [ :first, :second, :third, :fourth ]
hash = { :first => 0, :second => 1, :third => 2, :fourth => 3 }
hash.each_key { |x| puts "#{keys.index(x)}" }

The above method only work in 1.9 though.