Make hash keys match the order of a hash constant - ruby

Question

I have many hashes that contain the same keys but are all out of order from each other. I need to reorder each one to make them fit the proper order:
correct_order = {
:cat => "cat",
:dog => "dog",
:bear => "bear",
:zebra => "zebra",
:monkey => "monkey"
}
hash1 = {
:bear => "bear",
:cat => "cat"
}
hash2 = {
:cat => "cat",
:monkey => "monkey",
:zebra => "zebra",
:bear => "bear"
}
hash3 = {
:dog => "dog",
:monkey => "monkey",
:cat => "cat"
}
How would I go about comparing the key order of hash1, hash2, and hash3 to make each key match its place in the correct_order hash? hash2 would become:
hash2 {
:cat => "cat",
:bear => "bear",
:zebra => "zebra",
:monkey => "monkey"
}
Note that each new hash will not necessarily have all the keys. Each hash will be of varing size AND varying order.
Thank you

Answer 1

You can't "sort" a hash.
The doc says: Hashes enumerate their values in the order that the corresponding keys were inserted.
So you will have to create a new hash. For instance:
def sort_hash(ordered_keys, hash_to_sort)
new_hash= {}
ordered_keys.each do |key|
new_hash[key]= hash_to_sort[key] if hash_to_sort.has_key?(key)
end
new_hash
end
correct_order = {
:cat => "cat",
:dog => "dog",
:bear => "bear",
:zebra => "zebra",
:monkey => "monkey"
}
hash2 = {
:cat => "cat",
:monkey => "monkey",
:zebra => "zebra",
:bear => "bear"
}
puts sort_hash(correct_order.keys, hash2)
Note that the function returns a new hash. hash2 is not modified.

Answer 2

There is something that needs be said about the order of hash elements, but I'll address that after showing you how you can do what you want to do. This assumes you are using Ruby 1.9+, where the order of key insertion is maintained.
Code
def reorder(hash, order)
keys = order & hash.keys
Hash[keys.zip(hash.values_at(*keys))]
end
Examples
order = correct_order.keys
#=> [:cat, :dog, :bear, :zebra, :monkey]
reorder(hash1, order)
#=> {:cat=>"cat", :bear=>"bear"}
reorder(hash2, order)
#=> {:cat=>"cat", :bear=>"bear", :zebra=>"zebra", :monkey=>"monkey"}
reorder(hash3, order)
#=> {:cat=>"cat", :dog=>"dog", :monkey=>"monkey"}
Explanation
Let's see how the code deals with hash1:
hash = hash1
#=> {:bear=>"bear", :cat=>"cat"}
order
#=> [:cat, :dog, :bear, :zebra, :monkey]
keys = order & hash.keys
#=> [:cat, :dog, :bear, :zebra, :monkey] & [:bear, :cat]
#=> [:cat, :bear]
arr = keys.zip(hash.values_at(*keys))
#=> keys.zip({:bear=>"bear", :cat=>"cat"}.values_at(*[:cat, :bear]))
#=> keys.zip({:bear=>"bear", :cat=>"cat"}.values_at(:cat, :bear))
#=> keys.zip(["cat", "bear"])
#=> [[:cat, "cat"], [:bear, "bear"]]
Hash[arr]
#=> {:cat=>"cat", :bear=>"bear"}
In Ruby 1.9+ you can instead write the last step as:
arr.to_h
Note that the method Array#& preserves the order of the first array.
Hash order
Prior to Ruby 1.9 you could not depend on the order of key-value pairs (let's just say keys) in a hash. That changed in 1.9. Now you can be confident that when you enumerate the keys of a hash, the order of the enumeration will follow the order in which the keys were added to the hash. Be warned that that change was not embraced by all Rubyists. If you've been thinking of hashes as unordered collections for twenty years, it's a big change. Personally, I like the change and have found good uses for it.