How To Reverse a Ruby Hash

Oct 18, 2014 Ruby

It's easy to reverse a Ruby array.

a = %w{ red blue green }
# => ["red", "blue", "green"]

a.reverse
# => ["green", "blue", "red"]

But it's not as straightforward with a hash. We have to convert it to an array, reverse it, and send it back to a hash. This works for nested hashes as well.

my_hash = {
  :key_1 => 1,
  :key_2 => 2,
  :key_3 => [1, 2, 3],
  :key_4 => {
    :subkey_1 => 1,
    :subkey_2 => 2
  }
}
# => {:key_1=>1, :key_2=>2, :key_3=>[1, 2, 3], :key_4=>{:subkey_1=>1, :subkey_2=>2}}

new_hash = Hash[my_hash.to_a.reverse].to_hash
# => {:key_4=>{:subkey_1=>1, :subkey_2=>2}, :key_3=>[1, 2, 3], :key_2=>2, :key_1=>1}

If you want to keep it simple, you can add a method to the Hash class.

class Hash

  def reverse
    Hash[self.to_a.reverse]
  end

end

Then you could do this:

my_hash.reverse
# => {:key_4=>{:subkey_1=>1, :subkey_2=>2}, :key_3=>[1, 2, 3], :key_2=>2, :key_1=>1}

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