Reverse a hash in Ruby

Question

How would I reverse the elements in the hash, keeping the same values and keys, but reversing their order in the hash.

Like so:

{ \"4\" => \"happiness         


        

Answer 1

if need:

hash = {:a => :x, :b => :y, :c => :y, :d => :z}

to:

{:x => [:a], :y => [:b, c], :z => [:d] }

can:

h={};hash.to_a.each{|e|h[e[1]]||=[];h[e[1]]<<e[0]};h

Answer 2

h = { "4" => "happiness", "10" => "cool", "lala" => "54", "1" => "spider" }
p Hash[h.reverse_each.map{|e| e}]
#=> {"1"=>"spider", "lala"=>"54", "10"=>"cool", "4"=>"happiness"}

But this leaves a bad taste (just like the other answers, which work fine just like this one). If you have to do this, it could be an indication that a Hash was not the best choice.

Answer 3

Alternatively, you can use reduce and merge to add the item to the front of a new hash:

hash = { "4" => "happiness", "10" => "cool", "lala" => "54", "1" => "spider" }
hash.reduce({}){ |memo, object| Hash[*object].merge(memo) }

but, that's crazy :D

Answer 4

In Ruby 2.1+ you can combine reverse_each and to_h:

{foo: 1, bar: 2}.reverse_each.to_h
#=> {:bar=>2, :foo=>1}

Answer 5

You could convert the Hash to an Array, reverse that, and then convert it back to a Hash:

reversed_h = Hash[h.to_a.reverse]

Hash#to_a gives you an array of arrays, the inner arrays are simple [key,value] pairs, then you reverse that array using Array#reverse, and Hash[] converts the [key,value] pairs back into a Hash.

Ruby 2.1 adds an Array#to_h method so you can now say:

reversed_h = h.to_a.reverse.to_h

Answer 6

hash = { "4" => "happiness", "10" => "cool", "lala" => "54", "1" => "spider" }
reversed_hash = Hash[hash.to_a.reverse]