Hash invert in Ruby?

Question

I\'ve got a hash of the format:

{key1 => [a, b, c], key2 => [d, e, f]}

and I want to end up with:

{ a => key1, b =         


        

Answer 1

Ok, let's guess. You say you have an array but I agree with Benoit that what you probably have is a hash. A functional approach:

 h = {:key1 => ["a", "b", "c"], :key2 => ["d", "e", "f"]}
 h.map { |k, vs| Hash[vs.map { |v| [v, k] }] }.inject(:merge)
 #=> {"a"=>:key1, "b"=>:key1, "c"=>:key1, "d"=>:key2, "e"=>:key2, "f"=>:key2}

Also:

 h.map { |k, vs| Hash[vs.product([k])] }.inject(:merge)
 #=> {"a"=>:key1, "b"=>:key1, "c"=>:key1, "d"=>:key2, "e"=>:key2, "f"=>:key2}

Answer 2

If you want to correctly deal with duplicate values, then you should use the Hash#inverse from Facets of Ruby

Hash#inverse preserves duplicate values, e.g. it ensures that hash.inverse.inverse == hash

either:

• use Hash#inverse from here: http://www.unixgods.org/Ruby/invert_hash.html

• use Hash#inverse from FacetsOfRuby library 'facets'

usage like this:

require 'facets'

h = {:key1 => [:a, :b, :c], :key2 => [:d, :e, :f]}
 => {:key1=>[:a, :b, :c], :key2=>[:d, :e, :f]} 

h.inverse
 => {:a=>:key1, :b=>:key1, :c=>:key1, :d=>:key2, :e=>:key2, :f=>:key2} 

The code looks like this:

# this doesn't looks quite as elegant as the other solutions here,
# but if you call inverse twice, it will preserve the elements of the original hash

# true inversion of Ruby Hash / preserves all elements in original hash
# e.g. hash.inverse.inverse ~ h

class Hash

  def inverse
    i = Hash.new
    self.each_pair{ |k,v|
      if (v.class == Array)
        v.each{ |x|
          i[x] = i.has_key?(x) ? [k,i[x]].flatten : k
        }
      else
        i[v] = i.has_key?(v) ? [k,i[v]].flatten : k
      end
    }
    return i
  end

end


h = {:key1 => [:a, :b, :c], :key2 => [:d, :e, :f]}
 => {:key1=>[:a, :b, :c], :key2=>[:d, :e, :f]} 

h.inverse
 => {:a=>:key1, :b=>:key1, :c=>:key1, :d=>:key2, :e=>:key2, :f=>:key2} 

Answer 3

In the case where a value corresponds to more than one key, like "c" in this example...

{ :key1 => ["a", "b", "c"], :key2 => ["c", "d", "e"]}

...some of the other answers will not give the expected result. We will need the reversed hash to store the keys in arrays, like so:

{ "a" => [:key1], "b" => [:key1], "c" => [:key1, :key2], "d" => [:key2], "e" => [:key2] }

This should do the trick:

reverse = {}
hash.each{ |k,vs|
    vs.each{ |v|
        reverse[v] ||= []
        reverse[v] << k
    }
}

This was my use case, and I would have defined my problem much the same way as the OP (in fact, a search for a similar phrase got me here), so I suspect this answer may help other searchers.

Answer 4

hash = {:key1 => ["a", "b", "c"], :key2 => ["d", "e", "f"]}

first variant

hash.map{|k, v| v.map{|f| {f => k}}}.flatten
#=> [{"a"=>:key1}, {"b"=>:key1}, {"c"=>:key1}, {"d"=>:key2}, {"e"=>:key2}, {"f"=>:key2}] 

or

hash.inject({}){|h, (k,v)| v.map{|f| h[f] = k}; h}
#=> {"a"=>:key1, "b"=>:key1, "c"=>:key1, "d"=>:key2, "e"=>:key2, "f"=>:key2} 

UPD

ok, your hash is:

hash = {"status"=>{"1"=>["1", "14"], "2"=>["7", "12", "8", "13"]}}
hash["status"].inject({}){|h, (k,v)| v.map{|f| h[f] = k}; h}
#=> {"12"=>"2", "7"=>"2", "13"=>"2", "8"=>"2", "14"=>"1", "1"=>"1"}

Answer 5

new_hash={}
hash = {"key1" => ['a', 'b', 'c'], "key2" => ['d','e','f']}
hash.each_pair{|key, val|val.each{|v| new_hash[v] = key }}

This gives

new_hash # {"a"=>"key1", "b"=>"key1", "c"=>"key1", "d"=>"key2", "e"=>"key2", "f"=>"key2"}

Answer 6

If you're looking to reverse a hash formatted like this, the following may help you:

a = {:key1 => ["a", "b", "c"], :key2 => ["d", "e", "f"]}
a.inject({}) do |memo, (key, values)|
  values.each {|value| memo[value] = key }
  memo
end

this returns:

{"a"=>:key1, "b"=>:key1, "c"=>:key1, "d"=>:key2, "e"=>:key2, "f"=>:key2}