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

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}