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Keeping Multiple Values for the Same Hash Key
CodeExpectedActual
hash = Hash.new { |hash, key| hash[key] = [] }
raw_data = [ [1, 'a'], [1, 'b'], [1, 'c'], 
             [2, 'a'], [2, ['b', 'c']],
             [3, 'c'] ]
raw_data.each { |x,y| hash[x] << y }
hash
{1=>["a", "b", "c"], 2=>["a", ["b", "c"]], 3=>["c"]} {1=>["a", "b", "c"], 2=>["a", ["b", "c"]], 3=>["c"]} 
class MultiValuedHash < Hash
  def []=(key, value)
    if has_key?(key)      
      super(key, [value, self[key]].flatten)
    else
      super
    end  
  end
end
hash = MultiValuedHash.new
raw_data.each { |x,y|  hash[x] = y }
hash
 {1=>["c", "b", "a"], 2=>["b", "c", "a"], 3=>"c"} {1=>["c", "b", "a"], 2=>["b", "c", "a"], 3=>"c"}