Remapping the Keys and Values of a Hash

Problem

You have two hashes with common keys but differing values. You want to create a new hash that maps the values of one hash to the values of another.

Solution

	class Hash
	 def tied_with(hash)
	 remap do |h,key,value|
	 h[hash[key]] = value
	 end.delete_if { |key,value| key.nil? || value.nil? }
	 end

Here is the Hash#remap method:

	 def remap(hash={})
	 each { |k,v| yield hash, k, v }
	 hash
	 end
	end

Here's how to use Hash#tied_with to merge two hashes:

	a = {1 => 2, 3 => 4}
	b = {1 => 'foo', 3 => 'bar'}
	a.tied_with(b) # => {"foo"=>2, "bar"=>4}
	b.tied_with(a) # => {2=>"foo", 4=>"bar"}

 

Discussion

This remap method can be handy when you want to make a similar change to every item in a hash. It is also a good example of using the yield method.

Hash#remap is conceptually similar to Hash#collect, but Hash#collect builds up a nested array of key-value pairs, not a new hash.

See Also

  • The Facets library defines methods Hash#update_each and Hash#replace_each! for remapping the keys and values of a hash


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