Picking values from Ruby hashes

Written by

Matthew

in

Want to pick a certain set of key/value pairs from a Ruby hash? You might do this:

hash = {:foo => "bar", :bar => "baz", :baz => "boo"}
hash.select { |k,v| [:foo, :bar].include?(k) }

# returns [[:foo, "bar"], [:bar, "baz"]]

Kind of messy. We can do better by reopening the Hash class this way:

class Hash
  def pick(*values)
    select { |k,v| values.include?(k) }
  end
end

Now our selection works like this:

hash = {:foo => "bar", :bar => "baz", :baz => "boo"}
hash.pick(:foo, :bar)

# returns [[:foo, "bar"], [:bar, "baz"]]

Ruby is a wonderful language. This is one small example of how having access to existing classes can be incredibly powerful. With this power comes great responsibility. Wield your power wisely.

Comments

3 responses to “Picking values from Ruby hashes”

  1. Scott Moonen Avatar
    Scott Moonen

    If you’re using Rails, then ActiveSupport::CoreExtensions::Hash::Slice provides a slice() method to slice a hash, and this is automatically added to the Hash class for you. Its usage is exactly as given above, except that it returns a hash instead of an array of pairs.

  2. neerajdotname Avatar
    neerajdotname

    You can use facets Hash method slice.

    h.slice(:foo,:bar) #=> another hash

    h.slice(:foo,:bar).to_a #=> to convert the hash into array.

    http://facets.rubyforge.org/doc/index.html

  3. John Long Avatar
    John Long

    How about:


    class Hash
    def pick(*values)
    pairs = select { |k,v| values.include?(k) }
    Hash[*pairs.flatten]
    end
    end

    hash.pick(:foo, :bar)
    #returns {:bar=>"baz", :foo=>"bar"}

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