How do I extract a sub-hash from a hash?

Question

I have a hash:

h1 = {:a => :A, :b => :B, :c => :C, :d => :D} 

What is the best way to extract a sub-hash like this?

h1.extract_subhash(:b, :d, :e, :f) # => {:b => :B, :d => :D} h1 #=> {:a => :A, :c => :C} 

Answer 1

ActiveSupport, at least since 2.3.8, provides four convenient methods: #slice, #except and their destructive counterparts: #slice! and #except!. They were mentioned in other answers, but to sum them in one place:

x = {a: 1, b: 2, c: 3, d: 4} # => {:a=>1, :b=>2, :c=>3, :d=>4}  x.slice(:a, :b) # => {:a=>1, :b=>2}  x # => {:a=>1, :b=>2, :c=>3, :d=>4}  x.except(:a, :b) # => {:c=>3, :d=>4}  x # => {:a=>1, :b=>2, :c=>3, :d=>4} 

Note the return values of the bang methods. They will not only tailor existing hash but also return removed (not kept) entries. The Hash#except! suits best the example given in the question:

x = {a: 1, b: 2, c: 3, d: 4} # => {:a=>1, :b=>2, :c=>3, :d=>4}  x.except!(:c, :d) # => {:a=>1, :b=>2}  x # => {:a=>1, :b=>2} 

ActiveSupport does not require whole Rails, is pretty lightweight. In fact, a lot of non-rails gems depend on it, so most probably you already have it in Gemfile.lock. No need to extend Hash class on your own.

Answer 2

If you specifically want the method to return the extracted elements but h1 to remain the same:

h1 = {:a => :A, :b => :B, :c => :C, :d => :D} h2 = h1.select {|key, value| [:b, :d, :e, :f].include?(key) } # => {:b=>:B, :d=>:D}  h1 = Hash[h1.to_a - h2.to_a] # => {:a=>:A, :c=>:C}  

And if you want to patch that into the Hash class:

class Hash   def extract_subhash(*extract)     h2 = self.select{|key, value| extract.include?(key) }     self.delete_if {|key, value| extract.include?(key) }     h2   end end 

If you just want to remove the specified elements from the hash, that is much easier using delete_if.

h1 = {:a => :A, :b => :B, :c => :C, :d => :D} h1.delete_if {|key, value| [:b, :d, :e, :f].include?(key) } # => {:a=>:A, :c=>:C}  h1  # => {:a=>:A, :c=>:C}