Hash intersection in Ruby

hash_intersection.rb

first = {"a" => 4, "b" => 1, "c" => 3, "g" => 201, "h" => 501}
second = {"d" => 19, "e" => 18, "c" => 17, "g" => 201, "h" => 501}

keys_intersection = first.keys & second.keys
merged = first.dup.update(second)

intersection = {}
keys_intersection.each do |k|
  intersection[k]=merged[k] unless first[k] != second[k]
end

# same thing as freedom patch

class Hash
  def intersection(another_hash)
    keys_intersection = self.keys & another_hash.keys
    merged = self.dup.update(another_hash)

    intersection = {}
    keys_intersection.each {|k| intersection[k] = merged [k] unless self[k] != another_hash[k]}
    intersection
  end
end

{"a" => 4, "b" => 1, "c" => 3, "g" => 201, "h" => 501}.intersection({"d" => 19, "e" => 18, "c" => 17, "g" => 201, "h" => 501})
 => {"g"=>201, "h"=>501}

Why not

class Hash
  def intersection(another_hash)
    delete_if {|k, v| v != another_hash[k]}
  end
end

You'd want to use reject instead of delete_if to be nondestructive. And I'd personally prefer to implement it as & to match Array.

class Hash
  def &(other)
    reject { |k, v| !(other.include?(k) && other[k] == v) }
  end
end

The use of include? is necessary to avoid inclusion of surprise nil values in the result.

irb(main):006:0> {:x => 1, :y => 2, :z => nil} & {:y => 2}
=> {:y=>2}