Masatoshi Nishiguchi's Blog

Ruby Intersection of Object Arrays

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Masatoshi Nishiguchi
·

2 min read

By default, arrays of objects cannot be intersected in Ruby. By overriding eql?(other) and hash methods, we can make a Ruby object capable of intersecting.

class Color
  attr_reader :name

  def initialize(name)
    @name = name
  end
end

# Both lists have Blue in common so Blue should be the intersection.
list1 = [Color.new("Blue"), Color.new("Yellow"), Color.new("Black")].shuffle
list2 = [Color.new("Blue"), Color.new("Red"), Color.new("White")].shuffle

# But result is empty.
list1 & list2
#=> []

# Array-intersecting-capable class.
class Color
  attr_reader :name

  def initialize(name)
    @name = name
  end

  def eql?(other)
    other.kind_of?(self.class) && name == other.name
  end

  def hash
    name.hash
  end
end

# Both lists have Blue in common so Blue should be the intersection.
list1 = [Color.new("Blue"), Color.new("Yellow"), Color.new("Black")].shuffle
list2 = [Color.new("Blue"), Color.new("Red"), Color.new("White")].shuffle

# The result is as expected.
list1 & list2
#=> [#<Color:0x007f92001a5c18 @name="Blue">]

Surprisingly there was not much information on this topic. This old article from 2006 was helpful.

That's it.

Ruby