Identity and Equality

Part of the Object Oriented Programming section of Coddy's Ruby journey. Lesson 10 of 57.

A variable holds a reference to an object, not the object itself. Assigning it to another variable copies the reference, so both names then refer to the same object:

class Box
  attr_accessor :count

  def initialize
    @count = 0
  end
end

a = Box.new
b = a
b.count += 1
puts a.count

Output:

equal? asks whether two references point to the same object, and object_id is each object's unique number. For your own classes, == does the same by default, so two objects with the same values are not equal:

class Point
  attr_reader :x, :y

  def initialize(x, y)
    @x = x
    @y = y
  end
end

a = Point.new(1, 2)
b = Point.new(1, 2)
c = a
puts a == b
puts a.equal?(b)
puts a.equal?(c)
puts a.object_id == c.object_id

Output:

false
false
true
true

Define == to compare by value. include?, index and delete on arrays use ==, so they then find equal objects too. Check the class of other first, so comparing with a number or nil returns false instead of failing:

class Point
  attr_reader :x, :y

  def initialize(x, y)
    @x = x
    @y = y
  end

  def ==(other)
    other.is_a?(Point) && x == other.x && y == other.y
  end
end

a = Point.new(1, 2)
puts a == Point.new(1, 2)
puts a == 5
puts [Point.new(0, 0), Point.new(1, 2)].include?(a)
puts a.equal?(Point.new(1, 2))

Output:

true
false
true
false

dup makes a new object with copies of the instance variables. Assigning a new value in the copy leaves the original alone, but an array or string inside is still shared between the two, because only the reference was copied:

class Team
  attr_accessor :name, :players

  def initialize(name)
    @name = name
    @players = []
  end
end

a = Team.new("Reds")
b = a.dup
b.name = "Blues"
b.players << "Ada"
puts a.name
p a.players

Output:

Reds
["Ada"]

Challenge

Easy

Complete the class Point so two points with the same coordinates are equal with ==, and comparing a point with anything else returns false. to_s returns (1, 2).

The supplied code reads points as x,y lines. For each point it prints (1, 2) same as first: true or false, then it keeps a list of distinct points using include? and finally prints distinct: 3 and whether the first point equals the text "1,2".

Your code goes in point.rb. main.rb holds the supplied input/output code and cannot be edited.

Try it yourself

require_relative 'point'

# Supplied input/output code: keep it as it is
input = []
while (line = gets)
  input << line.chomp
end
points = input.map { |line| Point.new(*line.split(",").map(&:to_i)) }
points.each { |pt| puts "#{pt} same as first: #{pt == points[0]}" }
distinct = []
points.each { |pt| distinct << pt unless distinct.include?(pt) }
puts "distinct: #{distinct.size}"
puts "equals text: #{points[0] == input[0]}"