Comparable module usage in Ruby - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to include the Comparable module in the class.

Ruby

class Box
  include [1]
end

Drag options to blanks, or click blank then click option'

AComparable

BEnumerable

CKernel

DMath

Attempts:

3 left

💡 Hint

Common Mistakes

Including Enumerable instead of Comparable

Forgetting to include any module

Including unrelated modules like Math

The Comparable module must be included to enable comparison operators.

2fill in blank

medium

Complete the code to define the <=> method for comparing boxes by volume.

Ruby

class Box
  include Comparable

  def initialize(length, width, height)
    @length = length
    @width = width
    @height = height
  end

  def volume
    @length * @width * @height
  end

  def [1](other)
    volume <=> other.volume
  end
end

Drag options to blanks, or click blank then click option'

Aequals

Bcompare_to

C<=>

Dcompare

Attempts:

3 left

💡 Hint

Common Mistakes

Using method names like compare or equals instead of <=>

Not defining the <=> method at all

The <=> method is the spaceship operator used by Comparable to compare objects.

3fill in blank

hard

Fix the error in the comparison method name to enable Comparable functionality.

Ruby

class Box
  include Comparable

  def initialize(size)
    @size = size
  end

  def [1](other)
    @size <=> other.size
  end

  protected

  attr_reader :size
end

Drag options to blanks, or click blank then click option'

Acompare

Bequals

Ccompare_to

D<=>

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect method names like compare or equals

Misspelling the <=> operator

The method must be named <=> for Comparable to work correctly.

4fill in blank

hard

Fill both blanks to create a hash mapping box volumes to their sizes, filtering volumes greater than 100.

Ruby

boxes = [Box.new(2, 3, 4), Box.new(5, 5, 5), Box.new(1, 1, 1)]
volume_map = {box.[1] => box.[2] for box in boxes if box.volume > 100}

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Avolume

Bsize

Cheight

Dlength

Attempts:

3 left

💡 Hint

Common Mistakes

Using attributes that don't exist or are unrelated

Mixing up keys and values

The hash keys are volumes and values are sizes, filtered by volume > 100.

5fill in blank

hard

Fill all three blanks to create a hash of box sizes to volumes, including only boxes with volume less than 50.

Ruby

boxes = [Box.new(1, 2, 3), Box.new(2, 2, 2), Box.new(3, 3, 3)]
result = {box.[1] => box.[2] for box in boxes if box.volume [3] 50}

Drag options to blanks, or click blank then click option'

Asize

Bvolume

C<

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong comparison operators

Swapping keys and values

The hash maps sizes to volumes for boxes with volume less than 50.

Practice

1.

What is the main purpose of including the Comparable module in a Ruby class?

easy

A. To allow the class to inherit from another class

B. To automatically generate getter and setter methods

C. To enable comparison operators like <, <=, ==, >, >= for objects of that class

D. To add string manipulation methods to the class

Solution

1. Step 1: Understand Comparable module purpose

   The Comparable module provides comparison operators such as <, <=, ==, >, >= for objects.

2. Step 2: Recognize what including Comparable does

   Including Comparable requires defining the <=> method and then gives the class comparison operators automatically.

3. Final Answer:

   To enable comparison operators like <, <=, ==, >, >= for objects of that class -> Option C

4. Quick Check:

   Comparable adds comparison operators = A [OK]

Hint: Comparable adds comparison operators if <=> is defined [OK]

Common Mistakes:

• Thinking Comparable adds getters/setters
• Confusing Comparable with inheritance
• Assuming Comparable adds unrelated methods

2.

Which of the following is the correct way to define the comparison method <=> inside a Ruby class to use with Comparable?

class Box
  include Comparable

  def initialize(volume)
    @volume = volume
  end

  def <=>(other)
    ?
  end
end

easy

A. @volume <=> other.instance_variable_get(:@volume)

B. self.volume <=> other.volume

C. @volume <=> other.volume

D. @volume <=> other.@volume

Solution

1. Step 1: Understand instance variable access

   The instance variable @volume is private, so to compare, we must access it properly.

2. Step 2: Check each option for syntax and correctness

   self.volume <=> other.volume uses self.volume but no getter method is defined, so it fails. @volume <=> other.volume tries other.volume but no method exists. @volume <=> other.instance_variable_get(:@volume) uses other.instance_variable_get(:@volume), which accesses the variable correctly. @volume <=> other.@volume uses invalid syntax other.@volume.

3. Final Answer:

   @volume <=> other.instance_variable_get(:@volume) -> Option A

4. Quick Check:

   Access private var with instance_variable_get = B [OK]

Hint: Use instance_variable_get to access private vars in <=> [OK]

Common Mistakes:

• Trying to access other.@volume directly (syntax error)
• Assuming getter methods exist without defining them
• Using self.volume without a method

3.

What will be the output of the following Ruby code?

class Person
  include Comparable

  attr_reader :age

  def initialize(age)
    @age = age
  end

  def <=>(other)
    age <=> other.age
  end
end

p1 = Person.new(30)
p2 = Person.new(25)
puts p1 > p2

medium

A. false

B. true

C. nil

D. Error

Solution

1. Step 1: Understand the <=> method

   The <=> method compares the ages of two Person objects using age <=> other.age.

2. Step 2: Compare p1 and p2 using >

   p1.age is 30, p2.age is 25, so 30 > 25 is true.

3. Final Answer:

   true -> Option B

4. Quick Check:

   30 > 25 = true [OK]

Hint: Compare attributes inside <=> for correct operator results [OK]

Common Mistakes:

• Forgetting to include Comparable
• Not defining <=> method
• Confusing attribute names

4.

Identify the error in this Ruby class using Comparable and how to fix it:

class Item
  include Comparable

  def initialize(weight)
    @weight = weight
  end

  def <=>(other)
    @weight - other.weight
  end
end

medium

A. Method <=> returns a number, should return -1, 0, or 1

B. Syntax error in method definition spacing

C. Missing include Comparable line

D. Cannot call other.weight because no weight method is defined

Solution

1. Step 1: Check method calls inside <=>

   The code calls other.weight, but no weight method is defined in the class.

2. Step 2: Fix by adding an attr_reader for weight

   Adding attr_reader :weight allows other.weight to work correctly.

3. Final Answer:

   Cannot call other.weight because no weight method is defined -> Option D

4. Quick Check:

   Missing getter method = D [OK]

Hint: Define attr_reader for accessed attributes in <=> [OK]

Common Mistakes:

• Assuming instance variables are accessible without getters
• Returning subtraction result instead of using <=> operator
• Ignoring spacing in method definition (which is valid)

5.

You want to sort an array of Product objects by their price using Comparable. Which of the following is the best way to implement the <=> method?

class Product
  include Comparable

  attr_reader :price

  def initialize(price)
    @price = price
  end

  def <=>(other)
    ?
  end
end

hard

A. price <=> other.price

B. other.price <=> price

C. price - other.price

D. price > other.price ? 1 : -1

Solution

1. Step 1: Understand the <=> method contract

   The <=> method should return -1, 0, or 1 depending on comparison.

2. Step 2: Choose the correct comparison expression

   Using price <=> other.price correctly returns -1, 0, or 1. other.price <=> price reverses order, sorting descending. price - other.price returns a number but not guaranteed -1,0,1. price > other.price ? 1 : -1 returns only 1 or -1, missing 0 case.

3. Final Answer:

   price <=> other.price -> Option A

4. Quick Check:

   Use <=> operator for proper comparison = C [OK]

Hint: Use <=> operator directly for clean comparison [OK]

Common Mistakes:

• Reversing comparison order unintentionally
• Returning subtraction result instead of <=>
• Using ternary without zero equality case