Class.new for dynamic class creation in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Ruby code using Class.new?

Consider this Ruby code that creates a class dynamically and uses it:

Ruby

MyClass = Class.new do
  def greet
    "Hello from dynamic class!"
  end
end

obj = MyClass.new
puts obj.greet

A Hello from dynamic class!

B NoMethodError

C SyntaxError

D nil

💡 Hint

Look at how the method greet is defined inside the Class.new block.

The Class.new block defines a method greet. When we create an instance and call greet, it returns the string.

❓ Predict Output

intermediate

What does this code print when using Class.new with inheritance?

This Ruby code creates a subclass dynamically. What is printed?

Ruby

class Parent
  def message
    "Parent says hi"
  end
end

Child = Class.new(Parent) do
  def message
    super + " and Child says hello"
  end
end

puts Child.new.message

A NoMethodError

B Child says hello

C Parent says hi

D Parent says hi and Child says hello

💡 Hint

Remember that super calls the method from the parent class.

The Child class overrides message but calls super to get parent's message, then adds its own text.

🔧 Debug

advanced

Why does this dynamic class creation code raise an error?

This Ruby code tries to create a class dynamically but raises an error. Identify the cause.

Ruby

MyClass = Class.new do
  def initialize(name)
    @name = name
  end
end

obj = MyClass.new
puts obj.instance_variable_get(:@name)

A ArgumentError: wrong number of arguments (given 0, expected 1)

B SyntaxError: unexpected end-of-input

C NoMethodError: undefined method 'initialize'

D nil

💡 Hint

Check how many arguments are passed to new versus initialize method parameters.

The initialize method expects one argument, but new is called without arguments, causing ArgumentError.

📝 Syntax

advanced

Which option correctly defines a dynamic class with a class method using Class.new?

Select the Ruby code that correctly creates a class with a class method 'info' returning 'Dynamic class'.

A

MyClass = Class.new do
  def self.info
    'Dynamic class'
  end
end

B

MyClass = Class.new do
  def info
    'Dynamic class'
  end
end

MyClass.info

C

MyClass = Class.new do
  class << self
    def info
      'Dynamic class'
    end
  end
end

D

MyClass = Class.new do
  def info()
    'Dynamic class'
  end
end

MyClass.info

💡 Hint

Class methods can be defined inside 'class << self' block or with 'def self.method'.

Options A and C correctly define a class method (A using `def self.info`, C using `class << self`). Options B and D define an instance method but call it on the class, causing NoMethodError.

🚀 Application

expert

How many methods does this dynamically created class have?

Given this Ruby code, how many methods does the class MyClass respond to (excluding inherited methods)?

Ruby

MyClass = Class.new do
  def a
    1
  end

  def b
    2
  end

  def c
    3
  end
end

obj = MyClass.new
methods = obj.methods - Object.new.methods
puts methods.size

A 1

B 3

C 6

D 0

💡 Hint

Subtracting Object's methods from obj's methods leaves only methods defined in MyClass.

The class defines three instance methods: a, b, and c. Subtracting inherited methods leaves these three.

Practice

1. What does Class.new do in Ruby?

easy

A. Defines a method inside a class

B. Deletes an existing class

C. Creates a new instance of a class

D. Creates a new class dynamically at runtime

Solution

1. Step 1: Understand the purpose of Class.new

   Class.new is used to create a new class dynamically while the program runs, without writing a full class definition.

2. Step 2: Differentiate from other options

   It does not delete classes, create instances, or define methods alone. It creates a new class object.

3. Final Answer:

   Creates a new class dynamically at runtime -> Option D

4. Quick Check:

   Class.new = dynamic class creation [OK]

Hint: Class.new always creates a new class object [OK]

Common Mistakes:

• Confusing class creation with instance creation
• Thinking it deletes or modifies existing classes
• Assuming it only defines methods

2. Which of the following is the correct syntax to create a new class with Class.new and add a method greet that returns "Hello"?

easy

A. MyClass = Class.new do def greet return "Hello" end end

B. MyClass = Class.new { greet() = "Hello" }

C. MyClass = Class.new { def greet; "Hello"; end }

D. MyClass = Class.new { def greet() puts "Hello" }

Solution

1. Step 1: Check correct block syntax for Class.new

   The block can be given with curly braces or do-end. Inside, method definitions use def method_name; ...; end.

2. Step 2: Validate method definition and return value

   MyClass = Class.new { def greet; "Hello"; end } correctly defines greet returning "Hello". MyClass = Class.new do def greet return "Hello" end end is correct syntax but missing the 'end' for the block in MyClass = Class.new do def greet return "Hello" end end, so MyClass = Class.new do def greet return "Hello" end end is invalid as given. MyClass = Class.new { greet() = "Hello" } uses invalid method syntax. MyClass = Class.new { def greet() puts "Hello" } uses puts which prints but does not return the string.

3. Final Answer:

   MyClass = Class.new { def greet; "Hello"; end } -> Option C

4. Quick Check:

   Correct method syntax inside Class.new block = MyClass = Class.new { def greet; "Hello"; end } [OK]

Hint: Use def inside Class.new block with proper syntax [OK]

Common Mistakes:

• Using puts instead of returning a string
• Incorrect block syntax for Class.new
• Invalid method definition syntax

3. What will be the output of this Ruby code?

MyClass = Class.new do
  def greet
    "Hi"
  end
end

obj = MyClass.new
puts obj.greet

medium

A. Hi

B. Hello

C. Error: undefined method greet

D. nil

Solution

1. Step 1: Understand class creation and method definition

   The code creates a new class with a method greet returning "Hi".

2. Step 2: Create instance and call method

   Creating an instance obj and calling obj.greet returns "Hi" which is printed.

3. Final Answer:

   Hi -> Option A

4. Quick Check:

   Method greet returns "Hi" [OK]

Hint: Method returns string; puts prints it [OK]

Common Mistakes:

• Expecting 'Hello' instead of 'Hi'
• Thinking method is undefined
• Confusing return value with nil

4. Identify the error in this code snippet:

MyClass = Class.new {
  def greet
    "Hello"
  end
end

obj = MyClass.new
puts obj.greet

medium

A. Using curly braces instead of do-end for multi-line block

B. Missing parentheses in method definition

C. Cannot call new on Class.new result

D. Method greet should use puts instead of return

Solution

1. Step 1: Check block syntax for Class.new

   Curly braces are for single-line blocks. Multi-line blocks should use do-end syntax.

2. Step 2: Confirm other parts are correct

   Method definition syntax is valid, calling new is valid, and returning string is correct.

3. Final Answer:

   Using curly braces instead of do-end for multi-line block -> Option A

4. Quick Check:

   Multi-line block needs do-end [OK]

Hint: Use do-end for multi-line Class.new blocks [OK]

Common Mistakes:

• Using curly braces for multi-line blocks
• Thinking method needs parentheses
• Confusing method return with puts

5. You want to create a dynamic class with Class.new that has an initializer accepting a name and a method greet returning "Hello, <name>". Which code correctly implements this?

hard

A. Person = Class.new {
def initialize(name)
@name = name
end
def greet
"Hello, " + name
end
}

B. Person = Class.new do
def initialize(name)
@name = name
end
def greet
"Hello, #{@name}"
end
end

C. Person = Class.new do
def initialize(name)
name = @name
end
def greet
"Hello, " + @name
end
end

D. Person = Class.new do
def initialize(name)
@name = name
end
def greet
puts "Hello, #{@name}"
end
end

Solution

1. Step 1: Check initializer assigns instance variable

   Person = Class.new do def initialize(name) @name = name end def greet "Hello, #{@name}" end end correctly assigns @name = name in initialize. Person = Class.new do def initialize(name) name = @name end def greet "Hello, " + @name end end reverses assignment incorrectly. Person = Class.new { def initialize(name) @name = name end def greet "Hello, " + name end } uses wrong block syntax. Person = Class.new do def initialize(name) @name = name end def greet puts "Hello, #{@name}" end end uses puts in greet which prints but does not return the string.

2. Step 2: Check greet method returns correct string

   Person = Class.new do def initialize(name) @name = name end def greet "Hello, #{@name}" end end returns the string with interpolation. Person = Class.new { def initialize(name) @name = name end def greet "Hello, " + name end } uses undefined variable name. Person = Class.new do def initialize(name) @name = name end def greet puts "Hello, #{@name}" end end prints instead of returning.

3. Final Answer:

   Person = Class.new do def initialize(name) @name = name end def greet "Hello, #{@name}" end end -> Option B

4. Quick Check:

   Correct initializer and return string in greet [OK]

Hint: Assign @name in initialize; return string in greet [OK]

Common Mistakes:

• Reversing assignment in initialize
• Using puts instead of returning string
• Wrong block syntax with curly braces