Class declaration syntax in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple Ruby class method call

What is the output of this Ruby code?

Ruby

class Greeter
  def greet
    "Hello, friend!"
  end
end

g = Greeter.new
puts g.greet

A Error: undefined method

B greet

C Greeter

D Hello, friend!

💡 Hint

Look at what the greet method returns and what puts does.

The greet method returns the string "Hello, friend!" and puts prints it to the screen.

❓ Predict Output

intermediate

Output when accessing an instance variable

What will this Ruby code print?

Ruby

class Person
  def initialize(name)
    @name = name
  end

  def name
    @name
  end
end

p = Person.new("Alice")
puts p.name

A Error: undefined variable

B Person

C Alice

D @name

💡 Hint

The initialize method sets @name, and the name method returns it.

The instance variable @name is set to "Alice" and returned by the name method, so puts prints "Alice".

🔧 Debug

advanced

Identify the error in this class declaration

What error does this Ruby code produce?

Ruby

class Animal
  def speak
    puts "Roar"
  end

  def speak
    puts "Meow"
  end
end

cat = Animal.new
cat.speak

A Output: Meow

B NoMethodError: undefined method 'speak' for Animal

C RuntimeError: method redefinition error

D SyntaxError: unexpected end-of-input, expecting keyword_end

💡 Hint

Check if all methods and classes are properly closed with 'end'.

The second speak method is missing an 'end', causing a syntax error due to incomplete method definition.

❓ Predict Output

advanced

Output of class variable usage

What is the output of this Ruby code?

Ruby

class Counter
  @@count = 0

  def initialize
    @@count += 1
  end

  def self.count
    @@count
  end
end

c1 = Counter.new
c2 = Counter.new
puts Counter.count

A 2

B 0

C 1

D Error: undefined class variable

💡 Hint

Class variables are shared among all instances.

Each new instance increments @@count by 1. After two instances, @@count is 2.

🧠 Conceptual

expert

Understanding inheritance and method overriding

Given these Ruby classes, what will be printed when calling d.speak?

Ruby

class Dog
  def speak
    "Woof"
  end
end

class LoudDog < Dog
  def speak
    super.upcase
  end
end

d = LoudDog.new
puts d.speak

A Woof

B WOOF

C super

D Error: undefined method 'upcase' for nil

💡 Hint

The 'super' keyword calls the parent method, then upcase changes the string.

LoudDog's speak calls Dog's speak with super, which returns "Woof", then upcase converts it to "WOOF".

Practice

1. Which of the following is the correct way to start a class declaration in Ruby?

easy

A. class MyClass

B. Class myClass

C. def MyClass

D. module MyClass

Solution

1. Step 1: Recognize Ruby class syntax

   In Ruby, classes start with the keyword class followed by a capitalized name.

2. Step 2: Check each option

   class MyClass uses class and a capitalized name, which is correct. Options A, B, and D use wrong keywords or lowercase names.

3. Final Answer:

   class MyClass -> Option A

4. Quick Check:

   Class declaration starts with 'class' and capitalized name [OK]

Hint: Class names start with capital letter and use 'class' keyword [OK]

Common Mistakes:

• Using lowercase class names
• Using 'def' instead of 'class'
• Confusing 'module' with 'class'

2. Which of the following is the correct way to end a class declaration in Ruby?

easy

A. end

B. finish

C. stop

D. endclass

Solution

1. Step 1: Recall Ruby block endings

   Ruby uses the keyword end to close blocks like classes, methods, and conditionals.

2. Step 2: Identify correct ending keyword

   Only end uses end. The others are not valid Ruby keywords.

3. Final Answer:

   end -> Option A

4. Quick Check:

   Ruby blocks end with 'end' keyword [OK]

Hint: All Ruby blocks end with 'end' keyword [OK]

Common Mistakes:

• Using made-up keywords like 'stop' or 'finish'
• Writing 'endclass' instead of 'end'
• Omitting the 'end' keyword

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

class Dog
  def bark
    "Woof!"
  end
end

fido = Dog.new
puts fido.bark

medium

A. bark

B. Woof!

C. Dog

D. Error: undefined method

Solution

1. Step 1: Understand class and method

   The class Dog has a method bark that returns the string "Woof!".

2. Step 2: Create object and call method

   We create an object fido from Dog using new. Calling fido.bark returns "Woof!" which is printed.

3. Final Answer:

   Woof! -> Option B

4. Quick Check:

   Calling method returns its string output [OK]

Hint: Use .new to create object, call method to get output [OK]

Common Mistakes:

• Forgetting to use .new to create object
• Expecting method name instead of return value
• Confusing class name with method output

4. Identify the error in this Ruby class declaration:

class cat
  def meow
    "Meow!"
  end
end

medium

A. Missing 'end' keyword

B. Cannot use double quotes for strings

C. Method 'meow' should be outside the class

D. Class name should start with a capital letter

Solution

1. Step 1: Check class naming rules

   Ruby class names must start with a capital letter. Here, cat starts with lowercase.

2. Step 2: Verify other syntax

   The class has end keyword, method is inside class, and double quotes are valid for strings.

3. Final Answer:

   Class name should start with a capital letter -> Option D

4. Quick Check:

   Class names must be capitalized [OK]

Hint: Class names always start with uppercase letter [OK]

Common Mistakes:

• Using lowercase class names
• Thinking 'end' is missing
• Misplacing methods outside class

5. You want to create a class Car with a method start that returns "Engine started". Which code correctly declares this class and method?

hard

A. Car class
def start
"Engine started"
end
end

B. class car
def start
return Engine started
end
end

C. class Car
def start
"Engine started"
end
end

D. class Car
def start()
puts "Engine started"
end
end

Solution

1. Step 1: Check class declaration and naming

   class Car
     def start
       "Engine started"
     end
   end

   correctly uses class Car with capitalized name and ends with end.

2. Step 2: Check method definition and return value

   class Car
     def start
       "Engine started"
     end
   end

   defines method start returning the string "Engine started".

   class Car
     def start()
       puts "Engine started"
     end
   end

   uses puts which prints but returns nil, so not exactly the same.

3. Final Answer:

   class Car def start "Engine started" end end -> Option C

4. Quick Check:

   Class with capital name and method returning string [OK]

Hint: Class name capitalized; method returns string without puts [OK]

Common Mistakes:

• Using lowercase class names
• Using puts instead of return string
• Incorrect class declaration syntax