Instance methods in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of calling an instance method

What is the output of this Ruby code when calling the greet method on an instance of the Person class?

Ruby

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

  def greet
    "Hello, #{@name}!"
  end
end

person = Person.new("Alice")
puts person.greet

A Hello, Alice!

B Hello, @name!

C person.greet

D Error: undefined method greet

💡 Hint

Instance methods can access instance variables using @variable_name.

The greet method returns a string that includes the instance variable @name. Since person was initialized with "Alice", the output is "Hello, Alice!".

❓ Predict Output

intermediate

Instance method modifying state

What will be the output after running this Ruby code?

Ruby

class Counter
  def initialize
    @count = 0
  end

  def increment
    @count += 1
  end

  def current
    @count
  end
end

counter = Counter.new
counter.increment
counter.increment
puts counter.current

A 2

B 0

C 1

D Error: undefined method current

💡 Hint

Each call to increment adds 1 to @count.

The increment method adds 1 to the instance variable @count. After calling it twice, @count is 2, so current returns 2.

🔧 Debug

advanced

Why does this instance method raise an error?

This Ruby code raises an error when calling display. What is the cause?

Ruby

class Book
  def initialize(title)
    title = title
  end

  def display
    puts "Title: #{title}"
  end
end

book = Book.new("Ruby Guide")
book.display

A Missing return statement in initialize method

B Syntax error in initialize method

C Undefined local variable or method `title` in display method

D Cannot call puts inside instance method

💡 Hint

Instance variables start with @ and are needed to keep data between methods.

Inside initialize, title = title assigns the parameter to a local variable, not an instance variable. The display method tries to use title which is undefined in that scope, causing an error.

📝 Syntax

advanced

Identify the syntax error in this instance method

Which option correctly identifies the syntax error in this Ruby code?

Ruby

class Calculator
  def add(a, b)
    return a + b
  end

  def multiply(a, b)
    a * b
  # missing end here
end

calc = Calculator.new
puts calc.add(2, 3)

A Missing parentheses in method call add(2, 3)

B Missing <code>end</code> keyword for multiply method

C Incorrect use of return in add method

D Cannot create instance of Calculator without arguments

💡 Hint

Every method definition must be closed with end.

The multiply method is missing its closing end, causing a syntax error.

🚀 Application

expert

How many times is the instance method called?

Consider this Ruby code. How many times is the log instance method called during execution?

Ruby

class Logger
  def initialize
    @count = 0
  end

  def log(message)
    @count += 1
    puts "Log ##{@count}: #{message}"
  end

  def repeat_log(times, message)
    times.times { log(message) }
  end
end

logger = Logger.new
logger.log("Start")
logger.repeat_log(3, "Repeat")
logger.log("End")

A 6

B 4

C 3

D 5

💡 Hint

Count each explicit call to log, including inside loops.

The log method is called once before repeat_log, then 3 times inside repeat_log, and once more after. Total calls: 1 + 3 + 1 = 5.

Practice

1.

What is an instance method in Ruby?

easy

A. A method that belongs to an object created from a class

B. A method that belongs to the class itself, not objects

C. A method that runs only once when the program starts

D. A method that cannot be called on any object

Solution

1. Step 1: Understand what instance methods are

   Instance methods are defined inside a class and belong to objects created from that class.

2. Step 2: Differentiate from class methods

   Class methods belong to the class itself, not to individual objects.

3. Final Answer:

   A method that belongs to an object created from a class -> Option A

4. Quick Check:

   Instance method = belongs to object [OK]

Hint: Instance methods belong to objects, not the class itself [OK]

Common Mistakes:

• Confusing instance methods with class methods
• Thinking instance methods run only once
• Believing instance methods cannot be called on objects

2.

Which of the following is the correct way to define an instance method greet inside a Ruby class Person?

class Person
  ?
end

easy

A. def self.greet; puts 'Hello'; end

B. def greet; puts 'Hello'; end

C. def Person.greet; puts 'Hello'; end

D. def greet() puts 'Hello' end

Solution

1. Step 1: Identify instance method syntax

   Instance methods are defined with def method_name inside the class without self..

2. Step 2: Check options

   def greet; puts 'Hello'; end uses correct syntax. def self.greet; puts 'Hello'; end and def Person.greet; puts 'Hello'; end define class methods. def greet() puts 'Hello' end misses semicolons or line breaks, causing syntax issues.

3. Final Answer:

   def greet; puts 'Hello'; end -> Option B

4. Quick Check:

   Instance method = def method_name [OK]

Hint: Instance methods use def without self. [OK]

Common Mistakes:

• Using self. which defines class methods
• Omitting semicolons or line breaks incorrectly
• Defining methods outside the class

3.

What will be the output of the following Ruby code?

class Dog
  def bark
    'Woof!'
  end
end

fido = Dog.new
puts fido.bark

medium

A. Dog

B. bark

C. Woof!

D. NoMethodError

Solution

1. Step 1: Understand the class and method

   The class Dog has an instance method bark that returns the string 'Woof!'.

2. Step 2: Trace the method call

   An object fido is created from Dog, then fido.bark calls the bark method, returning 'Woof!'.

3. Final Answer:

   Woof! -> Option C

4. Quick Check:

   Calling instance method returns 'Woof!' [OK]

Hint: Instance method returns its string when called on object [OK]

Common Mistakes:

• Expecting method name instead of return value
• Confusing class name with output
• Thinking method is undefined

4.

Find the error in this Ruby code and select the fix that will make it work:

class Cat
  def meow
    puts 'Meow!'
  end
end

kitty = Cat.new
kitty.meow()

medium

A. Remove parentheses from kitty.meow() to kitty.meow

B. Change def meow to def self.meow

C. Add return before puts in method

D. No error; code works as is

Solution

1. Step 1: Check method definition and call

   The method meow is defined correctly as an instance method and called on an object.

2. Step 2: Verify syntax and usage

   Using parentheses in method call is allowed in Ruby, so kitty.meow() is valid.

3. Final Answer:

   No error; code works as is -> Option D

4. Quick Check:

   Parentheses optional in method calls [OK]

Hint: Parentheses optional in Ruby method calls [OK]

Common Mistakes:

• Thinking parentheses cause errors
• Confusing instance and class methods
• Adding unnecessary return before puts

5.

Given the class below, what will puts user.full_name print?

class User
  def initialize(first, last)
    @first_name = first
    @last_name = last
  end

  def full_name
    "#{@first_name} #{@last_name}".upcase
  end
end

user = User.new('Jane', 'Doe')

hard

A. JANE DOE

B. Error: undefined method full_name

C. "Jane Doe"

D. Jane Doe

Solution

1. Step 1: Understand instance variables and method

   The initialize method sets @first_name and @last_name. The full_name method returns their combination in uppercase.

2. Step 2: Trace the method call

   Calling user.full_name returns "JANE DOE" because of the upcase method.

3. Final Answer:

   JANE DOE -> Option A

4. Quick Check:

   full_name returns uppercase full name [OK]

Hint: String interpolation with upcase changes output to uppercase [OK]

Common Mistakes:

• Ignoring upcase method effect
• Expecting original case output
• Confusing string quotes in output