Instance methods in Ruby - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an instance method in Ruby?

An instance method is a function defined inside a class that can be called on objects (instances) created from that class. It operates on the data contained in the specific object.

beginner

How do you define an instance method in a Ruby class?

You define an instance method inside a class using the <code>def</code> keyword followed by the method name. For example:<br><pre>class Dog
  def bark
    puts "Woof!"
  end
end</pre>

beginner

How do you call an instance method in Ruby?

First, create an object (instance) of the class, then call the method on that object using dot notation. For example:

dog = Dog.new
dog.bark  # Calls the bark method

intermediate

What is the role of self inside an instance method?

Inside an instance method, self refers to the current object (instance) on which the method was called. It helps access or modify the object's data.

intermediate

Can instance methods access instance variables? How?

Yes. Instance methods can read or change instance variables using the @ symbol. For example:

def name
  @name
end

accesses the @name variable of the object.

How do you define an instance method in Ruby?

A Using <code>class method_name</code>

B Using <code>def self.method_name</code> inside a class

C Using <code>function method_name()</code>

D Using <code>def method_name</code> inside a class

Instance methods are defined with def method_name inside a class. Option B defines a class method.

What does self refer to inside an instance method?

A The current object (instance)

B A global variable

C The class itself

D The method name

self inside an instance method refers to the object that called the method.

How do you call an instance method named greet on an object person?

A <code>person.greet</code>

B <code>call greet on person</code>

C <code>Person.greet</code>

D <code>greet.person</code>

You call instance methods on objects using dot notation: person.greet.

Can instance methods access instance variables?

A No, only class methods can

B Only if they are global variables

C Yes, using the <code>@</code> symbol

D Only if passed as arguments

Instance methods can access instance variables using the @ symbol, like @name.

Which of these is an instance method definition?

A def self.run

B def run

C def Class.run

D def @run

Instance methods are defined with def method_name. Option A defines a class method.

Explain what an instance method is and how it is used in Ruby.

Think about how methods belong to objects.

Describe how self works inside an instance method and why it is useful.

Consider what 'self' means when you call a method on an object.

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