Self keyword behavior in Ruby - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the self keyword represent inside an instance method in Ruby?

Inside an instance method, self refers to the current object (instance) on which the method is called.

beginner

How does <code>self</code> behave inside a class method in Ruby?

<p>Inside a class method, <code>self</code> refers to the class itself, not an instance.</p>

intermediate

What is the value of <code>self</code> at the top level of a Ruby program (outside any class or method)?

At the top level, self refers to the main object, which is an instance of Object.

beginner

Can self be reassigned to another object inside a method?

No, self is a special keyword and cannot be reassigned to a different object inside a method.

intermediate

How does self help when defining setter methods in Ruby?

Using self.attribute = value inside a method calls the setter method instead of creating a local variable.

Inside an instance method, what does self refer to?

A The class itself

B The current object (instance)

C The main object

D A local variable

Inside an instance method, self always refers to the current instance of the class.

What does self refer to inside a class method?

A The current instance

B The main object

C A global variable

D The class itself

In a class method, self points to the class, not an instance.

At the top level of a Ruby program, self is:

A The main object

B An instance of the current class

C Nil

D The class object

At the top level, self is the main object, an instance of Object.

Why do we use self.attribute = value inside a method?

A To create a local variable

B To call a class method

C To call the setter method

D To reassign <code>self</code>

Using self.attribute = value calls the setter method instead of creating a local variable.

Can you assign a new object to self inside a method?

A No, <code>self</code> is special and cannot be reassigned

B Only in class methods

C Only at the top level

D Yes, anytime

self is a special keyword and cannot be reassigned inside methods.

Explain how the self keyword behaves differently inside instance methods, class methods, and at the top level in Ruby.

Think about what object is 'current' in each context.

Describe why using self.attribute = value inside a method is important compared to just writing attribute = value.

Consider how Ruby decides between local variables and method calls.

Practice

1. What does the self keyword represent inside an instance method in Ruby?

easy

A. The superclass of the current class

B. The class itself

C. A global variable

D. The current instance (object) of the class

Solution

1. Step 1: Understand instance methods context

   Inside an instance method, the code runs for a specific object created from the class.

2. Step 2: Identify what self points to

   In this context, self refers to that specific object (instance) running the method.

3. Final Answer:

   The current instance (object) of the class -> Option D

4. Quick Check:

   self in instance method = instance [OK]

Hint: Inside instance methods, think "self" means the object itself [OK]

Common Mistakes:

• Confusing self with the class inside instance methods
• Thinking self is a global or class variable
• Assuming self is always the class

2. Which of the following is the correct way to define a class method using self in Ruby?

easy

A. def self.my_method; 'hello'; end

B. def my_method.self; 'hello'; end

C. def my_method(self); 'hello'; end

D. def self:my_method; 'hello'; end

Solution

1. Step 1: Recall class method syntax

   Class methods are defined by prefixing the method name with self. inside the class.

2. Step 2: Check each option's syntax

   def self.my_method; 'hello'; end uses def self.my_method which is correct syntax. Others are invalid Ruby syntax.

3. Final Answer:

   def self.my_method; 'hello'; end -> Option A

4. Quick Check:

   Class method syntax = def self.method_name [OK]

Hint: Class methods start with def self.method_name [OK]

Common Mistakes:

• Placing self after method name
• Using colon instead of dot
• Passing self as a parameter

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

class Dog
  def speak
    self.class
  end
end

fido = Dog.new
puts fido.speak

medium

A. Dog

B. #

C. fido

D. Object

Solution

1. Step 1: Understand what self is inside speak

   Inside the instance method speak, self is the instance fido.

2. Step 2: Evaluate self.class

   Calling class on the instance returns its class, which is Dog.

3. Final Answer:

   Dog -> Option A

4. Quick Check:

   self.class in instance method = class name [OK]

Hint: self.class inside instance method returns the object's class [OK]

Common Mistakes:

• Thinking self.class returns the instance
• Confusing class with object name
• Expecting output to be an object instead of class name

4. Identify the error in this Ruby code snippet:

class Cat
  def self.meow
    puts "Meow!"
  end

  def meow
    puts "Meow from instance"
  end

  def self.instance_meow
    meow
  end
end

Cat.instance_meow

medium

A. Missing self in instance method definition

B. Calling instance method meow inside a class method without an instance

C. Incorrect syntax for class method definition

D. Using puts inside methods is not allowed

Solution

1. Step 1: Analyze method calls inside self.instance_meow

   This is a class method trying to call meow without specifying an instance.

2. Step 2: Understand method types

   meow without self. is an instance method, so it needs an object to call it on.

3. Step 3: Identify the error

   Calling an instance method directly inside a class method causes a NoMethodError because no instance is given.

4. Final Answer:

   Calling instance method meow inside a class method without an instance -> Option B

5. Quick Check:

   Class methods cannot call instance methods without an object [OK]

Hint: Class methods need an instance to call instance methods [OK]

Common Mistakes:

• Trying to call instance methods directly inside class methods
• Confusing self in class vs instance methods
• Ignoring need for object to call instance methods

5. Given the following Ruby code, what will be the output?

class Calculator
  def self.description
    "I am a Calculator"
  end

  def initialize(value)
    @value = value
  end

  def show_description
    self.class.description
  end
end

calc = Calculator.new(10)
puts calc.show_description

hard

A. NoMethodError

B. "Calculator"

C. "I am a Calculator"

D. "10"

Solution

1. Step 1: Understand self.class.description inside instance method

   Inside show_description, self is the instance calc. Calling class returns the class Calculator.

2. Step 2: Call class method description

   Calling description on the class returns the string "I am a Calculator".

3. Final Answer:

   "I am a Calculator" -> Option C

4. Quick Check:

   Instance calls class method via self.class = output string [OK]

Hint: Use self.class to call class methods from instances [OK]

Common Mistakes:

• Expecting instance variable value instead of class method output
• Confusing self with class inside instance methods
• Thinking NoMethodError will occur