Attr_reader, attr_writer, attr_accessor in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Ruby code using attr_reader?

Consider this Ruby class with attr_reader. What will be printed?

Ruby

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

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

A

Alice
Bob

B NoMethodError: undefined method `name' for #<Person:...>

C

Alice
NoMethodError: undefined method `name=' for #<Person:...>

D

nil
NoMethodError: undefined method `name=' for #<Person:...>

💡 Hint

Remember, attr_reader creates a getter method but no setter.

attr_reader creates a method to read the instance variable but does not allow writing. So p.name works and prints "Alice". But p.name = "Bob" raises a NoMethodError because the setter method is missing.

❓ Predict Output

intermediate

What happens when using attr_writer only?

Look at this Ruby code using attr_writer. What will be the output or error?

Ruby

class Car
  attr_writer :color
  def initialize(color)
    @color = color
  end
end

car = Car.new("red")
puts car.color
car.color = "blue"
puts car.instance_variable_get(:@color)

A NoMethodError: undefined method `color' for #<Car:...>

B NoMethodError: undefined method `color=' for #<Car:...>

C

nil
blue

D

red
blue

💡 Hint

attr_writer creates a setter but no getter method.

attr_writer creates a setter method color= but no getter color. So car.color raises NoMethodError. The setter car.color = "blue" works and changes the instance variable. Using instance_variable_get shows the updated value.

❓ Predict Output

advanced

What is the output when using attr_accessor?

Given this Ruby class with attr_accessor, what will be printed?

Ruby

class Book
  attr_accessor :title
  def initialize(title)
    @title = title
  end
end

book = Book.new("Ruby 101")
puts book.title
book.title = "Ruby 102"
puts book.title

A

Ruby 101
Ruby 102

B

Ruby 101
Ruby 101

C NoMethodError: undefined method `title=' for #<Book:...>

D NoMethodError: undefined method `title' for #<Book:...>

💡 Hint

attr_accessor creates both getter and setter methods.

attr_accessor creates both title and title= methods. So you can read and write the @title instance variable. The output shows the original title and the updated title.

🧠 Conceptual

advanced

Which statement about attr_reader, attr_writer, and attr_accessor is true?

Choose the correct statement about these Ruby attribute methods.

A <code>attr_reader</code> creates both getter and setter methods.

B <code>attr_accessor</code> creates getter and setter methods.

C <code>attr_writer</code> creates only a getter method.

D <code>attr_accessor</code> creates only a setter method.

💡 Hint

Think about which methods allow reading and writing.

attr_reader creates only getter methods. attr_writer creates only setter methods. attr_accessor creates both getter and setter methods.

🔧 Debug

expert

Why does this Ruby code raise NoMethodError when using attr_writer?

Look at this code snippet. Why does puts user.name raise NoMethodError?

Ruby

class User
  attr_writer :name
  def initialize(name)
    @name = name
  end
end

user = User.new("Eve")
puts user.name

A Because <code>attr_writer</code> creates only a getter method, no setter method exists.

B Because <code>attr_writer</code> creates both getter and setter methods, but the getter is private.

C Because the instance variable <code>@name</code> is not initialized.

D Because <code>attr_writer</code> only creates a setter method, no getter method exists.

💡 Hint

Check what methods attr_writer creates.

attr_writer creates only the setter method name=. It does not create a getter method name. So calling user.name raises NoMethodError.

Practice

1. What does attr_reader do in a Ruby class?

easy

A. Creates a method to read an attribute value

B. Creates a method to write an attribute value

C. Creates methods to both read and write an attribute

D. Deletes an attribute from the class

Solution

1. Step 1: Understand attr_reader purpose

   attr_reader defines a getter method for an instance variable, allowing you to read its value.

2. Step 2: Compare with other attr methods

   attr_writer creates a setter (write) method, and attr_accessor creates both getter and setter methods.

3. Final Answer:

   Creates a method to read an attribute value -> Option A

4. Quick Check:

   attr_reader = read method [OK]

Hint: attr_reader = read only method [OK]

Common Mistakes:

• Confusing attr_reader with attr_writer
• Thinking attr_reader allows writing values
• Mixing attr_accessor with attr_reader

2. Which of the following is the correct syntax to create both getter and setter methods for @name in Ruby?

easy

A. attr_writer :name

B. attr_accessor :name

C. attr_reader :name

D. attr_both :name

Solution

1. Step 1: Recall attr_accessor usage

   attr_accessor creates both getter and setter methods for the given symbol.

2. Step 2: Check other options

   attr_reader only creates getter, attr_writer only setter, and attr_both is invalid syntax.

3. Final Answer:

   attr_accessor :name -> Option B

4. Quick Check:

   attr_accessor = read + write [OK]

Hint: Use attr_accessor for read and write [OK]

Common Mistakes:

• Using attr_reader when write is needed
• Using invalid method names like attr_both
• Confusing attr_writer with attr_accessor

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

class Person
  attr_writer :age
  def initialize(age)
    @age = age
  end
  def show_age
    @age
  end
end

p = Person.new(30)
puts p.show_age
p.age = 40
puts p.show_age

medium

A. 30\n40

B. 30\n30

C. Error: undefined method `age='

D. Error: undefined method `show_age'

Solution

1. Step 1: Understand attr_writer effect

   attr_writer :age creates a setter method age=, but no getter method is created.

2. Step 2: Analyze method calls

   p.show_age returns @age which is 30 initially. Then p.age = 40 sets @age to 40. But p.show_age returns @age which is now 40.

3. Step 3: Check output

   Actually, p.show_age is called twice, before and after setting age. So outputs are 30 then 40.

4. Final Answer:

   30\n40 -> Option A

5. Quick Check:

   attr_writer allows setting, show_age reads @age [OK]

Hint: attr_writer only sets; custom getter needed [OK]

Common Mistakes:

• Assuming attr_writer creates getter method
• Expecting error on p.age= assignment
• Confusing show_age method with attr_reader

4. Identify the error in this Ruby class:

class Car
  attr_reader :model
  def initialize(model)
    @model = model
  end
  def model=(value)
    @model = value
  end
end

car = Car.new('Sedan')
car.model = 'SUV'
puts car.model

medium

A. Error: attr_reader conflicts with manual setter method

B. Error: undefined method 'model='

C. Error: attr_reader does not create setter method

D. No error, code runs and prints 'SUV'

Solution

1. Step 1: Check attr_reader and setter method

   attr_reader :model creates a getter method. The setter method model= is manually defined.

2. Step 2: Analyze method calls

   car.model = 'SUV' calls the manual setter, updating @model. puts car.model calls the getter from attr_reader, printing 'SUV'.

3. Final Answer:

   No error, code runs and prints 'SUV' -> Option D

4. Quick Check:

   attr_reader + manual setter works [OK]

Hint: attr_reader + manual setter is valid [OK]

Common Mistakes:

• Assuming attr_reader creates setter method
• Thinking attr_reader conflicts with manual setter
• Expecting error on model= assignment

5. You want a Ruby class Book where the title can be read but not changed, and the author can be both read and changed. Which attribute methods should you use?

hard

A. attr_writer :title
attr_writer :author

B. attr_writer :title
attr_reader :author

C. attr_reader :title
attr_accessor :author

D. attr_accessor :title
attr_writer :author

Solution

1. Step 1: Determine access for title

   Title should be readable but not writable, so use attr_reader :title.

2. Step 2: Determine access for author

   Author should be readable and writable, so use attr_accessor :author.

3. Final Answer:

   attr_reader :title attr_accessor :author -> Option C

4. Quick Check:

   Read-only + read-write = attr_reader + attr_accessor [OK]

Hint: Use attr_reader for read-only, attr_accessor for read-write [OK]

Common Mistakes:

• Using attr_writer for read-only attribute
• Using attr_accessor for read-only attribute
• Mixing up read and write access