Instance variables (@) in Ruby - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an instance variable in Ruby?

An instance variable in Ruby is a variable that belongs to a specific object (instance) of a class. It starts with the @ symbol and stores data unique to that object.

beginner

How do you declare an instance variable inside a Ruby class?

You declare an instance variable by prefixing the variable name with @ inside any method of the class, usually inside the initialize method.

beginner

Why do instance variables start with @ in Ruby?

The @ symbol tells Ruby that the variable is tied to a specific object instance, not a local or global variable. It helps Ruby keep track of data for each object separately.

intermediate

Can instance variables be accessed directly outside the class?

No, instance variables are private to the object. To access them outside the class, you need to use methods like getters or attr_reader.

beginner

Example: What does this code print?
<pre>class Dog
  def initialize(name)
    @name = name
  end
  def speak
    "Woof! My name is #{@name}"
  end
end

dog = Dog.new("Buddy")
puts dog.speak</pre>

It prints: Woof! My name is Buddy The @name instance variable stores the dog's name and is used inside the speak method.

What symbol is used to start an instance variable in Ruby?

A @

B $

C #

D &

Instance variables in Ruby always start with the @ symbol.

Where are instance variables typically declared in a Ruby class?

A Outside the class

B Inside the initialize method

C Inside a global method

D In a local variable

Instance variables are usually set inside the initialize method to store object-specific data.

Can you access an instance variable directly from outside the object?

A No, you need a method to access it

B Yes, always

C Only if it starts with $

D Only inside a module

Instance variables are private to the object; you must use methods like getters to access them.

What does @name represent inside a Ruby class?

A A constant

B A local variable

C A global variable

D An instance variable

The @ symbol means the variable is an instance variable tied to the object.

If you create two objects from the same class, do they share the same instance variables?

A Only if they are global

B Yes, instance variables are shared

C No, each object has its own instance variables

D Only if declared outside methods

Each object has its own copy of instance variables, so data is unique per object.

Explain what an instance variable is in Ruby and why it starts with @.

Think about how Ruby keeps data unique for each object.

Describe how you would use instance variables to store and access a person's name in a Ruby class.

Consider how to keep the name inside the object and show it outside.

Practice

1. What does an instance variable in Ruby always start with?

easy

A. An at symbol (@)

B. A dollar sign ($)

C. A hash symbol (#)

D. A percent sign (%)

Solution

1. Step 1: Recall Ruby instance variable syntax

   Instance variables in Ruby always begin with the at symbol (@) to distinguish them from local variables.

2. Step 2: Identify the correct symbol

   Among the options, only the at symbol (@) is used to start instance variables.

3. Final Answer:

   An at symbol (@) -> Option A

4. Quick Check:

   Instance variable starts with @ [OK]

Hint: Instance variables always start with @ in Ruby [OK]

Common Mistakes:

• Confusing instance variables with global variables ($)
• Using local variable syntax without @
• Thinking instance variables start with # or %

2. Which of the following is the correct way to define an instance variable inside a Ruby class?

easy

A. var @name = 'Alice'

B. @name = 'Alice'

C. name = @'Alice'

D. def @name = 'Alice'

Solution

1. Step 1: Understand instance variable assignment syntax

   In Ruby, instance variables are assigned by writing @variable_name = value inside methods or constructors.

2. Step 2: Check each option

   @name = 'Alice' correctly assigns an instance variable. The other options use invalid syntax.

3. Final Answer:

   @name = 'Alice' -> Option B

4. Quick Check:

   Assign instance variable with @name = value [OK]

Hint: Assign instance variables with @name = value inside methods [OK]

Common Mistakes:

• Adding var keyword (not used in Ruby)
• Placing @ inside quotes
• Using def keyword incorrectly

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

class Person
  def initialize(name)
    @name = name
  end
  def greet
    "Hello, #{@name}!"
  end
end

p = Person.new('Bob')
puts p.greet

medium

A. Error: undefined variable

B. Hello, @name!

C. Hello, name!

D. Hello, Bob!

Solution

1. Step 1: Understand instance variable assignment in initialize

   The initialize method sets @name to 'Bob' when Person.new('Bob') is called.

2. Step 2: Evaluate greet method output

   The greet method returns a string with #{@name} replaced by the instance variable value 'Bob'.

3. Final Answer:

   Hello, Bob! -> Option D

4. Quick Check:

   Instance variable @name holds 'Bob' so output is Hello, Bob! [OK]

Hint: Instance variables keep data per object, used inside methods with @ [OK]

Common Mistakes:

• Expecting literal '@name' in output
• Confusing local and instance variables
• Forgetting to use @ in string interpolation

4. Identify the error in this Ruby class code:

class Car
  def initialize(model)
    name = model
  end
  def show
    puts @name
  end
end

car = Car.new('Tesla')
car.show

medium

A. Syntax error in initialize method

B. Method show is missing return statement

C. Instance variable @name is not assigned, so nil is printed

D. Cannot call puts inside method

Solution

1. Step 1: Check variable assignment in initialize

   The code assigns local variable name = model, not instance variable @name.

2. Step 2: Understand effect on show method

   Since @name is never set, @name is nil, so puts prints an empty line (nil).

3. Final Answer:

   Instance variable @name is not assigned, so nil is printed -> Option C

4. Quick Check:

   Local variable assigned, not instance variable @name [OK]

Hint: Use @ to assign instance variables, not plain variable names [OK]

Common Mistakes:

• Assigning local variable instead of instance variable
• Expecting puts to fail without return
• Thinking syntax error occurs here

5. You want to create a Ruby class Book that stores a title and author as instance variables. You also want a method description that returns "Title by Author". Which code correctly implements this?

hard

A. class Book
def initialize(title, author)
@title = title
@author = author
end
def description
"#{@title} by #{@author}"
end
end

B. class Book
def initialize(title, author)
title = @title
author = @author
end
def description
"#{title} by #{author}"
end
end

C. class Book
def initialize(title, author)
@title = title
@author = author
end
def description
"title by author"
end
end

D. class Book
def initialize(title, author)
@title = title
@author = author
end
def description
"#{title} by #{author}"
end
end

Solution

1. Step 1: Assign instance variables correctly in initialize

   class Book def initialize(title, author) @title = title @author = author end def description "#{@title} by #{@author}" end end assigns @title and @author from parameters correctly. class Book def initialize(title, author) title = @title author = @author end def description "#{title} by #{author}" end end reverses assignment. The remaining options misuse variables in description.

2. Step 2: Use instance variables in description method

   class Book def initialize(title, author) @title = title @author = author end def description "#{@title} by #{@author}" end end uses #{@title} and #{@author} inside string interpolation, correctly accessing instance variables.

3. Final Answer:

   class Book def initialize(title, author) @title = title @author = author end def description "#{@title} by #{@author}" end end -> Option A

4. Quick Check:

   Assign and use @variables inside methods for correct output [OK]

Hint: Assign and use instance variables with @ inside all methods [OK]

Common Mistakes:

• Assigning variables backwards (param = @var)
• Using local variables instead of instance variables in methods
• Returning literal strings without interpolation