Instance_variable_get and set in Ruby - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does instance_variable_get do in Ruby?

It reads the value of an instance variable from an object, even if the variable is private or not accessible by normal methods.

beginner

How do you use instance_variable_set in Ruby?

You call it on an object with the variable name (as a symbol or string starting with '@') and the new value to assign to that instance variable.

intermediate

Why might you use instance_variable_get and instance_variable_set instead of normal getters and setters?

They let you access or change instance variables directly, even if no getter or setter methods exist or if the variables are private.

beginner

What is the correct syntax to get the value of an instance variable named @name from an object obj?

obj.instance_variable_get(:@name)

beginner

What happens if you try to get an instance variable that does not exist using instance_variable_get?

It returns nil because the variable is not set on the object.

Which symbol correctly represents an instance variable named 'age' for use with instance_variable_get?

A :@age

B :age

C age

D @age

Instance variable names must start with '@' and be passed as symbols or strings, so :@age is correct.

What will obj.instance_variable_get(:@score) return if @score is not set on obj?

A An error

B 0

C nil

D false

If the instance variable does not exist, instance_variable_get returns nil.

How do you set the instance variable @level to 5 on an object player?

A player.@level = 5

B player.instance_variable_get(:@level) = 5

C player.set_instance_variable(:level, 5)

D player.instance_variable_set(:@level, 5)

Use instance_variable_set with the variable name as a symbol starting with '@' and the value.

Why might using instance_variable_get and instance_variable_set be discouraged in normal code?

A They are slower than normal methods

B They break encapsulation and can make code harder to maintain

C They only work on global variables

D They require special gems

Directly accessing instance variables bypasses encapsulation, which can lead to fragile code.

Which of these is a valid use of instance_variable_get?

A obj.instance_variable_get(:@name)

B obj.instance_variable_get('@name')

C obj.instance_variable_get('name')

D obj.instance_variable_get(:name)

The variable name must start with '@' and be passed as a symbol or string; :@name is correct.

Explain how instance_variable_get and instance_variable_set work in Ruby and when you might use them.

Think about how you can read or change private data inside an object.

Describe the risks or downsides of using instance_variable_get and instance_variable_set in your Ruby programs.

Consider how direct access to internal data affects code design.

Practice

1. What does the Ruby method instance_variable_get do?

easy

A. It reads the value of an instance variable by its name.

B. It deletes an instance variable from an object.

C. It creates a new instance variable with a given name.

D. It lists all instance variables of an object.

Solution

1. Step 1: Understand the method purpose

   instance_variable_get is used to access the value of an instance variable by passing its name as a symbol or string.

2. Step 2: Compare with other options

   It does not delete, create, or list variables; it only reads the value.

3. Final Answer:

   It reads the value of an instance variable by its name. -> Option A

4. Quick Check:

   instance_variable_get reads variable value [OK]

Hint: Remember: get = read value of variable [OK]

Common Mistakes:

• Confusing get with set (changing value)
• Thinking it deletes variables
• Assuming it lists all variables

2. Which of the following is the correct syntax to set an instance variable @name to "Alice" using instance_variable_set?

easy

A. object.instance_variable_set(:nam, "Alice")

B. object.instance_variable_set("nam", "Alice")

C. object.instance_variable_set(@name, "Alice")

D. object.instance_variable_set(:@name, "Alice")

Solution

1. Step 1: Understand the argument format

   The first argument to instance_variable_set must be the instance variable name as a symbol or string, including the '@' prefix.

2. Step 2: Check each option

   A: object.instance_variable_set(:nam, "Alice") misses '@'. B: object.instance_variable_set("nam", "Alice") misses '@'. C: passes a variable instead of symbol/string. D: uses :@name symbol correctly.

3. Final Answer:

   object.instance_variable_set(:@name, "Alice") -> Option D

4. Quick Check:

   Use symbol with '@' prefix for setting [OK]

Hint: Use symbol with '@' prefix to set variable [OK]

Common Mistakes:

• Omitting '@' in variable name
• Passing variable instead of symbol/string
• Using string without '@' prefix

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

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

p = Person.new("Bob")
puts p.instance_variable_get(:@name)

medium

A. Bob

B. nil

C. @name

D. Error

Solution

1. Step 1: Understand instance variable initialization

   The @name variable is set to "Bob" in the constructor.

2. Step 2: Use instance_variable_get to read value

   Calling instance_variable_get(:@name) returns the value "Bob" stored in @name.

3. Final Answer:

   Bob -> Option A

4. Quick Check:

   instance_variable_get returns stored value [OK]

Hint: instance_variable_get returns actual stored value [OK]

Common Mistakes:

• Expecting variable name instead of value
• Confusing with nil if variable not set
• Thinking it raises error if variable exists

4. Identify the issue in this Ruby code snippet:

class Car
  def initialize
    @speed = 0
  end

  def set_speed(value)
    instance_variable_set(:speed, value)
  end
end

car = Car.new
car.set_speed(50)
puts car.instance_variable_get(:@speed)

medium

A. The variable @speed is not initialized.

B. Cannot call instance_variable_set inside a method.

C. Missing '@' in instance_variable_set argument name.

D. instance_variable_get should use string, not symbol.

Solution

1. Step 1: Check instance_variable_set argument

   The argument to instance_variable_set must include '@' prefix to identify the instance variable.

2. Step 2: Identify the error

   Here, :speed is missing '@', so Ruby will not find the variable and will create a new one without '@', causing unexpected behavior.

3. Final Answer:

   Missing '@' in instance_variable_set argument name. -> Option C

4. Quick Check:

   Always include '@' in variable name for instance_variable_set [OK]

Hint: Always prefix variable name with '@' in set/get methods [OK]

Common Mistakes:

• Omitting '@' in variable name
• Thinking instance_variable_set can't be used in methods
• Confusing symbol and string usage

5. Given a class Box with private instance variable @content, how can you change its value to "Books" from outside the class using instance_variable_set?

hard

A. box.instance_variable_set(:conten, "Books")

B. box.instance_variable_set(:@content, "Books")

C. box.@content = "Books"

D. box.set_content("Books")

Solution

1. Step 1: Understand private variable access

   Private instance variables cannot be accessed or changed directly from outside the class using normal assignment.

2. Step 2: Use instance_variable_set with correct name

   Using instance_variable_set with the symbol including '@' allows changing the private variable from outside.

3. Step 3: Check options

   box.instance_variable_set(:@content, "Books") correctly uses :@content. box.instance_variable_set(:conten, "Books") misses '@', box.@content = "Books" is invalid syntax, box.set_content("Books") assumes a setter method which may not exist.

4. Final Answer:

   box.instance_variable_set(:@content, "Books") -> Option B

5. Quick Check:

   Use instance_variable_set with '@' to change private vars [OK]

Hint: Use '@' prefix symbol to set private variables externally [OK]

Common Mistakes:

• Omitting '@' in variable name
• Trying direct assignment to private variable
• Assuming setter method exists