Instance_variable_get and set in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Ruby code using instance_variable_get?

Consider the following Ruby class and code. What will be printed when this code runs?

Ruby

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

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

A nil

B Error: undefined method `instance_variable_get' for Person

C Alice

D @name

💡 Hint

instance_variable_get reads the value of an instance variable by its name symbol.

The instance_variable_get method returns the value of the instance variable named by the symbol. Here, @name holds "Alice", so it prints Alice.

❓ Predict Output

intermediate

What does this code print after using instance_variable_set?

Look at this Ruby code. What will be the output?

Ruby

class Car
  def initialize
    @speed = 0
  end
end

car = Car.new
car.instance_variable_set(:@speed, 100)
puts car.instance_variable_get(:@speed)

A 100

B 0

C nil

D Error: undefined method `instance_variable_set' for Car

💡 Hint

instance_variable_set changes the value of an instance variable.

The instance_variable_set method sets the value of @speed to 100. Then instance_variable_get returns 100, which is printed.

❓ Predict Output

advanced

What error does this code raise?

What error will this Ruby code produce when run?

Ruby

class Box
  def initialize
    @content = "secret"
  end
end

box = Box.new
box.instance_variable_get("content")

A No error, returns nil

B NameError

C ArgumentError

D TypeError

💡 Hint

instance_variable_get expects a symbol or string starting with @.

instance_variable_get requires the variable name to start with '@'. Passing "content" without '@' causes a TypeError.

❓ Predict Output

advanced

What is the output of this code using instance_variable_set dynamically?

What will this Ruby code print?

Ruby

class User
  def initialize
    @data = {}
  end
  def set_var(name, value)
    instance_variable_set("@" + name.to_s, value)
  end
  def get_var(name)
    instance_variable_get("@" + name.to_s)
  end
end

user = User.new
user.set_var(:age, 30)
puts user.get_var(:age)

A age

B 30

C nil

D Error: undefined method `set_var' for User

💡 Hint

instance_variable_set and get can use string names with '@' prefix.

The set_var method sets @age to 30. The get_var method retrieves @age, printing 30.

🧠 Conceptual

expert

How many instance variables does this object have after running the code?

After running this Ruby code, how many instance variables does the object 'obj' have?

Ruby

class Sample
  def initialize
    @a = 1
    @b = 2
  end
end

obj = Sample.new
obj.instance_variable_set(:@c, 3)
obj.instance_variable_set(:@d, 4)
obj.instance_variable_get(:@a)
obj.instance_variable_get(:@b)

A 4

B 2

C 3

D 5

💡 Hint

instance_variable_set adds new instance variables if they don't exist.

Initially, obj has @a and @b. Then @c and @d are added by instance_variable_set. So total 4 instance variables.

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