Instance_variable_get and set in Ruby - Step-by-Step Execution

Concept Flow - Instance_variable_get and set

Create Object

↓

Set Instance Variable

↓

Get Instance Variable

↓

Use or Display Value

↓

End

This flow shows creating an object, setting an instance variable, getting its value, and using it.

Execution Sample

Ruby

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

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

Creates a Person object with a name and gets the value of the @name instance variable.

Execution Table

Step	Action	Code/Method	Instance Variable State	Output
1	Create Person object	Person.new("Alice")	@name = "Alice"	Person object created
2	Initialize @name	initialize("Alice")	@name = "Alice"	None
3	Get @name value	instance_variable_get(:@name)	@name = "Alice"	"Alice"
4	Set @age value	instance_variable_set(:@age, 30)	@name = "Alice", @age = 30	30
5	Get @age value	instance_variable_get(:@age)	@name = "Alice", @age = 30	30
6	Get non-existing @height	instance_variable_get(:@height)	@name = "Alice", @age = 30	nil
7	End	-	@name = "Alice", @age = 30	Execution ends

💡 All instance variables set or retrieved; no more actions.

Variable Tracker

Variable	Start	After Step 2	After Step 4	Final
@name	nil	"Alice"	"Alice"	"Alice"
@age	nil	nil	30	30
@height	nil	nil	nil	nil

Key Moments - 3 Insights

Why does instance_variable_get return nil for @height?

Because @height was never set on the object, instance_variable_get returns nil as shown in step 6 of the execution_table.

Can instance_variable_set create a new instance variable?

Yes, instance_variable_set creates or updates the variable. Step 4 shows @age being created and set to 30.

Does instance_variable_get require the variable name as a symbol or string?

It accepts the variable name as a symbol or string starting with @, like :@name, as used in steps 3 and 5.

Visual Quiz - 3 Questions

Look at the execution_table, what is the output of step 3?

A nil

B 30

C "Alice"

D Error

💡 Hint

Check the Output column for step 3 in the execution_table.

At which step is the @age instance variable created and set?

A Step 4

B Step 2

C Step 5

D Step 6

💡 Hint

Look at the Instance Variable State column to see when @age appears.

If you call instance_variable_get(:@height) before setting it, what will be returned?

A An error

B nil

C "height"

D 0

💡 Hint

Refer to step 6 in the execution_table and the variable_tracker row for @height.

Concept Snapshot

Instance_variable_get(:@var) returns the value of @var or nil if unset.
Instance_variable_set(:@var, value) sets or creates @var with value.
Use symbols or strings with @ prefix for variable names.
Useful to access or change variables dynamically.
Does not call getter/setter methods, works directly on variables.

Full Transcript

This visual execution shows how Ruby's instance_variable_get and instance_variable_set methods work. First, a Person object is created with @name set to "Alice". Using instance_variable_get(:@name) returns "Alice". Then, instance_variable_set(:@age, 30) creates a new instance variable @age with value 30. Getting @age returns 30. Trying to get an unset variable like @height returns nil. These methods let you read or write instance variables dynamically by name, without using normal getter or setter methods.

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