Frozen objects in Ruby - Mini Project: Build & Apply

Frozen objects

📖 Scenario: Imagine you are managing a list of favorite fruits in a program. Sometimes, you want to make sure this list does not change accidentally after you set it.

🎯 Goal: You will create a list of fruits, freeze it to prevent changes, and then try to modify it to see what happens.

📋 What You'll Learn

Create an array called fruits with the exact values: 'apple', 'banana', 'cherry'

Create a variable called frozen_fruits that is the frozen version of fruits

Attempt to add 'orange' to frozen_fruits using push

Print frozen_fruits to show the result

💡 Why This Matters

🌍 Real World

Freezing objects is useful when you want to keep data safe and unchanged, like configuration settings or fixed lists.

💼 Career

Understanding frozen objects helps you write safer Ruby code, which is important for roles like Ruby developer or backend engineer.

1

Create the fruits array

Create an array called fruits with these exact strings: 'apple', 'banana', 'cherry'

Ruby

# Create the fruits array
# Your code here

Hint

Use square brackets [] to create an array with the given strings separated by commas.

2

Freeze the fruits array

Create a variable called frozen_fruits and set it to the frozen version of the fruits array using the freeze method

Ruby

fruits = ['apple', 'banana', 'cherry']
# Freeze the fruits array
# Your code here

Hint

Call freeze on fruits and assign it to frozen_fruits.

3

Try to modify the frozen array

Use the push method to add the string 'orange' to the frozen_fruits array

Ruby

fruits = ['apple', 'banana', 'cherry']
frozen_fruits = fruits.freeze
# Try to add 'orange' to frozen_fruits
# Your code here

Hint

Call push('orange') on frozen_fruits to try adding an item.

4

Print the frozen array

Write a puts statement to print the frozen_fruits array

Ruby

fruits = ['apple', 'banana', 'cherry']
frozen_fruits = fruits.freeze
frozen_fruits.push('orange')
# Print frozen_fruits
# Your code here

Hint

Use puts frozen_fruits to print each fruit on its own line.

Note: Since the array is frozen, adding 'orange' will cause an error and the program will stop before printing.

Practice

1. What does the freeze method do to an object in Ruby?

easy

A. Makes the object mutable

B. Deletes the object from memory

C. Duplicates the object

D. Prevents any modifications to the object

Solution

1. Step 1: Understand the purpose of freeze

   The freeze method is used to make an object immutable, meaning it cannot be changed after freezing.

2. Step 2: Identify the effect on the object

   Once frozen, any attempt to modify the object will raise an error, so it prevents modifications.

3. Final Answer:

   Prevents any modifications to the object -> Option D

4. Quick Check:

   freeze makes object immutable = Prevents modifications [OK]

Hint: Freeze means no changes allowed to the object [OK]

Common Mistakes:

• Thinking freeze deletes the object
• Confusing freeze with duplication
• Believing freeze makes object mutable

2. Which of the following is the correct way to freeze a string object str in Ruby?

easy

A. str.freeze()

B. freeze str

C. str.freeze = true

D. freeze(str)

Solution

1. Step 1: Recall Ruby method call syntax

   In Ruby, methods are called on objects using dot notation, e.g., object.method.

2. Step 2: Identify the correct freeze syntax

   The correct way to freeze an object is by calling freeze on it: str.freeze(). The other options are invalid syntax.

3. Final Answer:

   str.freeze() -> Option A

4. Quick Check:

   Method call syntax = str.freeze() [OK]

Hint: Use dot notation to call freeze: object.freeze() [OK]

Common Mistakes:

• Using freeze as a standalone keyword
• Assigning freeze as a property
• Calling freeze like a function without dot

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

arr = [1, 2, 3]
arr.freeze
arr << 4
puts arr.inspect

medium

A. RuntimeError: can't modify frozen Array

B. [1, 2, 3, 4]

C. [1, 2, 3]

D. SyntaxError

Solution

1. Step 1: Understand freezing the array

   The array arr is frozen with arr.freeze, so it cannot be changed.

2. Step 2: Analyze the modification attempt

   The code tries to add an element with arr << 4, which modifies the array. Since it's frozen, this raises a runtime error.

3. Final Answer:

   RuntimeError: can't modify frozen Array -> Option A

4. Quick Check:

   Modifying frozen object = RuntimeError [OK]

Hint: Modifying frozen objects causes runtime errors [OK]

Common Mistakes:

• Expecting the array to change
• Thinking freeze allows modifications
• Confusing runtime error with syntax error

4. Identify the error in this Ruby code snippet:

name = "Alice"
name.freeze
name[0] = 'M'
puts name

medium

A. No error, output is "Mlice"

B. RuntimeError: can't modify frozen String

C. SyntaxError due to invalid assignment

D. TypeError: wrong type of argument

Solution

1. Step 1: Recognize string freezing

   The string name is frozen, so it cannot be changed after name.freeze.

2. Step 2: Understand the attempted modification

   Trying to change the first character with name[0] = 'M' modifies the string, which is forbidden and raises a runtime error.

3. Final Answer:

   RuntimeError: can't modify frozen String -> Option B

4. Quick Check:

   Changing frozen string = RuntimeError [OK]

Hint: Changing frozen strings causes runtime errors [OK]

Common Mistakes:

• Assuming strings can be changed after freeze
• Confusing syntax error with runtime error
• Ignoring freeze effect on string elements

5. Given this Ruby code, what will be the output?

hash = {a: [1, 2], b: [3, 4]}
hash.freeze
hash[:a] << 5
puts hash[:a].inspect

What does this tell us about freezing objects?

hard

A. RuntimeError: can't modify frozen Hash

B. [1, 2] - freeze prevents any changes including nested objects

C. [1, 2, 5] - freeze prevents changing the hash keys but not the objects inside

D. SyntaxError due to invalid hash modification

Solution

1. Step 1: Understand freezing the hash

   The hash object is frozen, so its structure (keys and values) cannot be changed directly.

2. Step 2: Analyze modification of nested array

   The value for key :a is an array, which is not frozen. Modifying this array with << 5 is allowed.

3. Step 3: Observe the output

   The array now contains [1, 2, 5], showing that freezing the hash does not freeze nested objects.

4. Final Answer:

   [1, 2, 5] - freeze prevents changing the hash keys but not the objects inside -> Option C

5. Quick Check:

   Freeze is shallow = nested objects can change [OK]

Hint: Freeze is shallow; nested objects stay mutable unless frozen [OK]

Common Mistakes:

• Assuming freeze affects nested objects
• Expecting runtime error on nested modification
• Confusing hash freezing with deep freeze