Immutable data with freeze in Ruby - Mini Project: Build & Apply

Immutable data with freeze

📖 Scenario: You are working on a simple Ruby program that manages a list of favorite fruits. To avoid accidental changes to this list later in the program, you want to make the list immutable.

🎯 Goal: Create a frozen array of fruits so that the list cannot be changed after creation. Then, try to add a new fruit to see what happens.

📋 What You'll Learn

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

Create a variable called immutable_fruits that holds the frozen version of fruits

Attempt to add 'orange' to immutable_fruits using the push method

Print the immutable_fruits array

💡 Why This Matters

🌍 Real World

Freezing data helps prevent accidental changes in programs, which is important in many real-world applications like configuration settings or constant values.

💼 Career

Understanding immutability and error handling is useful for writing safer, more reliable Ruby code in software development jobs.

1

Create the initial array of fruits

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

Ruby

# Create the array fruits with 'apple', 'banana', and 'cherry'
# Your code here

Hint

Use square brackets [] to create an array and separate items with commas.

2

Freeze the array to make it immutable

Create a variable called immutable_fruits that holds the frozen version of the fruits array using the freeze method.

Ruby

fruits = ['apple', 'banana', 'cherry']
# Freeze the fruits array and assign it to immutable_fruits
# Your code here

Hint

Use freeze method on the array to make it immutable.

3

Try to add a new fruit to the frozen array

Use the push method to try to add 'orange' to the immutable_fruits array.

Ruby

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

Hint

Use push method on immutable_fruits. Wrap it in begin...rescue to catch the error.

4

Print the frozen array and error message

Print the immutable_fruits array and then print the error_message variable to show the error caused by trying to modify the frozen array.

Ruby

fruits = ['apple', 'banana', 'cherry']
immutable_fruits = fruits.freeze
begin
  immutable_fruits.push('orange')
rescue => e
  error_message = e.message
end
# Print immutable_fruits and error_message
# Your code here

Hint

Use puts to print the array and the error message. Use inspect to show the array nicely.

Practice

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

easy

A. Prevents any further modifications to the object

B. Deletes the object from memory

C. Duplicates the object

D. Makes the object mutable

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 further changes.

3. Final Answer:

   Prevents any further modifications to the object -> Option A

4. Quick Check:

   freeze makes object immutable = Prevents changes [OK]

Hint: Freeze means no changes allowed after calling it [OK]

Common Mistakes:

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

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

easy

A. freeze(name)

B. name.frozen = true

C. name.freeze()

D. freeze name

Solution

1. Step 1: Recall Ruby method call syntax

   In Ruby, to call a method on an object, use dot notation like object.method().

2. Step 2: Identify the correct freeze syntax

   The correct way to freeze the string name is name.freeze(). Other options are invalid syntax or incorrect usage.

3. Final Answer:

   name.freeze() -> Option C

4. Quick Check:

   Method call with dot and parentheses = Correct freeze syntax [OK]

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

Common Mistakes:

• Using freeze as a standalone function
• Trying to assign frozen = true
• Omitting dot before freeze

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

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

medium

A. [1, 2, 3, 4]

B. RuntimeError: can't modify frozen Array

C. [1, 2, 3]

D. SyntaxError

Solution

1. Step 1: Understand freezing the array

   The array arr is frozen using arr.freeze, making it immutable.

2. Step 2: Analyze the modification attempt

   Trying to append 4 with arr << 4 will raise a RuntimeError because the array is frozen and cannot be changed.

3. Final Answer:

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

4. Quick Check:

   Frozen object + modification = RuntimeError [OK]

Hint: Modifying frozen object causes RuntimeError [OK]

Common Mistakes:

• Assuming frozen array can still be changed
• Expecting silent failure or no error
• Confusing output with syntax error

4. Identify the error in this Ruby code snippet:

str = "hello"
str.freeze
str[0] = 'H'
puts str

medium

A. RuntimeError: can't modify frozen String

B. SyntaxError due to invalid string modification

C. No error, output is "Hello"

D. Output is "hello" without changes

Solution

1. Step 1: Recognize string freezing effect

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

2. Step 2: Understand the attempted modification

   Trying to change the first character with str[0] = 'H' raises a RuntimeError because the string is frozen.

3. Final Answer:

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

4. Quick Check:

   Frozen string + modification = RuntimeError [OK]

Hint: Frozen string cannot be changed, causes RuntimeError [OK]

Common Mistakes:

• Thinking string changes silently fail
• Expecting output "Hello" without error
• Confusing syntax error with runtime error

5. Given this code, what is the best way to safely modify a frozen array without error?

original = [10, 20, 30].freeze
# Your code here to add 40

hard

A. original.freeze(false)

B. original << 40

C. original.push(40)

D. original = original.dup << 40

Solution

1. Step 1: Understand frozen array cannot be changed

   The array original is frozen, so direct modification like << or push will cause errors.

2. Step 2: Use duplication to create a modifiable copy

   Using original.dup creates a new unfrozen copy. Then appending 40 to this copy is allowed.

3. Step 3: Assign the modified copy back

   Assigning the new array with 40 added back to original variable safely updates it without error.

4. Final Answer:

   original = original.dup << 40 -> Option D

5. Quick Check:

   Duplicate frozen object before modifying = Safe change [OK]

Hint: Duplicate frozen object before changing it [OK]

Common Mistakes:

• Trying to modify frozen array directly
• Using non-existent freeze(false) method
• Assuming push works on frozen array