Frozen objects in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Ruby code with frozen string?

Consider this Ruby code snippet:

str = "hello".freeze
str << " world"
puts str

What will be the output or error?

Ruby

str = "hello".freeze
str << " world"
puts str

A RuntimeError: can't modify frozen String

B hello world

C TypeError: no implicit conversion of String into Integer

D hello

💡 Hint

Frozen objects cannot be changed after freezing.

The string is frozen, so trying to modify it with << causes a RuntimeError.

❓ Predict Output

intermediate

What happens when modifying a frozen array element?

Look at this Ruby code:

arr = [1, 2, 3].freeze
arr[0] = 10
puts arr.inspect

What will be the output or error?

Ruby

arr = [1, 2, 3].freeze
arr[0] = 10
puts arr.inspect

A RuntimeError: can't modify frozen Array

B [10, 2, 3]

C [1, 2, 3]

D TypeError: no implicit conversion of Integer into String

💡 Hint

Freezing an array prevents changing its elements.

The array is frozen, so assigning to an element raises RuntimeError.

❓ Predict Output

advanced

What is the output when freezing a hash and modifying a nested object?

Consider this Ruby code:

h = {a: [1, 2]}
h.freeze
h[:a] << 3
puts h[:a].inspect

What will be printed or what error occurs?

Ruby

h = {a: [1, 2]}
h.freeze
h[:a] << 3
puts h[:a].inspect

A [1, 2]

B [1, 2, 3]

C RuntimeError: can't modify frozen Hash

D TypeError: no implicit conversion of Integer into String

💡 Hint

Freezing a hash does not freeze its nested objects.

The hash is frozen, but its array value is not frozen, so << works and adds 3.

❓ Predict Output

advanced

What error occurs when freezing a string literal and modifying it?

Analyze this Ruby code:

def modify_str(s)
  s << "!"
end

str = "hello".freeze
modify_str(str)
puts str

What will happen when running this code?

Ruby

def modify_str(s)
  s << "!"
end

str = "hello".freeze
modify_str(str)
puts str

A hello!

B No output, program hangs

C RuntimeError: can't modify frozen String

D TypeError: no implicit conversion of String into Integer

💡 Hint

Frozen strings cannot be changed even inside methods.

The frozen string cannot be modified by << inside the method, causing RuntimeError.

🧠 Conceptual

expert

Why does freezing an object not freeze its nested objects in Ruby?

In Ruby, when you freeze an object like a hash or array, its nested objects remain mutable.

Why does Ruby behave this way?

A Because freezing an object disables garbage collection for nested objects.

B Because Ruby automatically freezes nested objects only if they are strings.

C Because freezing an object clones all nested objects and freezes the clones instead.

D Because freeze only marks the top-level object as immutable; nested objects are separate and must be frozen individually.

💡 Hint

Think about how objects and references work in Ruby.

Freezing marks only the object itself as immutable; nested objects are separate and need their own freeze calls.

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