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.