String freezing for immutability in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

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

Consider the following Ruby code snippet. What will it print?

Ruby

str = "hello"
str.freeze
begin
  str << " world"
rescue => e
  puts e.class
end

A TypeError

B FrozenError

C hello world

D hello

💡 Hint

Frozen strings cannot be modified. What error does Ruby raise when you try?

When a string is frozen in Ruby, any attempt to modify it raises a FrozenError. The code tries to append to the frozen string, so it raises FrozenError.

🧠 Conceptual

intermediate

Why freeze a string in Ruby?

Which of the following is the main reason to freeze a string in Ruby?

A To make the string immutable and prevent accidental changes

B To convert the string to uppercase automatically

C To allow the string to be garbage collected faster

D To enable string interpolation

💡 Hint

Think about what freezing means in everyday life: making something fixed or unchangeable.

Freezing a string makes it immutable, so it cannot be changed after freezing. This helps avoid bugs from accidental modifications.

❓ Predict Output

advanced

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

What will this Ruby code print?

Ruby

str = "data"
frozen_str = str.freeze
copy = frozen_str.dup
copy << "base"
puts copy
puts frozen_str

A

database
data

B

data
database

C

FrozenError
FrozenError

D

datadata
base

💡 Hint

Duplicating a frozen string creates a new unfrozen copy.

The frozen string cannot be changed, but dup creates a new unfrozen string copy. So modifying copy works, original stays frozen.

🔧 Debug

advanced

Identify the error in this Ruby code using frozen strings

What error will this code produce and why?

Ruby

def add_suffix(s)
  s.freeze
  s << "_end"
end

puts add_suffix("test")

A ArgumentError because freeze takes no arguments

B No error, prints 'test_end'

C TypeError because << is not defined for frozen strings

D FrozenError because the string is frozen before modification

💡 Hint

Freezing a string means you cannot change it afterwards.

The method freezes the string then tries to append to it, which raises FrozenError.

🚀 Application

expert

How many items are in the resulting array after freezing strings?

Given this Ruby code, how many elements does the array contain after execution?

Ruby

arr = []
3.times do |i|
  s = "item#{i}"
  arr << s.freeze
end
arr << "item3"
arr.freeze
arr << "item4"

A 3

B 5

C Raises FrozenError

D 4

💡 Hint

What happens if you try to add an element to a frozen array?

The array is frozen before the last push, so trying to add "item4" raises FrozenError.