Open classes (reopening classes) in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of reopening a class and adding a method?

Consider the following Ruby code that reopens the String class to add a new method. What will be printed when the code runs?

Ruby

class String
  def shout
    self.upcase + "!"
  end
end

puts "hello".shout

A HELLO!

B hello!

C Error: undefined method 'shout' for String

D hello

💡 Hint

Reopening a class allows you to add new methods to existing classes.

The String class is reopened and a new method shout is added. Calling shout on "hello" returns the uppercase version plus an exclamation mark.

❓ Predict Output

intermediate

What happens when you redefine an existing method in a reopened class?

Look at this Ruby code where the Array class is reopened and the push method is redefined. What will be the output?

Ruby

class Array
  def push(element)
    "Cannot add elements"
  end
end

arr = [1, 2]
puts arr.push(3)

A Error: wrong number of arguments for push

B "Cannot add elements"

C [1, 2, 3]

D 3

💡 Hint

Redefining a method replaces the original behavior.

The push method is redefined to always return the string "Cannot add elements" instead of adding to the array.

🔧 Debug

advanced

Why does this reopened class code raise an error?

This code reopens the Integer class to add a method, but it raises an error. What is the cause?

Ruby

class Integer
  def double
    self * 2
  end
end

puts 5.double

class Integer
  def double
    self + 2
  end
end

puts 5.double

A Error because Integer is a built-in class and cannot be reopened

B Error because method double is defined twice without aliasing

C No error; outputs 10 then 7

D Error because puts is called outside any method

💡 Hint

Ruby allows reopening built-in classes and redefining methods.

Ruby allows reopening Integer and redefining methods. The first puts prints 10 (5*2), the second prints 7 (5+2). No error occurs.

🧠 Conceptual

advanced

What is a risk of reopening core Ruby classes?

Reopening core Ruby classes like String or Array can be powerful but risky. Which of these is the main risk?

A It causes Ruby to crash immediately

B It makes the program run slower because classes are duplicated

C It prevents you from using built-in methods anymore

D It can cause conflicts if multiple libraries redefine the same method differently

💡 Hint

Think about what happens if two pieces of code change the same method.

Reopening core classes and redefining methods can cause conflicts and unexpected behavior if different code changes the same method in incompatible ways.

❓ Predict Output

expert

What is the output after reopening a class and using super?

Examine this Ruby code where the String class is reopened and the reverse method is redefined using super. What will be printed?

Ruby

class String
  def reverse
    "Reversed: " + super
  end
end

puts "abc".reverse

A Reversed: cba

B abc

C Error: super called outside method

D Reversed: abc

💡 Hint

super calls the original method from the class before reopening.

The new reverse method calls super to get the original reversed string, then adds "Reversed: " before it.

Practice

1. What does it mean to reopen a class in Ruby?

easy

A. You write the class again to add or change methods without deleting the original.

B. You create a new class with the same name but different methods.

C. You delete the old class and replace it with a new one.

D. You make a copy of the class and modify the copy only.

Solution

1. Step 1: Understand Ruby class behavior

   In Ruby, classes are open, meaning you can write the class again to add or change methods.

2. Step 2: Identify what reopening means

   Reopening a class means modifying the existing class by adding or replacing methods without deleting it.

3. Final Answer:

   You write the class again to add or change methods without deleting the original. -> Option A

4. Quick Check:

   Reopen class = add/change methods anytime [OK]

Hint: Reopen means add or change methods by rewriting class [OK]

Common Mistakes:

• Thinking reopening creates a new separate class
• Assuming reopening deletes the old class
• Confusing reopening with copying a class

2. Which of the following is the correct syntax to reopen the String class and add a method shout that returns the string in uppercase with exclamation?

easy

A. class String
def shout
self.upcase + "!"
end
end

B. def String.shout
self.upcase + "!"
end

C. class String
def shout
upcase!
end
end

D. class String
def shout
self + "!"
end
end

Solution

1. Step 1: Check class reopening syntax

   To reopen a class, write class ClassName again and define methods inside.

2. Step 2: Verify method definition and behavior

   Method shout should return uppercase string plus exclamation, so self.upcase + "!" is correct.

3. Final Answer:

   class String def shout self.upcase + "!" end end -> Option A

4. Quick Check:

   Reopen class + define method correctly = class String def shout self.upcase + "!" end end [OK]

Hint: Use class name again and define method inside [OK]

Common Mistakes:

• Defining method outside class
• Using destructive method like upcase! which changes string
• Not returning uppercase string with exclamation

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

class Array
  def first_two
    self[0..1]
  end
end

arr = [10, 20, 30, 40]
puts arr.first_two.inspect

medium

A. [10]

B. [10, 20]

C. [20, 30]

D. nil

Solution

1. Step 1: Understand method added to Array

   The method first_two returns elements at index 0 and 1 using self[0..1].

2. Step 2: Apply method on given array

   Array is [10, 20, 30, 40], so first_two returns [10, 20].

3. Final Answer:

   [10, 20] -> Option B

4. Quick Check:

   Array slice 0..1 = first two elements [OK]

Hint: Slice array 0..1 to get first two elements [OK]

Common Mistakes:

• Confusing index ranges and getting wrong slice
• Expecting single element instead of array
• Thinking method returns nil

4. This code tries to reopen the Integer class to add a method double. What is wrong?

class Integer
  def double
    self * 2
  end
end

puts 5.double()

medium

A. Method double should be defined as a class method, not instance method.

B. You cannot reopen built-in classes like Integer.

C. Nothing is wrong; it will print 10.

D. The method double uses wrong syntax for multiplication.

Solution

1. Step 1: Check if reopening Integer is allowed

   Ruby allows reopening built-in classes like Integer to add instance methods.

2. Step 2: Verify method and usage

   Method double multiplies self by 2, and calling 5.double() returns 10.

3. Final Answer:

   Nothing is wrong; it will print 10. -> Option C

4. Quick Check:

   Reopen Integer + instance method works = 10 output [OK]

Hint: Reopen built-in classes freely; instance methods work [OK]

Common Mistakes:

• Thinking built-in classes cannot be reopened
• Confusing instance and class methods
• Assuming syntax error in multiplication

5. You want to reopen the Hash class to add a method symbolize_keys that returns a new hash with all string keys converted to symbols. Which code correctly does this?

hard

A. class Hash
def symbolize_keys
each_with_object({}) do |(k, v), h|
h[k.to_sym] = v
end
end
end

B. class Hash
def symbolize_keys
map { |k, v| [k.to_sym, v] }
end
end

C. class Hash
def symbolize_keys
keys.map(&:to_sym)
end
end

D. class Hash
def symbolize_keys
self.transform_keys(&:to_sym)
end
end

Solution

1. Step 1: Understand the goal

   We want a method that returns a new hash with string keys converted to symbols.

2. Step 2: Check each option's correctness

   class Hash def symbolize_keys each_with_object({}) do |(k, v), h| h[k.to_sym] = v end end end manually builds a new hash but misses handling non-string keys safely. class Hash def symbolize_keys map { |k, v| [k.to_sym, v] } end end returns an array, not a hash. class Hash def symbolize_keys keys.map(&:to_sym) end end returns only keys as symbols, not a hash. class Hash def symbolize_keys self.transform_keys(&:to_sym) end end uses transform_keys(&:to_sym), a built-in method that correctly returns a new hash with symbol keys.

3. Final Answer:

   class Hash def symbolize_keys self.transform_keys(&:to_sym) end end -> Option D

4. Quick Check:

   Use transform_keys(&:to_sym) for symbol keys [OK]

Hint: Use transform_keys(&:to_sym) to convert keys easily [OK]

Common Mistakes:

• Returning array instead of hash
• Not returning a new hash
• Ignoring keys that are not strings