Open classes (reopening classes) in Ruby - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to reopen the String class and add a method.

Ruby

class String
  def [1]
    "Hello, " + self
  end
end

puts "World".greet

Drag options to blanks, or click blank then click option'

Agreet

Bhello

Csay_hello

Dgreeting

Attempts:

3 left

💡 Hint

Common Mistakes

Using a method name different from the one called on the string.

Forgetting to reopen the class before defining the method.

The method 'greet' is added to the String class and called on the string "World".

2fill in blank

medium

Complete the code to reopen the Integer class and add a method that doubles the number.

Ruby

class Integer
  def [1]
    self * 2
  end
end

puts 5.double

Drag options to blanks, or click blank then click option'

Amultiply

Btwice

Ctimes_two

Ddouble

Attempts:

3 left

💡 Hint

Common Mistakes

Using a method name that does not match the call.

Not returning the correct calculation.

The method 'double' is added to Integer and returns the number multiplied by 2.

3fill in blank

hard

Fix the error in reopening the Array class to add a method that returns the first element doubled.

Ruby

class Array
  def double_first
    [1] * 2
  end
end

puts [1, 2, 3].double_first

Drag options to blanks, or click blank then click option'

Afirst()

Bself[0]

Cself.first

Dself[1]

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'self[1]' which accesses the second element.

Using 'first()' without 'self'.

Using 'self.first' correctly accesses the first element of the array.

4fill in blank

hard

Fill both blanks to reopen the Hash class and add a method that returns keys with values greater than 10.

Ruby

class Hash
  def keys_with_large_values
    select { |[1], [2]| [2] > 10 }.keys
  end
end

puts({a: 5, b: 15, c: 20}.keys_with_large_values)

Drag options to blanks, or click blank then click option'

Akey

Bvalue

Ck

Dv

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'key' and 'value' which are valid but not the correct answer here.

Mixing up the order of key and value.

The block variables 'k' and 'v' represent key and value respectively in the Hash select method.

5fill in blank

hard

Fill all three blanks to reopen the Numeric class and add a method that returns the number squared if positive, else zero.

Ruby

class Numeric
  def positive_square
    if [1] > 0
      [2] [3] 2
    else
      0
    end
  end
end

puts 4.positive_square
puts (-3).positive_square

Drag options to blanks, or click blank then click option'

Aself

C**

D*

Attempts:

3 left

💡 Hint

Common Mistakes

Using '*' instead of '**' for squaring.

Not using 'self' to refer to the number.

Check if 'self' is positive, then return 'self ** 2' (square). The '*' operator is incorrect for exponentiation.

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