Ruby Control Structures: case/when

This tutorial will teach you how to use Ruby’s `case/when` control structure effectively. You’ll learn its syntax, practical applications, and how to solve real-world problems using this powerful tool.

Beginner40 minutes

Step 1: Introduction to `case/when`

The case/when statement in Ruby is a powerful alternative to multiple if/elsif conditions. It allows you to compare a value against multiple patterns and execute code based on the first match. It's clean, readable, and efficient.

day = 'Monday'

case day
when 'Monday'
  puts 'Start of the work week!'
when 'Friday'
  puts 'Almost the weekend!'
else
  puts 'Just another day.'
end

Practice Exercise

Write a case/when statement that checks the value of a variable weather and prints a message based on whether it's 'sunny', 'rainy', or 'cloudy'.

Show Solution

weather = 'sunny'

case weather
when 'sunny'
  puts 'Wear sunscreen and enjoy the day!'
when 'rainy'
  puts 'Don't forget your umbrella!'
when 'cloudy'
  puts 'A jacket might be a good idea.'
else
  puts 'Check the weather forecast!'
end

Step 2: Using Ranges in `case/when`

You can use ranges (e.g., 1..10) in when clauses to match a range of values. This is useful for categorizing numeric data or grouping similar conditions.

age = 25

case age
when 0..12
  puts 'Child'
when 13..19
  puts 'Teenager'
when 20..60
  puts 'Adult'
else
  puts 'Senior'
end

Practice Exercise

Write a case/when statement that categorizes a temperature variable into 'freezing' (below 0), 'cold' (0-15), 'moderate' (16-25), and 'hot' (above 25).

Show Solution

temperature = 10

case temperature
when -Float::INFINITY..0
  puts 'Freezing! Bundle up.'
when 0..15
  puts 'Cold. Wear a jacket.'
when 16..25
  puts 'Moderate. Perfect weather!'
else
  puts 'Hot. Stay hydrated!'
end

Step 3: Using Regular Expressions in `case/when`

Ruby allows you to use regular expressions in when clauses. This is particularly useful for matching strings against patterns.

email = 'user@example.com'

case email
when /@gmail.com$/
  puts 'Gmail user'
when /@yahoo.com$/
  puts 'Yahoo user'
else
  puts 'Other email provider'
end

Practice Exercise

Write a case/when statement that checks if a phone_number variable matches a valid US phone number format (e.g., (123) 456-7890).

Show Solution

phone_number = '(123) 456-7890'

case phone_number
when /\(\d{3}\) \d{3}-\d{4}/
  puts 'Valid US phone number.'
else
  puts 'Invalid phone number format.'
end

Step 4: Combining Conditions with `when`

You can combine multiple conditions in a single when clause using the && or || operators. This allows for more complex logic within a case statement.

age = 25
income = 50000

case
when age >= 18 && income > 40000
  puts 'Eligible for a premium credit card.'
when age >= 18 && income <= 40000
  puts 'Eligible for a standard credit card.'
else
  puts 'Not eligible for a credit card.'
end

Practice Exercise

Write a case/when statement that checks if a user is eligible for a discount based on their age (must be over 60) or membership_status (must be 'premium').

Show Solution

age = 65
membership_status = 'premium'

case
when age > 60 || membership_status == 'premium'
  puts 'Eligible for a discount!'
else
  puts 'Not eligible for a discount.'
end

Step 5: Using `case/when` with Objects

The case/when statement can also be used with objects and custom classes. Ruby uses the === operator to compare objects, which can be overridden for custom behavior.

class Animal
  attr_reader :type

  def initialize(type)
    @type = type
  end

  def ===(other)
    type == other.type
  end
end

animal = Animal.new('dog')

case animal
when Animal.new('cat')
  puts 'It's a cat!'
when Animal.new('dog')
  puts 'It's a dog!'
else
  puts 'Unknown animal.'
end

Practice Exercise

Create a Vehicle class with a type attribute. Write a case/when statement that checks the type of a vehicle object and prints a message based on whether it's a 'car', 'bike', or 'truck'.

Show Solution

class Vehicle
  attr_reader :type

  def initialize(type)
    @type = type
  end

  def ===(other)
    type == other.type
  end
end

vehicle = Vehicle.new('car')

case vehicle
when Vehicle.new('car')
  puts 'It's a car!'
when Vehicle.new('bike')
  puts 'It's a bike!'
when Vehicle.new('truck')
  puts 'It's a truck!'
else
  puts 'Unknown vehicle.'
end

Step 6: Advanced: Nested `case/when` Statements

You can nest case/when statements to handle more complex decision-making scenarios. This is useful when you need to evaluate multiple conditions in a hierarchical manner.

age = 25
income = 50000

case age
when 18..30
  case income
  when 0..30000
    puts 'Young adult with low income.'
  when 30001..60000
    puts 'Young adult with moderate income.'
  else
    puts 'Young adult with high income.'
  end
when 31..60
  puts 'Middle-aged adult.'
else
  puts 'Senior citizen.'
end

Practice Exercise

Write a nested case/when statement that categorizes a person based on their age and education_level (e.g., 'high school', 'college', 'graduate').

Show Solution

age = 22
education_level = 'college'

case age
when 18..22
  case education_level
  when 'high school'
    puts 'High school student or recent graduate.'
  when 'college'
    puts 'College student.'
  else
    puts 'Young adult with advanced education.'
  end
when 23..30
  puts 'Young professional.'
else
  puts 'Experienced professional.'
end

Step 7: Real-World Application: Menu Selection

The case/when statement is often used in real-world applications like menu selection systems. It provides a clean way to handle user input and execute corresponding actions.

puts 'Welcome! Choose an option: 1. Check balance 2. Deposit 3. Withdraw'
option = gets.chomp.to_i

case option
when 1
  puts 'Your balance is $1000.'
when 2
  puts 'How much would you like to deposit?'
when 3
  puts 'How much would you like to withdraw?'
else
  puts 'Invalid option.'
end

Practice Exercise

Create a simple menu system for a library. The user should be able to choose between '1. Borrow a book', '2. Return a book', and '3. Check available books'. Use case/when to handle the user's choice.

Show Solution

puts 'Welcome to the library! Choose an option: 1. Borrow a book 2. Return a book 3. Check available books'
option = gets.chomp.to_i

case option
when 1
  puts 'Which book would you like to borrow?'
when 2
  puts 'Which book are you returning?'
when 3
  puts 'Available books: Ruby Programming, Clean Code, Design Patterns'
else
  puts 'Invalid option.'
end

Step 8: Best Practices and Tips

Use case/when for readability when comparing a single variable against multiple values. 2. Avoid overly complex nested case/when statements. 3. Use ranges and regular expressions to simplify conditions. 4. Always include an else clause to handle unexpected cases.

Practice Exercise

Refactor the following if/elsif statement into a case/when statement:

if age < 13
  puts 'Child'
elsif age < 20
  puts 'Teenager'
elsif age < 65
  puts 'Adult'
else
  puts 'Senior'
end

Show Solution

case age
when 0..12
  puts 'Child'
when 13..19
  puts 'Teenager'
when 20..64
  puts 'Adult'
else
  puts 'Senior'
end