Ruby Control Structures

This tutorial provides a step-by-step guide to understanding and using Ruby’s if/else control structures. You’ll learn the basics, explore advanced use cases, and apply your knowledge through engaging practice exercises.

Beginner30 minutes

Step 1: Introduction to if/else

The if/else statement is a fundamental control structure in Ruby that allows your program to make decisions based on conditions. It evaluates a condition and executes a block of code if the condition is true. If the condition is false, it can execute an alternative block of code using else.

if 10 > 5
  puts '10 is greater than 5'
else
  puts '10 is not greater than 5'
end

Practice Exercise

Write a program that checks if a given number is positive. If it is, print 'Positive'. Otherwise, print 'Not positive'.

Show Solution

number = 7
if number > 0
  puts 'Positive'
else
  puts 'Not positive'
end
# Explanation: The program checks if the number is greater than 0 and prints 'Positive' if true. Otherwise, it prints 'Not positive'.

Step 2: Using elsif for Multiple Conditions

When you have multiple conditions to evaluate, you can use elsif to chain additional conditions. This allows you to handle more complex decision-making scenarios.

age = 18
if age < 13
  puts 'Child'
elsif age < 18
  puts 'Teenager'
else
  puts 'Adult'
end

Practice Exercise

Write a program that categorizes a person's age into 'Child' (0-12), 'Teenager' (13-17), or 'Adult' (18+).

Show Solution

age = 15
if age < 13
  puts 'Child'
elsif age < 18
  puts 'Teenager'
else
  puts 'Adult'
end
# Explanation: The program checks the age and prints the appropriate category based on the range it falls into.

Step 3: Nested if/else Statements

You can nest if/else statements inside one another to handle more complex logic. This is useful when you need to evaluate conditions within conditions.

temperature = 25
if temperature > 20
  if temperature > 30
    puts 'It’s hot outside!'
  else
    puts 'It’s warm outside.'
  end
else
  puts 'It’s cold outside.'
end

Practice Exercise

Write a program that checks if a number is even. If it is, check if it is also greater than 10. Print 'Even and greater than 10' or 'Even but not greater than 10' accordingly.

Show Solution

number = 14
if number % 2 == 0
  if number > 10
    puts 'Even and greater than 10'
  else
    puts 'Even but not greater than 10'
  end
else
  puts 'Not even'
end
# Explanation: The program first checks if the number is even. If true, it further checks if the number is greater than 10 and prints the appropriate message.

Step 4: Ternary Operator for Concise if/else

Ruby provides a shorthand for simple if/else statements using the ternary operator (? :). This is useful for writing concise code when the logic is straightforward.

age = 20
status = age >= 18 ? 'Adult' : 'Minor'
puts status

Practice Exercise

Rewrite the following if/else statement using the ternary operator: if x > 5; puts 'Greater'; else; puts 'Less'; end.

Show Solution

x = 7
puts x > 5 ? 'Greater' : 'Less'
# Explanation: The ternary operator checks if `x` is greater than 5 and prints 'Greater' if true, otherwise 'Less'.

Step 5: Using unless as an Alternative to if

The unless keyword is the opposite of if. It executes a block of code only if the condition is false. This can make your code more readable in certain scenarios.

raining = false
unless raining
  puts 'Let’s go outside!'
else
  puts 'Stay indoors.'
end

Practice Exercise

Write a program that checks if a user is not logged in. If they are not, print 'Please log in'. Otherwise, print 'Welcome back!'.

Show Solution

logged_in = false
unless logged_in
  puts 'Please log in'
else
  puts 'Welcome back!'
end
# Explanation: The program uses `unless` to check if the user is not logged in and prints 'Please log in' if true.

Step 6: Combining Conditions with Logical Operators

You can combine multiple conditions using logical operators like && (and), || (or), and ! (not). This allows you to create more complex decision-making logic.

age = 25
income = 50000
if age > 18 && income > 30000
  puts 'Eligible for loan'
else
  puts 'Not eligible'
end

Practice Exercise

Write a program that checks if a user is eligible for a discount. The user must be either a student (check if is_student is true) or have a purchase amount greater than $100.

Show Solution

is_student = true
purchase_amount = 120
if is_student || purchase_amount > 100
  puts 'Eligible for discount'
else
  puts 'Not eligible'
end
# Explanation: The program checks if the user is a student or has a purchase amount greater than $100 and prints 'Eligible for discount' if either condition is true.

Step 7: Real-World Example: User Authentication

Let’s apply what we’ve learned to a real-world scenario: user authentication. We’ll check if a user’s credentials are valid and provide appropriate feedback.

username = 'admin'
password = 'password123'
if username == 'admin' && password == 'password123'
  puts 'Login successful!'
else
  puts 'Invalid credentials.'
end

Practice Exercise

Write a program that simulates a login system. Check if the username is 'user' and the password is 'secret'. If both are correct, print 'Access granted'. Otherwise, print 'Access denied'.

Show Solution

username = 'user'
password = 'secret'
if username == 'user' && password == 'secret'
  puts 'Access granted'
else
  puts 'Access denied'
end
# Explanation: The program checks if the username and password match the expected values and prints 'Access granted' if they do.

Step 8: Advanced Challenge: Grade Calculator

Let’s tackle a more advanced problem: creating a grade calculator. The program will take a score and assign a grade based on predefined ranges.

score = 85
if score >= 90
  puts 'A'
elsif score >= 80
  puts 'B'
elsif score >= 70
  puts 'C'
elsif score >= 60
  puts 'D'
else
  puts 'F'
end

Practice Exercise

Write a program that calculates a student’s grade based on their score. Use the following ranges: A (90-100), B (80-89), C (70-79), D (60-69), F (0-59).

Show Solution

score = 78
if score >= 90
  puts 'A'
elsif score >= 80
  puts 'B'
elsif score >= 70
  puts 'C'
elsif score >= 60
  puts 'D'
else
  puts 'F'
end
# Explanation: The program evaluates the score and assigns a grade based on the range it falls into.