Practicing Test Driven Development with Ruby and RSpec

Introduction to Red-Green-Refactor Cycle

Welcome to your first lesson in this course dedicated to practicing Test Driven Development (TDD) using Ruby and RSpec . Test Driven Development is an effective approach that prioritizes writing tests before coding. This guides you to develop your application around testing, ensuring that each component functions correctly as intended. In this lesson, you'll learn the fundamentals of TDD and the Red-Green-Refactor cycle and understand their roles in creating consistent and maintainable code. In this course, emphasis is placed on hands-on practice, where you'll receive requirements through tests, one at a time. Your objective is to implement code that makes each test pass, simulating a real-world TDD environment. As the course guide, it's akin to being your pair programmer, providing test-driven prompts to hone your skills as you progress.

Understanding the Red-Green-Refactor Cycle

As a reminder, the Red-Green-Refactor cycle is central to TDD, guiding your development process: Red : Start by writing a failing test to define the next step. Green : Implement just enough code to pass the test, focusing on functionality. Refactor : Optimize the code for clarity and efficiency without changing its behavior. The combination of these phases fosters a disciplined and iterative approach to development. By starting with a failing test, you solidify the requirement and focus on addressing only what is necessary to meet that specific need. The act of writing minimal code ensures that each part of the application is built to clear specifications, reducing the potential for errors. The refactoring phase allows you to clean up and reorganize code, ensuring it remains efficient and comprehensible, contributing to the overall robustness and quality of the software. Together, these phases provide a structured method to incrementally build, test, and polish code, ensuring high standards throughout the development process.

Requirements for `calculate_discount` Method

In this unit, we will outline the test cases that guide the implementation of the calculate_discount method by employing the Red-Green-Refactor cycle of Test Driven Development (TDD). Each test case serves as a specific requirement, starting with writing a failing test ("Red") to define a behavior, followed by implementing minimal code changes to pass the test ("Green"), and finalizing with code optimization ("Refactor"). We will cover essential scenarios, such as applying correct percentage-based discounts, handling zero discount cases, managing decimal discount precision, and ensuring valid input handling to guarantee robustness and correctness.

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Ruby

def calculate_discount(original_price, discount_percentage)
  raise ArgumentError, 'Price cannot be negative' if original_price < 0
  raise ArgumentError, 'Discount percentage cannot be > 100%' if discount_percentage > 100
  raise ArgumentError, 'Discount cannot be negative' if discount_percentage < 0

  discounted_price = original_price * (1 - discount_percentage / 100.0)
  discounted_price.round(2)
end

def calculate_discount(original_price, discount_percentage)
  raise ArgumentError, 'Price cannot be negative' if original_price < 0
  raise ArgumentError, 'Discount percentage cannot be > 100%' if discount_percentage > 100
  raise ArgumentError, 'Discount cannot be negative' if discount_percentage < 0

  discounted_price = original_price * (1 - discount_percentage / 100.0)
  discounted_price.round(2)
end

Ruby

require 'rspec'

RSpec.describe 'calculate_discount' do
  it 'applies the correct discount to the price' do
    # Arrange
    original_price = 100
    discount_percentage = 20
    expected_discounted_price = 80

    # Act
    result = calculate_discount(original_price, discount_percentage)

    # Assert
    expect(result).to eq(expected_discounted_price)
  end
end

require 'rspec'

RSpec.describe 'calculate_discount' do
  it 'applies the correct discount to the price' do
    # Arrange
    original_price = 100
    discount_percentage = 20
    expected_discounted_price = 80

    # Act
    result = calculate_discount(original_price, discount_percentage)

    # Assert
    expect(result).to eq(expected_discounted_price)
  end
end

Ruby

require 'rspec'

RSpec.describe 'calculate_discount' do
  it 'returns the original price when discount is zero' do
    # Arrange
    original_price = 50
    discount_percentage = 0

    # Act
    result = calculate_discount(original_price, discount_percentage)

    # Assert
    expect(result).to eq(original_price)
  end
end

require 'rspec'

RSpec.describe 'calculate_discount' do
  it 'returns the original price when discount is zero' do
    # Arrange
    original_price = 50
    discount_percentage = 0

    # Act
    result = calculate_discount(original_price, discount_percentage)

    # Assert
    expect(result).to eq(original_price)
  end
end

Ruby

require 'rspec'

RSpec.describe 'calculate_discount' do
  it 'handles decimal discounts correctly' do
    # Arrange
    original_price = 100
    discount_percentage = 33.333
    expected_discounted_price = 66.67

    # Act
    result = calculate_discount(original_price, discount_percentage)

    # Assert
    expect(result.round(2)).to eq(expected_discounted_price)
  end
end

require 'rspec'

RSpec.describe 'calculate_discount' do
  it 'handles decimal discounts correctly' do
    # Arrange
    original_price = 100
    discount_percentage = 33.333
    expected_discounted_price = 66.67

    # Act
    result = calculate_discount(original_price, discount_percentage)

    # Assert
    expect(result.round(2)).to eq(expected_discounted_price)
  end
end

Ruby

require 'rspec'

RSpec.describe 'calculate_discount' do
  it 'throws an error for negative prices' do
    # Arrange
    original_price = -50
    discount_percentage = 10

    # Act, Assert
    expect { calculate_discount(original_price, discount_percentage) }.to raise_error(ArgumentError, 'Price cannot be negative')
  end
end

require 'rspec'

RSpec.describe 'calculate_discount' do
  it 'throws an error for negative prices' do
    # Arrange
    original_price = -50
    discount_percentage = 10

    # Act, Assert
    expect { calculate_discount(original_price, discount_percentage) }.to raise_error(ArgumentError, 'Price cannot be negative')
  end
end

Ruby

require 'rspec'

RSpec.describe 'calculate_discount' do
  it 'throws an error for discounts greater than 100 percent' do
    # Arrange
    original_price = 100
    discount_percentage = 110

    # Act, Assert
    expect { calculate_discount(original_price, discount_percentage) }.to raise_error(ArgumentError, 'Discount percentage cannot be > 100%')
  end
end

require 'rspec'

RSpec.describe 'calculate_discount' do
  it 'throws an error for discounts greater than 100 percent' do
    # Arrange
    original_price = 100
    discount_percentage = 110

    # Act, Assert
    expect { calculate_discount(original_price, discount_percentage) }.to raise_error(ArgumentError, 'Discount percentage cannot be > 100%')
  end
end

Ruby

require 'rspec'

RSpec.describe 'calculate_discount' do
  it 'throws an error for discounts less than zero' do
    # Arrange
    original_price = 100
    discount_percentage = -10
    
    # Act, Assert
    expect { calculate_discount(original_price, discount_percentage) }.to raise_error(ArgumentError, 'Discount cannot be negative')
  end
end

require 'rspec'

RSpec.describe 'calculate_discount' do
  it 'throws an error for discounts less than zero' do
    # Arrange
    original_price = 100
    discount_percentage = -10
    
    # Act, Assert
    expect { calculate_discount(original_price, discount_percentage) }.to raise_error(ArgumentError, 'Discount cannot be negative')
  end
end

Ruby

def calculate_discount(original_price, discount_percentage)
  raise ArgumentError, 'Price cannot be negative' if original_price < 0
  raise ArgumentError, 'Discount percentage cannot be > 100%' if discount_percentage > 100
  raise ArgumentError, 'Discount cannot be negative' if discount_percentage < 0

  discounted_price = original_price * (1 - discount_percentage / 100.0)
  discounted_price.round(2)
end

Ruby

require 'rspec'

RSpec.describe 'calculate_discount' do
  it 'applies the correct discount to the price' do
    # Arrange
    original_price = 100
    discount_percentage = 20
    expected_discounted_price = 80

    # Act
    result = calculate_discount(original_price, discount_percentage)

    # Assert
    expect(result).to eq(expected_discounted_price)
  end
end

Ruby

require 'rspec'

RSpec.describe 'calculate_discount' do
  it 'returns the original price when discount is zero' do
    # Arrange
    original_price = 50
    discount_percentage = 0

    # Act
    result = calculate_discount(original_price, discount_percentage)

    # Assert
    expect(result).to eq(original_price)
  end
end

Ruby

require 'rspec'

RSpec.describe 'calculate_discount' do
  it 'handles decimal discounts correctly' do
    # Arrange
    original_price = 100
    discount_percentage = 33.333
    expected_discounted_price = 66.67

    # Act
    result = calculate_discount(original_price, discount_percentage)

    # Assert
    expect(result.round(2)).to eq(expected_discounted_price)
  end
end

Ruby

require 'rspec'

RSpec.describe 'calculate_discount' do
  it 'throws an error for negative prices' do
    # Arrange
    original_price = -50
    discount_percentage = 10

    # Act, Assert
    expect { calculate_discount(original_price, discount_percentage) }.to raise_error(ArgumentError, 'Price cannot be negative')
  end
end

Ruby

require 'rspec'

RSpec.describe 'calculate_discount' do
  it 'throws an error for discounts greater than 100 percent' do
    # Arrange
    original_price = 100
    discount_percentage = 110

    # Act, Assert
    expect { calculate_discount(original_price, discount_percentage) }.to raise_error(ArgumentError, 'Discount percentage cannot be > 100%')
  end
end

Ruby

require 'rspec'

RSpec.describe 'calculate_discount' do
  it 'throws an error for discounts less than zero' do
    # Arrange
    original_price = 100
    discount_percentage = -10
    
    # Act, Assert
    expect { calculate_discount(original_price, discount_percentage) }.to raise_error(ArgumentError, 'Discount cannot be negative')
  end
end

Introduction to Red-Green-Refactor Cycle

Understanding the Red-Green-Refactor Cycle

Requirements for `calculate_discount` Method

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Implementation of `calculate_discount` Method

1. Correct Discount Application

2. Zero Discount Handling

3. Decimal Discount Precision

4. Negative Price Handling

5. Discount Percentage Greater Than 100%

6. Negative Discount Percentage

Summary and Preparation for Practice Exercises