Welcome to the very first lesson of the "Applying Clean Code Principles" course! In this lesson, we will focus on a fundamental concept in clean coding: the DRY ("Don't Repeat Yourself") principle. Understanding DRY is crucial for writing efficient, maintainable, and clean code. This principle is not just important for coding interviews but also in everyday software development. Today, we will dive deep into issues caused by repetitive code and explore strategies to combat redundancy. 🚀

Repetitive functionality in code can introduce several issues that affect the efficiency and maintainability of your software:

• Code Bloat: Repeating similar code across different parts of your application unnecessarily increases the size of the codebase. This makes the code harder to navigate and increases the chances of introducing errors.

• Risk of Inconsistencies: When similar pieces of logic are scattered across different areas, it's easy for them to become out of sync during updates or bug fixes. This can result in logic discrepancies and potentially introduce new problems.

• Maintenance Challenges: Updating code often requires modifications in multiple places, leading to increased work and a higher likelihood of errors. Redundant code makes it difficult for developers to ensure all necessary changes have been made consistently.

To adhere to the DRY principle and avoid repeating yourself, several strategies can be employed:

• Extracting Method: Move repeated logic into a dedicated method that can be called wherever needed. This promotes reuse and simplifies updates.

• Extracting Variable: Consolidate repeated expressions or values into variables. This centralizes change, reducing the potential for errors.

• Replace Temp with Query: Use a method to compute values on demand rather than storing them in temporary variables, aiding in readability and reducing redundancy.

Consider the following problematic code snippet where repetitive logic is used for calculating the total price based on different shipping methods:

Ruby
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1def calculate_click_and_collect_total(order)
2  items_total = 0
3  # Iteration to calculate items_total
4  order.items.each do |item|
5    items_total += item.price * item.quantity
6  end
7  shipping_cost = items_total > 100 ? 0 : 5
8  items_total + shipping_cost + order.tax
9end
10
11def calculate_post_shipment_total(order, is_express)
12  items_total = 0
13  # Duplicate: Iteration to calculate items_total
14  order.items.each do |item|
15    items_total += item.price * item.quantity
16  end
17  shipping_cost = is_express ? items_total * 0.1 : items_total * 0.05
18  items_total + shipping_cost + order.tax
19end

Both methods contain duplicated logic for calculating the total price of items, making them error-prone and hard to maintain. Now, let's refactor this code.

By consolidating the shared logic into a separate method, we can eliminate redundancy and streamline updates:

Ruby
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1def calculate_click_and_collect_total(order)
2  # Calculate items total (duplicated logic)
3  items_total = calculate_items_total(order)
4  shipping_cost = items_total > 100 ? 0 : 5
5  items_total + shipping_cost + order.tax
6end
7
8def calculate_post_shipment_total(order, is_express)
9  # Calculate items total (duplicated logic)
10  items_total = calculate_items_total(order)
11  shipping_cost = is_express ? items_total * 0.1 : items_total * 0.05
12  items_total + shipping_cost + order.tax
13end
14
15def calculate_items_total(order)
16  # Extracted method for items total calculation
17  items_total = 0
18  order.items.each do |item|
19    items_total += item.price * item.quantity
20  end
21  items_total
22end

By extracting the calculate_items_total method, we centralize the logic of item total calculation, leading to cleaner, more maintainable code.

Let's look at another example dealing with repeated calculations for discount rates:

Ruby
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1def apply_discount(price, customer)
2  loyalty_discount = customer.loyalty_level * 0.02
3  price *= (1 - loyalty_discount)
4  # Additional discounts
5  seasonal_discount = 0.10
6  price *= (1 - seasonal_discount)
7  price
8end

Here, the discount rates are scattered throughout the code, which complicates management and updates.

We can simplify this by extracting the discount rates into variables:

Ruby
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1def apply_discount(price, customer)
2  loyalty_discount = customer.loyalty_level * 0.02
3  seasonal_discount = 0.10
4
5  total_discount = 1 - loyalty_discount - seasonal_discount
6  price *= total_discount
7  
8  price
9end

Now, with the total_discount variable, the logic is cleaner, more readable, and allows changes in just one place. 🎉

Our final example involves temporary variables that lead to repetition:

Ruby
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1def eligible_for_discount?(customer)
2  # Logic to determine new_customer
3  new_customer = customer.sign_up_date > Date.today << 3
4  new_customer && customer.purchase_history.size > 5
5end
6
7def eligible_for_loyalty_program?(customer)
8  # Duplicate: Logic to determine new_customer
9  new_customer = customer.sign_up_date > Date.today << 3
10  new_customer || customer.loyalty_level > 3
11end

The variable new_customer is used in multiple places, causing duplicated logic.

Let's refactor by extracting the logic into a method, reducing duplication and enhancing modularity:

Ruby
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1def eligible_for_discount?(customer)
2  new_customer?(customer) && customer.purchase_history.size > 5
3end
4
5def eligible_for_loyalty_program?(customer)
6  new_customer?(customer) || customer.loyalty_level > 3
7end
8
9def new_customer?(customer)
10  customer.sign_up_date > Date.today << 3
11end

By creating the new_customer? method, we've simplified the code and made it more maintainable. 🚀

In this lesson, you learned about the DRY principle and strategies like Extracting Method, Extracting Variable, and Replace Temp with Query to eliminate code redundancy. These strategies help to create code that is easier to maintain, enhance, and understand. Next, you'll have the opportunity to apply these concepts in practical exercises, strengthening your ability to refactor code and uphold clean coding standards. Happy coding! 😊