Welcome to Module 4! Up until now, if you wanted to perform a specific task (like calculating a tax or greeting a user) three different times in your program, you had to copy and paste that code three times.

In programming, we have a rule called DRY: Don’t Repeat Yourself. To follow this rule, we use Functions.

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1. What is a Function?

A function is a named block of code that performs a specific task. You “define” it once, and then you can “call” it (use it) whenever you need it.

Think of it like a recipe in a cookbook. The recipe is the definition, and actually cooking the meal is the call.

The Code:

Python

# 1. Defining the function
def say_hello():
    print("Hello there!")
    print("Welcome to Module 4.")

# 2. Calling the function
say_hello()
say_hello()

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2. Parameters: Giving Data to Functions

Functions become much more powerful when you give them information to work with. These pieces of information are called Parameters.

The Code:

Python

def greet_person(name):
    print("Hello, " + name + "!")

greet_person("Alice")
greet_person("Bob")

In this example, name is the parameter. “Alice” and “Bob” are the data we send into that parameter.

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3. The return Statement

Sometimes, you don’t want the function to just print something. You want it to do a calculation and give the answer back to you so you can use it later.

The Code:

Python

def add_numbers(a, b):
    return a + b

# We save the 'returned' answer into a variable
total = add_numbers(10, 5)
print("The total is: " + str(total))

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4. Local vs. Global Scope

A quick but important rule: variables created inside a function stay inside that function. This is called Local Scope.

Python

def my_function():
    x = 10  # This x only exists inside here
    print(x)

my_function()
# print(x)  <-- This would cause an error!

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🎓 Module 4 Final Project: “The Tip Calculator” 💰

Let’s build a tool that helps friends split a bill at a restaurant.

The Challenge:

1. Create a function called calculate_tip.
2. The function should take two parameters: bill_amount and tip_percentage.
3. The function should return the total amount to pay (Bill + Tip).
4. Ask the user for their bill and tip, call the function, and print the final result.

The Solution:

Try to structure the function before looking at the code.

Python

# 1. Define the function
def calculate_tip(bill, tip_percent):
    tip_amount = bill * (tip_percent / 100)
    return bill + tip_amount

2. Get user input

user_bill = float(input(“How much was the bill? “)) user_tip = float(input(“What percentage tip (e.g. 15)? “))

3. Use the function

final_price = calculate_tip(user_bill, user_tip) print(“Your total bill with tip is: $” + str(final_price))

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Next Up: In Module 5, we’ll learn how to handle large amounts of data using Lists and Dictionaries!