Day 4: Functions - One Week Python

📚 What You'll Learn Today

Functions are one of the most powerful concepts in programming:

What functions are and why to use them
Defining and calling functions
Parameters and arguments
Return values
Default parameters

⚙️ What is a Function?

A function is a reusable block of code that does a specific task. Instead of writing the same code over and over, you can put it in a function and call it when needed.

💡 Analogy: Think of a function like a recipe. You write the recipe once (define the function), and then you can follow it anytime you need it (call the function).

Why Use Functions?

Don't repeat yourself (DRY) - Write once, use many times
Organization - Break code into logical chunks
Easy to test - Test each function separately
Reusability - Use functions in other programs

📝 Defining a Function

Use the def keyword to create a function:

Basic Syntax:

def function_name():
    # Code inside the function
    print("Hello!")
            

Calling a Function:

def greet():
    print("Hello!")

# Call the function
greet()
greet()
greet()
            

Hello! Hello! Hello!

📥 Parameters and Arguments

Functions can accept input values called parameters:

Function with Parameter:

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

greet("Alex")
greet("Jordan")
greet("Taylor")
            

Hello, Alex! Hello, Jordan! Hello, Taylor!

Multiple Parameters:

def add(a, b):
    print(a + b)

add(5, 3)
add(10, 20)
add(-1, 1)
            

8 30 0

↩️ Return Values

Functions can send back a value using return:

Basic Return:

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

result = add(5, 3)
print(result)
            

Using Return in Conditionals:

def is_adult(age):
    if age >= 18:
        return True
    else:
        return False

if is_adult(25):
    print("Can vote!")
if is_adult(16):
    print("Can vote!")
else:
    print("Cannot vote yet.")
            

Can vote! Cannot vote yet.

📋 Default Parameters

Give parameters default values that get used if no argument is provided:

With Default Values:

def greet(name, greeting="Hello"):
    print(greeting + ", " + name + "!")

greet("Alex")  # Uses default greeting
greet("Jordan", "Hi")  # Uses custom greeting
greet("Taylor", "Welcome")  # Uses custom greeting
            

Hello, Alex! Hi, Jordan! Welcome, Taylor!

⚠️ Important: Parameters with default values must come AFTER parameters without defaults.

🔄 Scope

Variables inside a function are local - they don't affect variables outside:

def double(x):
    x = x * 2
    print("Inside function:", x)

number = 5
double(number)
print("Outside function:", number)
            

Inside function: 10 Outside function: 5

🏋️ Exercise: Calculator Function

Create a calculator function that performs basic operations:

# Create a function called calculate
# It should take three parameters: num1, operation, num2
# operation can be: "+", "-", "*", "/"
# Return the result

# Example:
# calculate(10, "+", 5) should return 15
# calculate(10, "*", 5) should return 50
            

Bonus Challenge:

Handle division by zero and return "Cannot divide by zero!"

# Example solution:
def calculate(num1, operation, num2):
    if operation == "+":
        return num1 + num2
    elif operation == "-":
        return num1 - num2
    elif operation == "*":
        return num1 * num2
    elif operation == "/":
        if num2 == 0:
            return "Cannot divide by zero!"
        return num1 / num2
    else:
        return "Invalid operation"

print(calculate(10, "+", 5))  # 15
print(calculate(10, "/", 0))  # Error message
            

✅ Key Takeaways

def keyword defines a function
Parameters go inside parentheses after function name
return sends a value back
Arguments are values you pass when calling
Default parameters have values unless overridden
Variables inside functions are local (scope)
Write once, use anywhere - that's DRY!

🎉 Ready for Day 5?

Tomorrow you'll learn about Lists and Dictionaries - storing collections of data!

Marking complete will unlock Day 5!

⚙️ Day 4: Functions

📚 What You'll Learn Today

⚙️ What is a Function?

Why Use Functions?

📝 Defining a Function

Basic Syntax:

Calling a Function:

📥 Parameters and Arguments

Function with Parameter:

Multiple Parameters:

↩️ Return Values

Basic Return:

Using Return in Conditionals:

📋 Default Parameters

With Default Values:

🔄 Scope

🏋️ Exercise: Calculator Function

Bonus Challenge:

✅ Key Takeaways

🎉 Ready for Day 5?