Functions with Parameters and Return Values: Reusable Logic

Functions are more than shortcuts. They’re the foundation of modular, scalable code. When you add parameters and return values, your functions become flexible tools that adapt to different inputs and deliver useful results.

1. Why Parameters and Return Values Matter

• Parameters let you customize behavior
• Return values let you reuse results
• Together, they turn static code into dynamic logic

Real-Life Analogy

Think of a blender:

• You choose ingredients (parameters)
• You press blend (function call)
• You get a smoothie (return value)

2. Defining Functions with Parameters

python

def greet(name):
    print(f"Hello, {name}!")

You can now greet anyone:

python

greet("Sherif")
greet("Amina")

Multiple Parameters

python

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

Call it with different numbers:

python

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

3. Returning Values from Functions

python

def multiply(a, b):
    return a * b

result = multiply(4, 6)
print(result)  # Output: 24

You can store, reuse, or pass the result to other functions.

4. Function Anatomy Recap

python

def function_name(parameters):
    # logic
    return result

• def — defines the function
• function_name — what you call it
• parameters — input values
• return — output value

5. Real-World Examples

Calculate Area of a Rectangle

python

def area(length, width):
    return length * width

Convert Celsius to Fahrenheit

python

def c_to_f(celsius):
    return (celsius * 9/5) + 32

Check if a Number Is Prime

python

def is_prime(n):
    if n < 2:
        return False
    for i in range(2, int(n**0.5)+1):
        if n % i == 0:
            return False
    return True

6. Default Parameters

You can set default values:

python

def greet(name="Guest"):
    print(f"Hello, {name}!")

Call it with or without a name:

python

greet("Sherif")
greet()

7. Keyword Arguments

Specify parameters by name:

python

def introduce(name, age):
    print(f"{name} is {age} years old.")

introduce(age=28, name="Sherif")

8. Return vs. Print

• print() shows output on screen
• return sends output back to the caller

python

def square(n):
    return n * n

print(square(5))  # Output: 25

9. Using Returned Values in Other Functions

python

def double(n):
    return n * 2

def triple(n):
    return n * 3

def combine(x):
    return double(x) + triple(x)

print(combine(4))  # Output: 20

10. Functions That Return Multiple Values

python

def stats(numbers):
    return min(numbers), max(numbers), sum(numbers)

low, high, total = stats([3, 7, 2, 9])
print(low, high, total)

11. Real-World Projects Using Functions

• Budget calculator: Input expenses, return total
• Quiz app: Input answers, return score
• Weather converter: Input Celsius, return Fahrenheit
• Login system: Input credentials, return access status
• Inventory tracker: Input item, return availability

12. Common Mistakes

• Forgetting to use return
• Mixing up print() and return
• Not passing required parameters
• Using global variables instead of parameters
• Returning inside a loop without a base case

13. Exercises for Mastery

1. Write a function that returns the square of a number
2. Write a function that returns the longest word in a sentence
3. Write a function that returns the average of a list
4. Write a function that returns True if a string is a palindrome
5. Write a function that returns the factorial of a number

14. Best Practices

• Use descriptive names (calculate_tax, not ct)
• Keep functions short and focused
• Use comments or docstrings
• Avoid side effects (changing global state)
• Test with different inputs

15. Final Thoughts

Functions with parameters and return values are the building blocks of real-world programming. They let you write clean, reusable, testable code, and build systems that adapt to different inputs and deliver meaningful results.