Writing Your Own Functions

Functions in Python

Functions are defined with def. They accept parameters and can return values with return:

def greet(name, hour=12):
    if hour < 12:
        return f"Good morning, {name}!"
    elif hour < 18:
        return f"Good afternoon, {name}!"
    else:
        return f"Good evening, {name}!"

print(greet("Alice"))       # Good afternoon, Alice!
print(greet("Bob", 8))      # Good morning, Bob!
print(greet("Carol", 20))   # Good evening, Carol!

Here hour has a default value (12) — if not provided, Python uses it.

Multiple return values

def min_max(numbers):
    return min(numbers), max(numbers)

low, high = min_max([3, 1, 7, 2, 9])
print(low, high)  # 1 9

Your task

Write a function is_prime(n) that returns True if n is a prime number, otherwise False.

A prime number is divisible only by 1 and itself. 2, 3, 5, 7, 11 are primes. 1 is not.