Functions

Chapter 6

FREE Functions — Reusable Code Blocks

Article · 15 min

Have you noticed yourself writing the same code over and over? Functions let you package code into reusable blocks. Once defined, you can use them anywhere!

🔧 Defining Your First Function

Syntax:

def function_name():
    # code to run
    # when the function is called

Example:

def greet():
    print("Hello! Welcome to Python 🎉")

# Call the function
greet()  # Hello! Welcome to Python 🎉
greet()  # Hello! Welcome to Python 🎉

🔑 def keyword → function name → parentheses () → colon : → indented body.

Practice 1

✏️ Try it yourself!

1 Code

Write a function called say_hello that prints "Hi there!" three times.

2 Code

Write a function called show_menu that prints a menu with 3 options.

📥 Functions with Parameters

Syntax:

def function_name(parameter1, parameter2):
    # use parameter1 and parameter2

Example:

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

greet_person("Alice")   # Hello, Alice! 👋
greet_person("Bob")     # Hello, Bob! 👋

Practice 2

✏️ Try it yourself!

1 Code

Write a function double(n) that prints twice the number passed to it.

2 Code

Write a function welcome(name, subject) that prints "Welcome, [name]! Let's learn [subject]."

📤 Returning Values

Syntax:

def function_name():
    return value

Examples:

def square(x):
    return x * x

result = square(5)
print(result)  # 25

def is_even(n):
    return n % 2 == 0

print(is_even(10))  # True
print(is_even(7))   # False

💡 return vs print: return sends a value back to the caller. print displays something on screen. A function without return returns None.

Practice 3

✏️ Try it yourself!

1 Code

Write a function add(a, b) that returns the sum of two numbers.

2 Code

Write a function is_positive(n) that returns True if n is greater than 0.

3 Code

Write a function rectangle_area(length, width) that returns the area.

🎯 Default Parameters

def power(base, exponent=2):
    return base ** exponent

print(power(5))      # 25   (5²)
print(power(2, 10))  # 1024 (2¹⁰)

Practice 4

✏️ Try it yourself!

1 Code

Write a function multiply(a, b=1) that returns a × b. If b is not given, return a.

📋 Procedure vs Function

In IGCSE, a procedure does something but doesn't return a value. A function returns a value.

# PROCEDURE — no return
def display_menu():
    print("1. Add student")
    print("2. View scores")
    print("3. Quit")

# FUNCTION — has return
def get_average(scores):
    total = 0
    for s in scores:
        total += s
    return total / len(scores)

# Calling them
display_menu()
avg = get_average([70, 80, 90])
print(f"Average: {avg}")

Practice 5

✏️ Try it yourself!

1 Code

Write a procedure print_separator() that prints "---".

2 Code

Write a function celsius_to_fahrenheit(c) that returns the converted temperature.

🌍 Local vs Global Variables

score = 10   # GLOBAL
def update_score():
    score = 5   # LOCAL — different variable!
    print(f"Inside: {score}")  # 5

update_score()
print(f"Outside: {score}")    # 10 (unchanged!)

Practice 6

✏️ Try it yourself!

1 Code

What will this print? x = 10; def change(): x = 20; change(); print(x)

📚 Library Routines: MOD, DIV, ROUND, RANDOM

# MOD (%) — remainder
print(17 % 5)   # 2

# DIV (//) — integer division
print(17 // 5)  # 3

# ROUND
print(round(3.7))       # 4
print(round(3.14159, 2))  # 3.14

# RANDOM
import random
print(random.randint(1, 10))   # random 1-10
print(random.random())         # random 0.0-1.0

Practice 7

✏️ Try it yourself!

1 Code

Check if 37 is even using MOD.

2 Code

Generate a random number between 1 and 6 (like a dice).

3 Code

Round 3.14159 to 2 decimal places.

📝 Writing Maintainable Code

# ❌ BAD — what does this do?
def f(a, b):
    c = a
    for d in range(b - 1):
        c = c * a
    return c

# ✅ GOOD — meaningful names + docstring!
def calculate_power(base, exponent):
    """Return base raised to the power of exponent."""
    result = base
    for i in range(exponent - 1):
        result = result * base
    return result

Practice 8

✏️ Try it yourself!

1 Code

Rewrite this poorly-named function with meaningful names: def x(a,b): return a*b/2

2 Code

Add a docstring to your rectangle_area function.

FREE String Handling

Article · 15 min

📝 Homework 12

In IGCSE Computer Science, you need to know four key string operations: length, substring, upper case, and lower case. Let's master them in Python!

📏 Length — `len()`

Syntax:

len(string)

Example:

word = "Hello"
print(len(word))  # 5

sentence = "Hello World"
print(len(sentence))  # 11  (spaces count!)

Practice 1

✏️ Try it yourself!

1 Code

Find the length of your full name.

2 Code

Check if the word "Python" has more than 5 characters.

✂️ Substring — Extracting Part of a String

Syntax — slicing:

string[start:end]     # from start to end-1
string[start:]        # from start to end
string[:end]          # from beginning to end-1

Examples:

text = "Python Programming"

print(text[0:6])   # Python  (chars 0 to 5)
print(text[7:])    # Programming  (from char 7)
print(text[-3:])   # ing  (last 3 chars)
print(text[:6])    # Python  (first 6 chars)

📝 IGCSE Note: In Python, strings are 0-indexed (first char = [0]). In pseudocode, MID(str, start, length) is used. Know both!

Practice 2

✏️ Try it yourself!

1 Code

Extract the first 5 characters from "International".

2 Code

Extract the last 4 characters from "computerscience@gmail.com".

3 Code

Extract "Science" from "Computer Science".

🔤 Upper and Lower Case

Syntax:

string.upper()    # convert to uppercase
string.lower()    # convert to lowercase

Examples:

msg = "Hello World"
print(msg.upper())   # HELLO WORLD
print(msg.lower())   # hello world

# Case-insensitive comparison
answer = input("Continue? (yes/no): ")
if answer.lower() == "yes":
    print("Let's go! 🚀")

Practice 3

✏️ Try it yourself!

1 Code

Ask the user for a colour and check if it's "red" (ignore case).

2 Code

Convert "i love python" to uppercase and print it.

🎯 Exam-Style String Questions

# 1. Count vowels
word = "education"
vowels = 0
for ch in word:
    if ch.lower() in "aeiou":
        vowels += 1
print(f"Vowels: {vowels}")  # 5

# 2. Check file extension
filename = "report.pdf"
if filename[-4:] == ".pdf":
    print("It's a PDF!")

# 3. Extract first name
full_name = "Alice Smith"
space_pos = full_name.find(" ")
first_name = full_name[:space_pos]
print(f"First name: {first_name}")  # Alice

# 4. Reverse a string
word = "Python"
reversed_word = word[::-1]
print(reversed_word)  # nohtyP

Practice 4

✏️ Try it yourself!

1 Code

Count how many times the letter "e" appears in "International Baccalaureate".

2 Code

Extract the domain name from "user@example.com" (everything after @ and before .).

3 Code

Check if a word entered by the user is a palindrome (reads same forwards and backwards).

4 Code

Write a program that capitalises the first letter of each word in a sentence.

Functions

FREE Functions — Reusable Code Blocks

🔧 Defining Your First Function

Practice 1

📥 Functions with Parameters

Practice 2

📤 Returning Values

Practice 3

🎯 Default Parameters

Practice 4

📋 Procedure vs Function

Practice 5

🌍 Local vs Global Variables

Practice 6

📚 Library Routines: MOD, DIV, ROUND, RANDOM

Practice 7

📝 Writing Maintainable Code

Practice 8

FREE String Handling

📏 Length — len()

Practice 1

✂️ Substring — Extracting Part of a String

Practice 2

🔤 Upper and Lower Case

Practice 3

🎯 Exam-Style String Questions

Practice 4

📏 Length — `len()`