In real life, we make decisions every day: "If it's raining, take an umbrella. Otherwise, wear sunglasses."

In programming, we use selection statements to make decisions. Python gives us if, elif, and else.

🧠 The if Statement

Syntax:

if condition:
    # code runs if condition is True

Example:

age = 18
if age >= 18:
    print("You are an adult.")   # ✅ This runs

age = 15
if age >= 18:
    print("This won't print")    # ❌ Condition is False

🔑 Notice the colon : at the end of the if line, and the indentation (4 spaces) before print. Python uses indentation to know which code belongs to the if.

🧠 if...else

Syntax:

if condition:
    # runs if True
else:
    # runs if False

Example:

score = 45
if score >= 50:
    print("Pass ✅")
else:
    print("Fail ❌ — better luck next time!")

🧠 if...elif...else Chain

Syntax:

if condition1:
    # runs if condition1 is True
elif condition2:
    # runs if condition1 is False and condition2 is True
else:
    # runs if none of the above are True

Example — grade calculator:

mark = 75

if mark >= 90:
    grade = "A*"
elif mark >= 80:
    grade = "A"
elif mark >= 70:
    grade = "B"
elif mark >= 60:
    grade = "C"
else:
    grade = "D"

print(f"Your grade is {grade}")  # B

🎯 Logical Operators: and, or, not

Syntax:

if condition1 and condition2:   # both True
if condition1 or condition2:    # at least one True
if not condition:               # the opposite

Examples:

age = 20
has_ticket = True

if age >= 18 and has_ticket:
    print("Welcome to the show! 🎭")

if age < 12 or age > 65:
    print("You get a discount! 🎉")

if not has_ticket:
    print("Please buy a ticket first.")

🔗 Relational Operators

🪆 Nested IF Statements

Syntax — an if inside an if:

if outer_condition:
    if inner_condition:
        # both are True

Example:

age = 20
has_id = True

if age >= 18:
    if has_id:
        print("Welcome! 🎉")
    else:
        print("Need ID first.")
else:
    print("Too young.")

🔑 Each level of nesting adds 4 more spaces. Keep it to 3 levels max in exams!

Writing code is only half the story. You also need to check inputs are correct and test your program thoroughly. Let's look at three key IGCSE topics.

✅ Validation — Checking Input Data

Validation checks if data is reasonable and complete before the program uses it.

1. Range check — is the value within limits?

age = int(input("Enter age (0-120): "))
if age < 0 or age > 120:
    print("Invalid age! ❌")

2. Type check — is it the right kind of data?

try:
    score = int(input("Enter score: "))
    print(f"Valid score: {score}")
except ValueError:
    print("That's not a number! ❌")

3. Length check — is it the right length?

password = input("Create password (min 8 chars): ")
if len(password) < 8:
    print("Too short! ❌")

4. Presence check — was something entered?

name = input("Enter your name: ")
if name == "":
    print("Name cannot be empty! ❌")

5. Format check — does it match a pattern?

email = input("Enter email: ")
if "@" not in email or "." not in email:
    print("Invalid email format! ❌")

🧪 Test Data — Normal, Abnormal, Boundary, Extreme

When testing, you need different kinds of test data:

📋 Trace Tables — Dry-Run Your Code

A trace table lets you step through an algorithm on paper, tracking how variable values change.

Example — find the highest score:

scores = [3, 7, 2]
highest = scores[0]
for i in range(1, len(scores)):
    if scores[i] > highest:
        highest = scores[i]
print(highest)