Chapter 4: If Statements

How If Statements Work

The Basic Idea

An if statement asks a question that has a yes or no answer. In Python, that answer is either True or False — these are called boolean values.

• If the condition is True, the indented block of code runs.
• If the condition is False, the indented block is skipped.

# You can see boolean values directly
print(10 > 5)     # True
print(3 == 7)     # False
print(4 != 4)     # False
print(6 <= 6)     # True

Comparison Operators

These operators compare two values and return True or False. In your GCSE, these are also called relational operators:

Logical Operators: Combining Conditions

Sometimes one condition is not enough. You can combine conditions using and, or, and not:

• and — both conditions must be true. "If it is Saturday and it is sunny, go to the park."
• or — at least one condition must be true. "If it is Saturday or Sunday, have a lie-in."
• not — flips the result. "If it is not raining, walk to school."

age = 15
has_permission = True

if age >= 13 and has_permission:
    print("You can create an account.")
else:
    print("Sorry, you cannot sign up yet.")

# Try changing age or has_permission to see different results

Truth Tables

A truth table shows every possible combination of inputs for a logical operator and what the result will be. These are essential for your GCSE — you need to be able to read and create them. Think of A and B as two conditions.

AND Truth Table

With and, the result is True only when both inputs are True:

OR Truth Table

With or, the result is True when at least one input is True:

NOT Truth Table

With not, the result is simply the opposite of the input:

# Verify the truth tables yourself!
print("--- AND ---")
print(True and True)    # True
print(True and False)   # False
print(False and True)   # False
print(False and False)  # False

print("--- OR ---")
print(True or True)     # True
print(True or False)    # True
print(False or True)    # True
print(False or False)   # False

print("--- NOT ---")
print(not True)         # False
print(not False)        # True

Indentation Matters!

Unlike many other programming languages, Python uses indentation (spaces at the start of a line) to define which lines belong to an if block. Every line inside the block must be indented by the same amount — typically four spaces.

# Correct - the indented lines belong to the if block
weather = "rainy"

if weather == "rainy":
    print("Take an umbrella!")
    print("Wear a waterproof coat.")

print("Have a good day!")  # This always runs (not indented)

Common Misconception: = vs ==

# Assignment vs Comparison - run this to see the difference
favourite_colour = "blue"    # = assigns the value

if favourite_colour == "blue":   # == checks the value
    print("Great choice! Blue it is.")
else:
    print("That's a nice colour too.")

Selection in the Real World

Selection is not just a classroom concept — it is everywhere in the software you use every day:

• Login systems: If the username and password both match the stored values, grant access. Otherwise, show an error. This is a classic use of the and operator.
• E-commerce: If the basket total is greater than £50, apply free delivery. Online shops use selection to calculate discounts and promotional offers.
• Games: If player_health <= 0, trigger "game over". Games use thousands of if statements to manage health, scores, and level progression.
• Smart homes: If the temperature is below 18°C and the time is after 6pm, turn on the heating. Smart thermostats like Nest use selection logic constantly.
• Social media: If a user's age is less than 13, block account creation. This is required by law under rules like COPPA (Children's Online Privacy Protection Act).

Key Vocabulary

Make sure you understand these key terms — they will appear in your GCSE exam:

Worked Example: Building a Grade Calculator

Let us walk through a complete example step by step. We need a program that takes a student's score (0–100) and gives them a grade:

• A* — 90 and above
• A — 80 to 89
• B — 70 to 79
• C — 60 to 69
• D — 50 to 59
• U — below 50 (ungraded)

Step 1: Think about the order. We start with the highest grade and work downwards. Because Python stops at the first true condition, we only need to check the lower boundary of each grade.

Step 2: Handle invalid input. What if someone enters 105 or -3? A good program checks for this.

Step 3: Add a motivational message. We can use another if/elif/else to print encouragement based on the grade.

# Worked Example: Grade Calculator
score = 73

# Step 2: Check for invalid input first
if score < 0 or score > 100:
    print("Error: Score must be between 0 and 100.")
else:
    # Step 1: Determine the grade (highest boundary first)
    if score >= 90:
        grade = "A*"
    elif score >= 80:
        grade = "A"
    elif score >= 70:
        grade = "B"
    elif score >= 60:
        grade = "C"
    elif score >= 50:
        grade = "D"
    else:
        grade = "U"

    print("Score: " + str(score) + "/100")
    print("Grade: " + grade)

    # Step 3: Motivational message
    if grade == "A*" or grade == "A":
        print("Outstanding work! Keep it up!")
    elif grade == "B" or grade == "C":
        print("Good effort. A bit more revision could push you higher!")
    elif grade == "D":
        print("You passed, but there is room for improvement.")
    else:
        print("Don't give up! Let's work on the areas you found tricky.")

Common Mistakes to Avoid