Selection & Operators

Before you can write a real Paper 4 program, you need the building blocks: storing information, receiving it from a user, and making decisions. Python follows the same logic as pseudocode — the only difference is exact syntax.

Python has five data types you need for this syllabus:

# Type casting — converting between types
x = "42"          # str — cannot do arithmetic
y = int(x)        # now y is the integer 42
z = float(x)      # z is 42.0

int("15")     # -> 15
float("3.14") # -> 3.14
str(100)      # -> "100"
bool(0)       # -> False
bool(1)       # -> True

Almost every Paper 4 program starts by reading values from the user and ends by printing results. Forgetting to convert input() is the single most common mistake in Paper 4.

Output — print() and f-strings

# Basic print
print("Hello, World!")
print(42)

# f-string — variables go inside { }
name = "Omar"
age = 16
print(f"Name: {name}, Age: {age}")

Input — input() always returns a string

# input() always returns str — convert immediately if you need arithmetic
name = input("Enter your name: ")          # str, no conversion
age = int(input("Enter your age: "))        # convert to int
price = float(input("Enter price: "))       # convert to float

Lab Tasks — Practice I/O

Conditional statements are how a program makes decisions. In Paper 4 you will almost always need at least one if. Cambridge marks carefully: correct indentation and the right branch structure are both required.

# Pseudocode: IF age >= 18 THEN ... ENDIF
age = int(input("Enter your age: "))
if age >= 18:
    print("You are eligible to vote.")

age = int(input("Enter your age: "))
if age >= 18:
    print("You are eligible to vote.")
else:
    print("You are not old enough to vote.")

if-elif-else — more than two outcomes

Use elif when there are more than two possible outcomes. Python tests conditions top to bottom and runs the first branch that is True — then skips the rest.

marks = int(input("Enter marks: "))
if marks >= 90:
    print("Grade: A*")
elif marks >= 80:
    print("Grade: A")
elif marks >= 70:
    print("Grade: B")
elif marks >= 60:
    print("Grade: C")
else:
    print("Grade: F")

A nested if is an if placed inside another if block. Use nesting when you need to check a second condition that only makes sense once the first is already True.

age = int(input("Enter age: "))
hasLicense = input("Have a license? (yes/no): ")

if age >= 17:
    if hasLicense == "yes":
        print("You can drive.")
    else:
        print("You need a license to drive.")
else:
    print("You are not old enough to drive.")

Logical operators — and / or / not

Combine multiple conditions using these three operators:

Cambridge Paper 4 specifically tests // and % because they map directly to DIV and MOD in pseudocode.

Arithmetic operators

Comparison & logical operators

Common mistakes that lose marks

Exam-style Lab Tasks

Fifteen exam-style tasks. Click Hint for bullet-point guidance, then Help to reveal a worked Python solution.