3.4 · Selection (if / elif / else)

Goal: make decisions with binary and multi-way selection.

Binary selection

if score >= 50:
    print("Pass")
else:
    print("Fail")

Multi-way selection

if score >= 80:
    grade = "A"
elif score >= 70:
    grade = "B"
elif score >= 60:
    grade = "C"
else:
    grade = "F"
print(grade)

Important: Python evaluates elif only when previous conditions are false. The order matters!

Boolean combinations

if age >= 18 and has_id:
    print("Eligible")

if temperature < 0 or temperature > 40:
    print("Extreme")

if not member:
    print("Please sign up")

Indentation rules

Python uses indentation (4 spaces conventionally) to define blocks. Wrong indentation → syntax error or logic error.

if score >= 50:
    print("Pass")     # part of the if
    print("Well done")
print("Goodbye")       # always runs (outside if)

Nested if (allowed and common)

if logged_in:
    if user_type == "admin":
        print("Welcome, admin")
    else:
        print("Welcome, user")
else:
    print("Please log in")

For complex logic, prefer combining conditions with and/or rather than deep nesting.

Worked example · Day-of-week classification

day = input("Day? ").lower()

if day in ("sat", "sun"):
    print("Weekend")
elif day in ("mon", "tue", "wed", "thu", "fri"):
    print("Weekday")
else:
    print("Unknown day")

Common student mistakes

• Confusing = (assignment) and == (comparison).
• Forgetting the : after the if condition.
• Misaligning indentation (mixing tabs and spaces).
• Writing if 0 < x < 10: works in Python (chained comparison) but is unusual in other languages.

Exam-style question

Q (5 marks): Write a Python program that reads an integer and prints whether it is positive, negative, or zero. Validate the input is an integer.

Sample answer:

raw = input("Enter an integer: ")
try:
    n = int(raw)
    if n > 0:
        print("Positive")
    elif n < 0:
        print("Negative")
    else:
        print("Zero")
except ValueError:
    print("Not a valid integer")

Key takeaways

• if, elif, else with mandatory : and indented blocks.
• Use and, or, not to combine conditions.
• Order of elif matters when conditions overlap.

➡️ Next: 3.5 Iteration