Control Flow: if, elif, else

The if Statement

age = 18

if age >= 18:
    print("You are an adult")

if-else

temperature = 25

if temperature > 30:
    print("It's hot outside")
else:
    print("It's comfortable")

if-elif-else

score = 75

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

print(f"Grade: {grade}")  # Grade: C

Nested if Statements

x = 15

if x > 0:
    if x % 2 == 0:
        print("Positive even number")
    else:
        print("Positive odd number")
else:
    print("Non-positive number")

Ternary Operator (Conditional Expression)

age = 20
status = "adult" if age >= 18 else "minor"
print(status)  # adult

# Nested ternary (use sparingly!)
n = 0
sign = "positive" if n > 0 else "negative" if n < 0 else "zero"
print(sign)  # zero

Pattern Matching (match-case) — Python 3.10+

command = "quit"

match command:
    case "quit":
        print("Quitting...")
    case "help":
        print("Available commands: quit, help, run")
    case "run":
        print("Running...")
    case _:
        print(f"Unknown command: {command}")

Match with Guards

point = (3, 4)

match point:
    case (0, 0):
        print("Origin")
    case (x, 0):
        print(f"On x-axis at {x}")
    case (0, y):
        print(f"On y-axis at {y}")
    case (x, y) if x == y:
        print(f"On diagonal at {x}")
    case (x, y):
        print(f"Point at ({x}, {y})")

Exercises

Exercise 1: FizzBuzz

For numbers 1–20, print "Fizz" if divisible by 3, "Buzz" if by 5, "FizzBuzz" if by both, else the number.

Solution:

for i in range(1, 21):
    if i % 15 == 0:
        print("FizzBuzz")
    elif i % 3 == 0:
        print("Fizz")
    elif i % 5 == 0:
        print("Buzz")
    else:
        print(i)

Exercise 2: BMI Calculator

Calculate BMI = weight(kg) / height(m)². Classify:

• < 18.5: Underweight
• 18.5–24.9: Normal
• 25–29.9: Overweight
• ≥ 30: Obese

Solution:

weight = 70   # kg
height = 1.75 # meters
bmi = weight / height ** 2

if bmi < 18.5:
    category = "Underweight"
elif bmi < 25:
    category = "Normal"
elif bmi < 30:
    category = "Overweight"
else:
    category = "Obese"

print(f"BMI: {bmi:.1f} — {category}")  # BMI: 22.9 — Normal

Exercise 3: Traffic Light

Write a match statement for traffic light colors: Red (stop), Yellow (slow down), Green (go).

Solution:

light = "green"

match light:
    case "red":
        action = "Stop"
    case "yellow":
        action = "Slow down"
    case "green":
        action = "Go"
    case _:
        action = "Unknown signal"

print(action)  # Go