python-by-example/02_control_flow/02_elif.py

"""
================================================================================
File: 02_elif.py
Topic: Multiple Conditions with elif
================================================================================

This file demonstrates how to handle multiple conditions using 'elif' (else if).
When you have more than two possible outcomes, elif allows you to check
multiple conditions in sequence.

Key Concepts:
- elif chains for multiple conditions
- Order matters - first True condition wins
- Default case with else

================================================================================
"""

# -----------------------------------------------------------------------------
# 1. Basic elif Structure
# -----------------------------------------------------------------------------
# Check multiple conditions in sequence

score = 85

print("--- Grade Calculator ---")

if score >= 90:
    grade = "A"
    print("Excellent work!")
elif score >= 80:
    grade = "B"
    print("Good job!")
elif score >= 70:
    grade = "C"
    print("Satisfactory.")
elif score >= 60:
    grade = "D"
    print("You passed, but need improvement.")
else:
    grade = "F"
    print("You need to study harder.")

print(f"Your grade: {grade}")

# -----------------------------------------------------------------------------
# 2. Order Matters in elif
# -----------------------------------------------------------------------------
# The first condition that evaluates to True will be executed

print("\n--- Order Matters Example ---")

number = 15

# Correct order (most specific first)
if number > 20:
    print("Greater than 20")
elif number > 10:
    print("Greater than 10")  # This will execute
elif number > 5:
    print("Greater than 5")
else:
    print("5 or less")

# -----------------------------------------------------------------------------
# 3. Day of the Week Example
# -----------------------------------------------------------------------------
# Classic example showing multiple discrete options

print("\n--- Day of the Week ---")

day = 3  # 1=Monday, 2=Tuesday, etc.

if day == 1:
    day_name = "Monday"
elif day == 2:
    day_name = "Tuesday"
elif day == 3:
    day_name = "Wednesday"
elif day == 4:
    day_name = "Thursday"
elif day == 5:
    day_name = "Friday"
elif day == 6:
    day_name = "Saturday"
elif day == 7:
    day_name = "Sunday"
else:
    day_name = "Invalid day"

print(f"Day {day} is {day_name}")

# -----------------------------------------------------------------------------
# 4. Temperature Categories
# -----------------------------------------------------------------------------
# Real-world example with ranges

print("\n--- Temperature Categories ---")

celsius = 22

if celsius < 0:
    category = "Freezing"
    advice = "Stay indoors if possible!"
elif celsius < 10:
    category = "Cold"
    advice = "Wear a warm jacket."
elif celsius < 20:
    category = "Cool"
    advice = "A light jacket is recommended."
elif celsius < 30:
    category = "Warm"
    advice = "Perfect weather for outdoor activities."
else:
    category = "Hot"
    advice = "Stay hydrated!"

print(f"Temperature: {celsius}°C")
print(f"Category: {category}")
print(f"Advice: {advice}")

# -----------------------------------------------------------------------------
# 5. BMI Calculator Example
# -----------------------------------------------------------------------------
# Practical health-related example

print("\n--- BMI Calculator ---")

weight = 70  # kg
height = 1.75  # meters

bmi = weight / (height ** 2)

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

print(f"BMI: {bmi:.1f}")
print(f"Category: {category}")

# -----------------------------------------------------------------------------
# 6. Age Group Classification
# -----------------------------------------------------------------------------
# Using elif for demographic categories

print("\n--- Age Group Classification ---")

age = 25

if age < 0:
    group = "Invalid age"
elif age < 13:
    group = "Child"
elif age < 20:
    group = "Teenager"
elif age < 30:
    group = "Young Adult"
elif age < 60:
    group = "Adult"
else:
    group = "Senior"

print(f"Age {age} belongs to: {group}")

# -----------------------------------------------------------------------------
# 7. Combined Conditions with elif
# -----------------------------------------------------------------------------
# Using logical operators within elif

print("\n--- Combined Conditions ---")

hour = 14  # 24-hour format
is_weekend = False

if hour < 6:
    greeting = "It's very early!"
elif hour < 12 and not is_weekend:
    greeting = "Good morning, time to work!"
elif hour < 12 and is_weekend:
    greeting = "Good morning, enjoy your day off!"
elif hour < 18:
    greeting = "Good afternoon!"
elif hour < 22:
    greeting = "Good evening!"
else:
    greeting = "Good night!"

print(f"Hour: {hour}:00 - {greeting}")

# -----------------------------------------------------------------------------
# 8. Handling User Input Example
# -----------------------------------------------------------------------------

print("\n--- Menu Selection Example ---")

# Simulating user choice (in real code, you'd use input())
choice = "B"

if choice == "A" or choice == "a":
    print("You selected: Start Game")
elif choice == "B" or choice == "b":
    print("You selected: Load Game")
elif choice == "C" or choice == "c":
    print("You selected: Settings")
elif choice == "Q" or choice == "q":
    print("Goodbye!")
else:
    print("Invalid choice. Please try again.")