Chapter 7: If-Elif-Else Statements¶

🚀 Open Notebook¶

📺 Video Tutorial¶

If statements in Python are easy (if, elif, else) 🤔 (12:07)

📚 What You’ll Learn¶

Make your programs smart with conditional logic! Learn how to make decisions based on different conditions.

🎯 Learning Objectives¶

Understand if, elif, and else statements
Use comparison operators (==, !=, >, <, >=, <=)
Create programs that make decisions
Chain multiple conditions together
Understand the flow of conditional execution

📖 Concept Explanation¶

What are Conditional Statements?¶

Conditional statements let your program make decisions and execute different code based on whether conditions are True or False.

Syntax¶

Basic If Statement¶

if condition:
    # This code runs if condition is True
    print("Condition is true!")

If-Else Statement¶

if condition:
    # Runs if condition is True
else:
    # Runs if condition is False

If-Elif-Else Chain¶

if condition1:
    # Runs if condition1 is True
elif condition2:
    # Runs if condition1 is False and condition2 is True
elif condition3:
    # Runs if condition1 and condition2 are False, and condition3 is True
else:
    # Runs if all conditions above are False

Comparison Operators¶

Operator	Meaning	Example	Result
`==`	Equal to	`5 == 5`	`True`
`!=`	Not equal to	`5 != 3`	`True`
`>`	Greater than	`5 > 3`	`True`
`<`	Less than	`3 < 5`	`True`
`>=`	Greater than or equal	`5 >= 5`	`True`
`<=`	Less than or equal	`3 <= 5`	`True`

Important: Indentation Matters!¶

if age >= 18:
    print("Adult")      # Indented - part of if block
    print("Can vote")   # Indented - part of if block
print("Program ends")   # Not indented - always runs

💡 Examples¶

Example 1: Simple If-Else¶

temperature = 25

if temperature > 30:
    print("It's hot outside!")
else:
    print("It's nice weather!")

Example 2: Multiple Conditions (Elif)¶

score = 85

if score >= 90:
    print("Grade: A")
elif score >= 80:
    print("Grade: B")
elif score >= 70:
    print("Grade: C")
elif score >= 60:
    print("Grade: D")
else:
    print("Grade: F")

Example 3: User Authentication¶

password = input("Enter password: ")

if password == "secret123":
    print("Access granted!")
else:
    print("Access denied!")

Example 4: Even or Odd¶

number = int(input("Enter a number: "))

if number % 2 == 0:
    print(f"{number} is even")
else:
    print(f"{number} is odd")

✍️ Practice Exercises¶

Exercise 1: Voting Eligibility¶

# Ask for age and determine if person can vote (18+)
age = int(input("Enter your age: "))
# Add if-else logic

Exercise 2: Grade Calculator¶

# Ask for a test score (0-100) and print the letter grade
# A: 90-100, B: 80-89, C: 70-79, D: 60-69, F: below 60

Exercise 3: Temperature Advisory¶

# Ask for temperature and give advice:
# > 30: "It's hot! Stay hydrated"
# 20-30: "Nice weather!"
# 10-19: "A bit cool, bring a jacket"
# < 10: "It's cold! Bundle up!"

Exercise 5: Movie Rating System¶

# Ask for age and determine what movies they can watch
# G: All ages
# PG: 7+
# PG-13: 13+
# R: 17+

🔍 Common Mistakes¶

Mistake 1: Using = Instead of ==¶

# Wrong:
if age = 18:  # Error! This is assignment, not comparison
    print("18!")

# Correct:
if age == 18:  # This is comparison
    print("18!")

Mistake 2: Forgetting the Colon¶

# Wrong:
if age > 18  # Error! Missing colon
    print("Adult")

# Correct:
if age > 18:  # Colon is required
    print("Adult")

Mistake 3: Incorrect Indentation¶

# Wrong:
if age > 18:
print("Adult")  # Error! Not indented

# Correct:
if age > 18:
    print("Adult")  # Indented with 4 spaces or 1 tab

Mistake 4: Unreachable Conditions¶

# Wrong order:
if age > 0:
    print("Positive age")
elif age > 18:  # This will never execute!
    print("Adult")

# Correct order (specific conditions first):
if age > 18:
    print("Adult")
elif age > 0:
    print("Positive age")

📝 Logical Operators¶

You can combine conditions using logical operators:

AND - Both conditions must be True¶

if age >= 18 and age < 65:
    print("Working age")

OR - At least one condition must be True¶

if age < 13 or age > 65:
    print("Reduced ticket price")

NOT - Reverses the condition¶

if not is_raining:
    print("Go outside!")

🎮 Real-World Examples¶

Example: ATM Withdrawal¶

balance = 1000
withdrawal = int(input("Amount to withdraw: "))

if withdrawal <= 0:
    print("Invalid amount")
elif withdrawal > balance:
    print("Insufficient funds")
else:
    balance -= withdrawal
    print(f"Withdrawal successful. New balance: ${balance}")

Example: Discount Calculator¶

total = float(input("Enter purchase total: "))

if total >= 100:
    discount = total * 0.20  # 20% off
    print("20% discount applied!")
elif total >= 50:
    discount = total * 0.10  # 10% off
    print("10% discount applied!")
else:
    discount = 0
    print("No discount")

final_price = total - discount
print(f"Final price: ${final_price:.2f}")

🚀 Try It Yourself¶

Create a number guessing game that checks if user’s guess is too high, too low, or correct
Make a BMI calculator that categorizes the result (underweight, normal, overweight, obese)
Build a simple quiz with multiple choice questions
Create a parking fee calculator based on hours parked

📊 Flow Control¶

Remember: In an if-elif-else chain:

Only ONE block will execute
Python checks conditions from top to bottom
Once a True condition is found, it executes that block and skips the rest
The else block is optional and runs if all conditions are False

🎓 Key Takeaways from Video¶

Use if-elif-else for conditional logic
Follow along with the video for hands-on practice
Experiment with the code examples to deepen understanding

💡 These points cover the main concepts from the video tutorial to help reinforce your learning.

🔗 Next Chapter¶

Continue to Chapter 8: Calculator to build a functional calculator program!