Chapter 6: Conditions

Teaching Objectives

By the end of this chapter, students should:

• Understand the concept of conditional statements
• Master the syntax and usage of if, elif, and else statements
• Know how to use comparison and logical operators within conditions
• Apply nested conditionals to solve more complex problems
• Understand the concept of conditional expressions (ternary operator)

Preparation

Before teaching this chapter, ensure:

• Students understand basic operators (comparison and logical)
• You have prepared examples with clear, relatable scenarios
• You have created diagrams to visualize how conditional execution works

Lesson Overview

1. Introduction to Conditional Statements (15 minutes)

Start by explaining what conditionals are and why they're important:

• Conditionals allow programs to make decisions based on certain conditions
• They enable different code paths to execute depending on whether conditions are true or false
• They are fundamental to programming logic and problem-solving

Analogy: Conditionals are like crossroads in a journey - based on specific conditions, you choose which path to take.

Real-world examples:

• Checking the weather to decide what to wear
• Deciding whether to bring an umbrella based on the forecast
• Checking if you have enough money before buying something

2. Basic If Statements (20 minutes)

Introduce the basic structure of an if statement:

# Basic if statement
age = 15
if age >= 13:
    print("You can watch a PG-13 movie")
    
# Another example
temperature = 28
if temperature > 25:
    print("It's hot today!")
    print("Remember to drink water")

Explain the components:

• The if keyword begins the conditional block
• The condition (age >= 13 or temperature > 25) evaluates to either True or False
• The indented block runs only if the condition is True
• Multiple statements can be included in the indented block

Teaching points:

• Emphasize the importance of proper indentation
• Show how the condition must evaluate to a boolean value
• Demonstrate how to use comparison operators in conditions

3. If-Else Statements (15 minutes)

Expand to include the else clause:

# If-else example
age = 10
if age >= 13:
    print("You can watch a PG-13 movie")
else:
    print("Sorry, this movie is not suitable for your age")
    
# Another example
has_ticket = True
if has_ticket:
    print("Welcome to the concert!")
else:
    print("You need to purchase a ticket first")

Teaching approach:

• Explain that the else block executes when the condition is False
• Use flowcharts to visually represent the decision paths
• Demonstrate that exactly one of the blocks will always execute

4. If-Elif-Else Statements (20 minutes)

Introduce multiple conditions with elif:

# Grade example
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")
    
# Weather example
weather = "cloudy"

if weather == "sunny":
    print("Wear sunglasses")
elif weather == "rainy":
    print("Bring an umbrella")
elif weather == "snowy":
    print("Wear a warm coat")
elif weather == "cloudy":
    print("You might want a light jacket")
else:
    print("Check the weather forecast")

Teaching points:

• Explain that elif is short for "else if"
• Conditions are checked in order, and the first true condition has its block executed
• Once a condition is met, the remaining conditions are skipped
• The else block is optional and runs only if all conditions are False

5. Nested Conditionals (15 minutes)

Show how conditionals can be placed inside other conditionals:

# Nested if example
age = 15
has_permission = True

if age >= 13:
    if has_permission:
        print("You can play the game")
    else:
        print("You need parental permission")
else:
    print("You are too young for this game")
    
# Ticket pricing example
age = 65
is_student = False

if age < 18:
    print("Child ticket: $5")
else:
    if age >= 65:
        print("Senior ticket: $8")
    else:
        if is_student:
            print("Student ticket: $8")
        else:
            print("Adult ticket: $10")

Teaching approach:

• Use indentation to clearly show the nesting structure
• Explain that each level of nesting represents a further refinement of conditions
• Show how nested conditions can often be rewritten using logical operators

# Rewriting the nested condition with logical operators
if age >= 13 and has_permission:
    print("You can play the game")
elif age >= 13:  # and not has_permission
    print("You need parental permission")
else:
    print("You are too young for this game")

6. Using Logical Operators in Conditions (15 minutes)

Demonstrate combining conditions with logical operators:

# Using 'and' operator
age = 25
income = 50000

if age > 21 and income > 40000:
    print("You qualify for the premium credit card")
else:
    print("You do not qualify for the premium credit card")
    
# Using 'or' operator
is_weekend = True
is_holiday = False

if is_weekend or is_holiday:
    print("The store is open from 10 AM to 6 PM")
else:
    print("The store is open from 9 AM to 9 PM")
    
# Using 'not' operator
is_raining = False

if not is_raining:
    print("You can go for a walk")
else:
    print("Better stay indoors")

Teaching tips:

• Review truth tables for logical operators
• Explain the short-circuit evaluation of logical operators
• Show how to use parentheses to clarify complex conditions

# Complex condition with parentheses
if (age > 18 and income > 30000) or has_guarantor:
    print("Loan approved")
else:
    print("Loan denied")

7. Conditional Expressions (Ternary Operator) (10 minutes)

Introduce the conditional expression syntax for simple conditions:

# Traditional if-else
age = 20
if age >= 18:
    status = "adult"
else:
    status = "minor"
    
# Same logic using conditional expression
age = 20
status = "adult" if age >= 18 else "minor"

print(status)  # Outputs: adult

# Another example
num = -5
absolute = num if num >= 0 else -num
print(absolute)  # Outputs: 5

Teaching approach:

• Explain that this is a more compact way to write simple if-else statements
• Emphasize that it should only be used for simple conditions to maintain readability
• Show the general syntax: value_if_true if condition else value_if_false

8. Guided Practice (20 minutes)

Have students work through these exercises:

1. Movie ticket prices:

   # Determine ticket price based on age
   age = int(input("Enter your age: "))
   
   if age < 5:
       price = 0
   elif age < 13:
       price = 8
   elif age < 65:
       price = 12
   else:
       price = 10
       
   print(f"Your ticket costs ${price}")
   

2. Simple calculator:

   # Basic calculator with error handling
   num1 = float(input("Enter first number: "))
   num2 = float(input("Enter second number: "))
   operation = input("Enter operation (+, -, *, /): ")
   
   if operation == "+":
       result = num1 + num2
   elif operation == "-":
       result = num1 - num2
   elif operation == "*":
       result = num1 * num2
   elif operation == "/":
       if num2 == 0:
           print("Error: Cannot divide by zero")
       else:
           result = num1 / num2
   else:
       print("Invalid operation")
       result = None
       
   if result is not None:
       print(f"Result: {result}")
   

9. Problem-Solving Activities (15 minutes)

Present students with more challenging problems:

1. Leap year checker:

   # Check if a year is a leap year
   year = int(input("Enter a year: "))
   
   if (year % 4 == 0 and year % 100 != 0) or (year % 400 == 0):
       print(f"{year} is a leap year")
   else:
       print(f"{year} is not a leap year")
   

2. BMI calculator:

   # Calculate BMI and determine category
   weight = float(input("Enter weight in kg: "))
   height = float(input("Enter height in meters: "))
   
   bmi = weight / (height ** 2)
   print(f"Your BMI is: {bmi:.1f}")
   
   if bmi < 18.5:
       category = "Underweight"
   elif bmi < 25:
       category = "Normal weight"
   elif bmi < 30:
       category = "Overweight"
   else:
       category = "Obese"
       
   print(f"Category: {category}")
   

10. Review and Discussion (10 minutes)

• Review the key concepts covered
• Ask students to explain in their own words:
  • What is a conditional statement?
  • When would you use elif versus multiple if statements?
  • How do logical operators help with conditions?

Common Challenges and Solutions

• Indentation errors: Emphasize the importance of consistent indentation in Python.
• Condition confusion: Help students understand the difference between = (assignment) and == (comparison).
• Logical operator misconceptions: Review logical operator precedence and short-circuit evaluation.
• Nested conditionals complexity: Encourage students to refactor nested conditionals when possible for readability.

Extension Activities

For students who finish early:

• Challenge them to create a rock-paper-scissors game using conditionals
• Have them build a simple quiz program with score tracking
• Ask them to implement a password strength checker with multiple criteria

Assessment

Look for these indicators of understanding:

• Students can write syntactically correct conditional statements
• They can create programs that make appropriate decisions based on inputs
• They understand how to combine conditions using logical operators
• They can trace the execution path of code with conditionals

Resources

• Python Documentation on Control Flow
• Interactive Conditional Visualizer
• Practice Problems with Conditionals