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Interactive Python Programming Textbook

Module 1: Python Programming

Control Flow: Decisions and Loops

Last Updated: December 2024 Author:Dr. Deepak D. Shudhalwar, Professor PSSCIVE

Understanding Program Flow

In our daily lives, we encounter situations where we must follow fixed sequences of steps to complete tasks, while in others we have choices. Understanding these patterns is crucial for programming.

Airport Boarding Process

Show ticket and identity proof at the main entrance of the Airport.
Scanning of luggage at the security checkpoint.
Take your boarding pass from the check-in counter.
Pass security check including personal screening.
Finally, board the plane at the designated gate.

This fixed sequence represents sequential flow in programming.

Note: Passengers can take their boarding pass from the airline's check-in counter OR they can also get a boarding pass at an electronic kiosk nearby. This represents conditional execution in programming.

Programming Analogy

Just like passengers have multiple ways to get boarding passes (counter OR kiosk), programs often need to make decisions:

IF the passenger chooses counter, THEN follow counter procedure
ELSE IF passenger chooses kiosk, THEN follow kiosk procedure
ELSE print error message

Try It Yourself: Python Control Flow

Practice the concepts you've learned by modifying the code below. This interactive editor simulates Python execution in your browser.

Python Code Editor

# Airport Boarding Process Simulation
# Try modifying the conditions below!

boarding_method = "kiosk"  # Try changing to "counter" or "invalid"

print("=== Airport Boarding Process ===")
print("1. Show ticket and ID at entrance ✓")
print("2. Luggage scanning ✓")

# Decision making - similar to passenger choices
if boarding_method == "counter":
    print("3. Get boarding pass from check-in counter")
elif boarding_method == "kiosk":
    print("3. Get boarding pass from electronic kiosk")
else:
    print("3. ERROR: Invalid boarding method selected!")

print("4. Pass security check ✓")
print("5. Board the plane ✓")
print("\nProcess complete! Have a safe flight! ✈️")

# Try adding more conditions below:
# Add your own code here to practice

Output will appear here after running the code...

Page 1

Interactive Python Programming Textbook

Module 1: Python Programming

Session 1: Control Structures in Python

Last Updated: December 2024 Author: Dr. Deepak D. Shudhalwar, Professor PSSCIVE

Understanding Control Structures

Control structures are the building blocks of programming that determine the flow of execution. They allow programs to make decisions, repeat actions, and handle different scenarios.

Types of Control Structures

Sequential Structures: Code executes line by line
Selection Structures: if, if...else, if...elif...else
Iteration Structures: for loops, while loops
Jump Structures: break, continue, return

Key Concept: Control structures allow your program to respond to different situations dynamically, making programs intelligent and adaptable.

The if Statement

The if statement is the most basic control structure. It allows your program to execute code only when a certain condition is true.

Syntax of if Statement

if condition:
    # Code to execute if condition is True
    statement1
    statement2
    # ... more statements

The indented block under if only executes when the condition evaluates to True.

Practice: if Statement

# Understanding if statement
# Change the age value and see what happens

age = 18

print("=== Voting Eligibility Check ===")
print(f"Age: {age}")

# if statement to check voting eligibility
if age >= 18:
    print("✓ You are ELIGIBLE to vote!")
    print("✓ Please register to vote.")
else:
    print("✗ You are NOT ELIGIBLE to vote.")
    print(f"✗ You need {18 - age} more year(s) to vote.")

print("\nProgram execution complete!")

# Try these age values: 16, 18, 20, 25

Output will appear here after running the code...

Comparison Operators

Comparison operators are used in conditions to compare values. They return either True or False.

Python Comparison Operators

== 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

Page 2

Interactive Python Programming Textbook

Module 1: Python Programming

Example 1.1: Voting Eligibility Check

Last Updated: December 2024 Author:Dr. Deepak D. Shudhalwar, Professor PSSCIVE

Practical Example: Voting Eligibility

Let's examine a complete example to understand the control flow of the if statement. This program checks if a person is eligible to vote based on their age.

Voting Rule: In most countries, if the age is 18 or older, the person is eligible to vote.

Complete Python Program

Example 1.1: Voting Eligibility Program

# Example 1.1: Voting Eligibility Check
# Demonstrating if statement in action

def check_voting_eligibility(age):
    """
    Function to check voting eligibility based on age
    Returns: Eligibility status
    """
    print(f"\nChecking eligibility for age {age}:")
    print("-" * 30)
    
    # if statement to check voting eligibility
    if age >= 18:
        print("✓ You are ELIGIBLE to vote!")
        print("✓ Please register to vote.")
        return "ELIGIBLE"
    else:
        print("✗ You are NOT ELIGIBLE to vote.")
        print(f"✗ You need {18 - age} more year(s) to vote.")
        return "NOT ELIGIBLE"

# Main program
print("VOTING ELIGIBILITY CHECKER")
print("=" * 40)

# Test with different ages
test_ages = [20, 18, 17]

for age in test_ages:
    result = check_voting_eligibility(age)
    print(f"Result: {result}\n")

print("Program execution completed successfully!")

Output will appear here after running the code...

Program Analysis

Age: 20

if 20 >= 18:
→ True ✓

ELIGIBLE

Age: 18

if 18 >= 18:
→ True ✓

ELIGIBLE

Age: 17

if 17 >= 18:
→ False ✗

NOT ELIGIBLE

Interactive Test

Try the program with different ages to see how the if statement works:

Test Voting Eligibility

Enter an age to check voting eligibility:

Try ages like: 16, 18, 20, 25

Page 3

Interactive Python Programming Textbook

Module 1: Python Programming

1.1.2: if...else Statement

Last Updated: December 2024 Author: Dr. Deepak D. Shudhalwar, Professor PSSCIVE

Working with Alternative Paths

The if...else statement allows writing two alternative paths. The control condition determines which path gets executed.

Syntax of if...else Statement

if test_expression:
    # Body of if (executes when True)
    statement1
    statement2
else:
    # Body of else (executes when False)
    statement3
    statement4

How it works: The if...else statement evaluates the test expression and executes the body of if only when the condition is True. If the condition is False, the body of else is executed.

Example 1.2: Modified Voting Eligibility

Let's modify Example 1.1 to illustrate the use of if...else structure:

Example 1.2: if...else Voting Check

# Example 1.2: if...else statement
# Modified from Example 1.1

def check_voting_eligibility(age):
    """Check voting eligibility using if...else"""
    print(f"Checking eligibility for age {age}:")
    print("-" * 40)
    
    # if...else statement
    if age >= 18:
        print("✓ You are ELIGIBLE to vote!")
        print("✓ Please register to vote.")
        return "ELIGIBLE"
    else:
        print("✗ You are NOT ELIGIBLE to vote.")
        years_needed = 18 - age
        print(f"✗ You need {years_needed} more year(s) to vote.")
        return "NOT ELIGIBLE"

# Main program
print("VOTING ELIGIBILITY CHECKER")
print("=" * 50)

# Test with different ages
test_ages = [20, 18, 17, 15, 21]

for age in test_ages:
    result = check_voting_eligibility(age)
    print(f"Result: {result}")
    print("=" * 50)

print("\n✅ Program completed!")

Output will appear here after running the code...

if vs if...else Comparison

if Statement Only

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

When False: Nothing happens

if...else Statement

if age >= 18:
    print("Eligible")
else:
    print("Not Eligible")

When False: else block executes

Practice if...else Statements

Write and test your own if...else code:

Your Practice Editor

# Practice if...else statements
# Example: Number classifier

number = 10

if number > 0:
    print(f"{number} is positive")
else:
    print(f"{number} is not positive")

# Try modifying the code above:
# 1. Change the number value
# 2. Add more conditions
# 3. Create different classifiers

# Your code here:

Your output will appear here...

Page 4

Interactive Python Programming Textbook

Module 1: Python Programming

1.1.3: if...elif...else Statement

Last Updated: December 2024 Author:Dr. Deepak D. Shudhalwar, Professor PSSCIVE

Handling Multiple Conditions

When you have multiple independent conditions to check, you can use elif statements (short for "else if") between the if and else control structures.

Syntax of if...elif...else

if condition1:
    # Executes when condition1 is True
    statement1
elif condition2:
    # Executes when condition1 is False and condition2 is True
    statement2
elif condition3:
    # Executes when both condition1 and condition2 are False
    # and condition3 is True
    statement3
else:
    # Executes when all conditions are False
    statement4

Important Notes:

Only one block executes (the first True condition)
Conditions are checked from top to bottom
Once a True condition is found, the rest are skipped
The else block is optional

Interactive Grade Calculator

A practical example of if...elif...else with a grade calculator:

Calculate Your Grade

Enter your score (0-100):

Try scores like: 95, 85, 75, 65, 55

Grade Calculator Code

# Grade Calculator using if...elif...else

def calculate_grade(score):
    """Calculate grade based on score"""
    print(f"\nScore: {score}")
    print("-" * 30)
    
    # if...elif...else statement for grade calculation
    if score >= 90:
        grade = "A+"
        message = "Excellent! Outstanding performance!"
    elif score >= 80:
        grade = "A"
        message = "Very good! Keep up the good work!"
    elif score >= 70:
        grade = "B"
        message = "Good work! Room for improvement."
    elif score >= 60:
        grade = "C"
        message = "Satisfactory. Needs more effort."
    elif score >= 40:
        grade = "D"
        message = "Passing grade. Study harder."
    else:
        grade = "F"
        message = "Needs improvement. Please seek help."
    
    print(f"Grade: {grade}")
    print(f"Message: {message}")
    return grade

# Main program
print("GRADE CALCULATOR")
print("=" * 40)

# Test with different scores
test_scores = [95, 85, 75, 65, 45, 35]

for score in test_scores:
    grade = calculate_grade(score)
    print(f"Final Grade: {grade}")
    print("=" * 40)

print("\n✅ All grades calculated successfully!")

Output will appear here after running the code...

Practice if...elif...else

Your Practice Editor

# Practice if...elif...else statements
# Example: Traffic light system

light_color = "green"

if light_color == "red":
    print("STOP")
elif light_color == "yellow":
    print("GET READY")
elif light_color == "green":
    print("GO")
else:
    print("INVALID COLOR")

# Try changing light_color to:
# "red", "yellow", "green", "blue"

# Create your own multiple condition example:
# Your code here:

Your output will appear here...

Page 5

Interactive Python Programming Textbook

Module 1: Python Programming

Advanced: Multiple elif Blocks

Last Updated: December 2024 Author:Dr. Deepak D. Shudhalwar, Professor PSSCIVE

Advanced if...elif...else Structure

You can have multiple elif blocks to handle many independent conditions. Among the several if...elif...else blocks, only one block is executed according to the condition.

Key Concepts

Only One Block Executes

First True condition wins, rest are skipped

Order Matters

Conditions checked from top to bottom

Multiple elif Blocks

You can have as many elif as needed

Single else Block

Only one else allowed per if statement

Example 1.3: Age Category Classifier

Let's create a comprehensive age classifier with multiple elif blocks:

Example 1.3: Age Category Classifier

# Example 1.3: Age Category Classifier
# Using multiple elif blocks

def classify_age(age):
    """Classify age into categories using if...elif...else"""
    print(f"\nAge: {age}")
    print("-" * 30)
    
    # if...elif...elif...else statement with multiple conditions
    if age < 13:
        print("Category: Child")
        print("Message: Enjoy your childhood!")
        return "Child"
    elif age < 20:
        print("Category: Teenager")
        print("Message: Focus on studies and growth!")
        return "Teenager"
    elif age < 40:
        print("Category: Young Adult")
        print("Message: Build your career!")
        return "Young Adult"
    elif age < 60:
        print("Category: Middle-aged")
        print("Message: Share your wisdom!")
        return "Middle-aged"
    elif age < 80:
        print("Category: Senior")
        print("Message: Enjoy retirement!")
        return "Senior"
    else:
        print("Category: Elderly")
        print("Message: Cherished wisdom!")
        return "Elderly"

# Main program
print("AGE CATEGORY CLASSIFIER")
print("=" * 50)

# Test with different ages
test_ages = [8, 16, 25, 45, 65, 85]

for age in test_ages:
    category = classify_age(age)
    print(f"Result: {category}")
    print("=" * 50)

print("\n✅ Program completed!")
print("Multiple elif blocks handled 6 different age ranges!")

Output will appear here after running the code...

Output Comparison

Age: 8

if age < 13:
→ True ✓

Child

Age: 16

elif age < 20:
→ True ✓

Teenager

Age: 45

elif age < 60:
→ True ✓

Middle-aged

Practice Multiple elif Blocks

Your Practice Editor

# Practice multiple elif blocks
# Example: Day of week classifier

day_number = 3  # 1=Monday, 7=Sunday

if day_number == 1:
    print("Monday: Start of the week!")
elif day_number == 2:
    print("Tuesday: Keep going!")
elif day_number == 3:
    print("Wednesday: Middle of the week!")
elif day_number == 4:
    print("Thursday: Almost there!")
elif day_number == 5:
    print("Friday: Weekend is coming!")
elif day_number == 6:
    print("Saturday: Enjoy the weekend!")
elif day_number == 7:
    print("Sunday: Rest and prepare!")
else:
    print("Invalid day number! Enter 1-7")

# Try changing day_number to different values

# Create your own multiple condition example:
# Your code here:

Your output will appear here...

Page 6

Table of Contents

Understanding Program Flow

Airport Boarding Process

Try It Yourself: Python Control Flow

Understanding Control Structures

Types of Control Structures

The if Statement

Comparison Operators

Python Comparison Operators

Practical Example: Voting Eligibility

Complete Python Program

Interactive Test

Test Voting Eligibility

Working with Alternative Paths

Example 1.2: Modified Voting Eligibility

if vs if...else Comparison

if Statement Only

if...else Statement

Practice if...else Statements

Handling Multiple Conditions

Interactive Grade Calculator

Calculate Your Grade

Practice if...elif...else

Advanced if...elif...else Structure

Key Concepts

Only One Block Executes

Order Matters

Multiple elif Blocks

Single else Block

Example 1.3: Age Category Classifier

Practice Multiple elif Blocks