4.1 Control Structures in Python¶

From Algorithms to Code¶

In Chapter 2, we learned about control structures in algorithms: sequence, selection, and iteration. Now it’s time to implement these concepts in Python! This section shows you how to translate your algorithmic thinking into working Python code.

Remember the three fundamental control structures:

Sequence: Instructions executed one after another
Selection: Making decisions with if/else statements
Iteration: Repeating instructions with loops

Sequence (Sequential Execution)¶

What It Is¶

Sequence is the simplest control structure - instructions are executed one line after another, from top to bottom.

# Simple sequence - each line executes in order
print("Starting the program...")
name = input("What's your name? ")
age = int(input("What's your age? "))
print(f"Hello {name}, you are {age} years old!")
print("Program finished.")

Grade Calculator Example¶

Grade Calculator Example

# Grade calculator using sequence
print("=== Grade Calculator ===")

# Get student information
student_name = input("Enter student name: ")
subject = input("Enter subject: ")

# Get assessment scores
assignment1 = float(input("Enter Assignment 1 score (out of 100): "))
assignment2 = float(input("Enter Assignment 2 score (out of 100): "))
test_score = float(input("Enter test score (out of 100): "))

# Calculate average
total_points = assignment1 + assignment2 + test_score
average = total_points / 3

# Display results
print(f"\n--- Results for {student_name} ---")
print(f"Subject: {subject}")
print(f"Assignment 1: {assignment1}")
print(f"Assignment 2: {assignment2}")
print(f"Test Score: {test_score}")
print(f"Average: {average:.1f}")

Selection (Conditional Statements)¶

Simple If Statements¶

Selection allows your program to make decisions and execute different code based on conditions.

# Basic if statement
age = int(input("Enter your age: "))

if age >= 18:
    print("You are eligible to vote!")
    print("Don't forget to register!")

If-Else Statements¶

# If-else provides an alternative path
score = float(input("Enter your test score: "))

if score >= 50:
    print("Congratulations! You passed!")
else:
    print("You need to retake the test.")
    print("Study hard and try again!")

If-Elif-Else (Multiple Conditions)¶

# Multiple conditions with elif
def get_letter_grade(score):
    """Convert a numerical score to a letter grade."""
    if score >= 90:
        return "A"
    elif score >= 80:
        return "B"
    elif score >= 70:
        return "C"
    elif score >= 60:
        return "D"
    else:
        return "F"

# Test the function
student_score = 85
letter = get_letter_grade(student_score)
print(f"Score: {student_score} = Grade: {letter}")

Nested If Statements¶

# Nested conditions for more complex logic
age = int(input("Enter your age: "))
has_license = input("Do you have a driver's license? (yes/no): ").lower()

if age >= 16:
    if has_license == "yes":
        print("You can drive!")
    else:
        print("You're old enough, but you need a license first.")
else:
    print("You're too young to drive.")
    years_to_wait = 16 - age
    print(f"Wait {years_to_wait} more years.")

Logical Operators¶

# Combining conditions with and, or, not
username = input("Username: ")
password = input("Password: ")
age = int(input("Age: "))

# Using 'and' - both conditions must be true
if username == "admin" and password == "secret123":
    print("Admin access granted!")

# Using 'or' - either condition can be true
if age < 13 or age > 65:
    print("You qualify for a discount!")

# Using 'not' - reverse the condition
if not (age >= 18):
    print("You are under 18")

Iteration (Loops)¶

For Loops (Definite Iteration)¶

For loops are used when you know how many times you want to repeat something.

# Simple for loop with range
print("Counting to 5:")
for i in range(1, 6):
    print(f"Count: {i}")

# For loop with a list
students = ["Alice", "Bob", "Charlie", "Diana"]
print("\nClass roster:")
for student in students:
    print(f"- {student}")

# For loop with string
word = "Python"
print(f"\nLetters in '{word}':")
for letter in word:
    print(letter)

Range Function Variations¶

# Different ways to use range()

# range(stop) - starts at 0
print("0 to 4:")
for i in range(5):
    print(i, end=" ")

# range(start, stop) - custom starting point
print("\n3 to 7:")
for i in range(3, 8):
    print(i, end=" ")

# range(start, stop, step) - custom increment
print("\nEven numbers 0 to 10:")
for i in range(0, 11, 2):
    print(i, end=" ")

# Counting backwards
print("\nCountdown:")
for i in range(5, 0, -1):
    print(i, end=" ")
print("Blast off!")

While Loops (Indefinite Iteration)¶

While loops continue until a condition becomes false.

# Simple while loop
count = 1
print("Counting while count <= 5:")
while count <= 5:
    print(f"Count: {count}")
    count += 1  # Same as count = count + 1

print("Done!")

Input Validation with While¶

# Keep asking until valid input
age = -1  # Initialize with invalid value
while age < 0 or age > 120:
    age = int(input("Enter a valid age (0-120): "))
    if age < 0 or age > 120:
        print("Invalid age! Please try again.")

print(f"Thank you! You entered age: {age}")

# Interactive menu using while loop
def show_menu():
    print("\n=== Student Grade System ===")
    print("1. Add a grade")
    print("2. View all grades")
    print("3. Calculate average")
    print("4. Exit")

grades = []
choice = ""

while choice != "4":
    show_menu()
    choice = input("Choose an option (1-4): ")

    if choice == "1":
        grade = float(input("Enter grade: "))
        grades.append(grade)
        print(f"Added grade: {grade}")

    elif choice == "2":
        if grades:
            print("All grades:", grades)
        else:
            print("No grades entered yet.")

    elif choice == "3":
        if grades:
            average = sum(grades) / len(grades)
            print(f"Average grade: {average:.1f}")
        else:
            print("No grades to calculate.")

    elif choice == "4":
        print("Goodbye!")

    else:
        print("Invalid choice. Please try again.")

Mapping from Pseudocode to Python¶

Example 1: Number Guessing Game¶

Number Guessing Game

PseudocodePython Implementation

BEGIN NumberGuessingGame
    SET secret_number = 7
    SET guess = 0
    SET attempts = 0

    WHILE guess != secret_number DO
        PROMPT "Enter your guess: "
        READ guess
        SET attempts = attempts + 1

        IF guess < secret_number THEN
            PRINT "Too low!"
        ELIF guess > secret_number THEN
            PRINT "Too high!"
        ELSE
            PRINT "Correct!"
        END IF
    END WHILE

    PRINT "You won in " + attempts + " attempts!"
END

# Number guessing game
secret_number = 7
guess = 0
attempts = 0

while guess != secret_number:
    guess = int(input("Enter your guess: "))
    attempts += 1

    if guess < secret_number:
        print("Too low!")
    elif guess > secret_number:
        print("Too high!")
    else:
        print("Correct!")

print(f"You won in {attempts} attempts!")

Example 2: Grade Classifier¶

Grade Classifier

PseudocodePython Implementation

BEGIN GradeClassifier
    FOR each student in class DO
        READ student_name
        READ student_score

        IF student_score >= 90 THEN
            SET grade = "A"
        ELIF student_score >= 80 THEN
            SET grade = "B"
        ELIF student_score >= 70 THEN
            SET grade = "C"
        ELIF student_score >= 60 THEN
            SET grade = "D"
        ELSE
            SET grade = "F"
        END IF

        PRINT student_name + ": " + grade
    END FOR
END

# Grade classifier for multiple students
students = ["Alice", "Bob", "Charlie"]
scores = [92, 78, 85]

for i in range(len(students)):
    student_name = students[i]
    student_score = scores[i]

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

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

Combining Control Structures¶

Password Strength Checker¶

def check_password_strength(password):
    """Check if password meets security requirements."""

    # Initialize checks
    has_upper = False
    has_lower = False
    has_digit = False
    has_special = False

    # Check each character in password
    for char in password:
        if char.isupper():
            has_upper = True
        elif char.islower():
            has_lower = True
        elif char.isdigit():
            has_digit = True
        elif char in "!@#$%^&*":
            has_special = True

    # Determine strength
    if len(password) >= 8 and has_upper and has_lower and has_digit and has_special:
        return "Strong"
    elif len(password) >= 6 and (has_upper or has_lower) and has_digit:
        return "Medium"
    else:
        return "Weak"

# Test password strength
test_passwords = ["abc", "Password1", "P@ssw0rd123"]

for password in test_passwords:
    strength = check_password_strength(password)
    print(f"'{password}' is {strength}")

Shopping Cart Calculator¶

def shopping_cart():
    """Simple shopping cart with discount logic."""

    cart = []
    total = 0

    while True:
        print(f"\nCurrent total: ${total:.2f}")
        print("1. Add item")
        print("2. View cart")
        print("3. Checkout")

        choice = input("Choose option: ")

        if choice == "1":
            item_name = input("Item name: ")
            item_price = float(input("Item price: $"))
            cart.append({"name": item_name, "price": item_price})
            total += item_price
            print(f"Added {item_name} for ${item_price:.2f}")

        elif choice == "2":
            if cart:
                print("\n--- Your Cart ---")
                for item in cart:
                    print(f"- {item['name']}: ${item['price']:.2f}")
            else:
                print("Cart is empty")

        elif choice == "3":
            break

        else:
            print("Invalid choice")

    # Apply discount logic
    if total > 100:
        discount = total * 0.1  # 10% discount
        total -= discount
        print(f"10% discount applied: -${discount:.2f}")

    print(f"Final total: ${total:.2f}")
    print("Thank you for shopping!")

# Run the shopping cart
shopping_cart()

Common Patterns and Best Practices¶

Input Validation Pattern¶

def get_valid_integer(prompt, min_value=None, max_value=None):
    """Get a valid integer from user with optional range checking."""

    while True:
        try:
            value = int(input(prompt))

            # Check range if specified
            if min_value is not None and value < min_value:
                print(f"Value must be at least {min_value}")
                continue

            if max_value is not None and value > max_value:
                print(f"Value must be at most {max_value}")
                continue

            return value

        except ValueError:
            print("Please enter a valid integer")

# Use the validation function
age = get_valid_integer("Enter your age (0-120): ", 0, 120)
score = get_valid_integer("Enter test score (0-100): ", 0, 100)

Flag-Controlled Loop Pattern¶

def find_target_in_list(target_list, target_value):
    """Search for a value in a list using a flag pattern."""

    found = False
    position = -1

    for i in range(len(target_list)):
        if target_list[i] == target_value:
            found = True
            position = i
            break  # Exit loop early when found

    if found:
        print(f"Found {target_value} at position {position}")
    else:
        print(f"{target_value} not found in list")

    return found, position

# Test the search function
numbers = [10, 25, 30, 45, 50]
find_target_in_list(numbers, 30)  # Found at position 2
find_target_in_list(numbers, 99)  # Not found

Practice Exercises¶

Exercise 1: Temperature Converter

Write a program that:

Uses a while loop for a menu
Converts between Celsius and Fahrenheit
Uses if/elif for menu choices
Validates input temperature ranges

Sample Solution

def celsius_to_fahrenheit(celsius):
    """Convert Celsius to Fahrenheit."""
    return (celsius * 9/5) + 32

def fahrenheit_to_celsius(fahrenheit):
    """Convert Fahrenheit to Celsius."""
    return (fahrenheit - 32) * 5/9

def main():
    while True:
        print("\nTemperature Converter Menu:")
        print("1. Celsius to Fahrenheit")
        print("2. Fahrenheit to Celsius")
        print("3. Exit")

        choice = input("Enter your choice (1-3): ").strip()

        if choice == "1":
            try:
                celsius = float(input("Enter temperature in Celsius (-273.15 to 1000): "))
                if celsius < -273.15:
                    print("Error: Temperature cannot be below absolute zero!")
                else:
                    fahrenheit = celsius_to_fahrenheit(celsius)
                    print(f"{celsius}°C = {fahrenheit:.2f}°F")
            except ValueError:
                print("Error: Please enter a valid number!")

        elif choice == "2":
            try:
                fahrenheit = float(input("Enter temperature in Fahrenheit (-459.67 to 1832): "))
                if fahrenheit < -459.67:
                    print("Error: Temperature cannot be below absolute zero!")
                else:
                    celsius = fahrenheit_to_celsius(fahrenheit)
                    print(f"{fahrenheit}°F = {celsius:.2f}°C")
            except ValueError:
                print("Error: Please enter a valid number!")

        elif choice == "3":
            print("Goodbye!")
            break
        else:
            print("Error: Invalid choice. Please select 1, 2, or 3.")

if __name__ == "__main__":
    main()

This program demonstrates:

While loop for the main menu
If/elif/else for menu choices and input validation
Try/except for error handling
Function definitions for the conversion logic
Input validation for temperature ranges

Exercise 2: Simple ATM

Create an ATM simulation with:

PIN validation (3 attempts max)
Menu options: check balance, withdraw, deposit
Balance updates with validation
Loop until user chooses to exit

Sample Solution

def main():
    # Initialize account
    correct_pin = "1234"
    balance = 1000.00
    max_attempts = 3

    # PIN validation
    for attempt in range(max_attempts):
        pin = input("Enter your PIN: ").strip()
        if pin == correct_pin:
            print("PIN accepted. Welcome!")
            break
        else:
            remaining = max_attempts - attempt - 1
            if remaining > 0:
                print(f"Incorrect PIN. {remaining} attempts remaining.")
            else:
                print("Too many incorrect attempts. Card blocked.")
                return
    else:
        return  # Exit if PIN validation failed

    # Main ATM menu
    while True:
        print(f"\nCurrent Balance: ${balance:.2f}")
        print("\nATM Menu:")
        print("1. Check Balance")
        print("2. Withdraw")
        print("3. Deposit")
        print("4. Exit")

        choice = input("Enter your choice (1-4): ").strip()

        if choice == "1":
            print(f"Your current balance is: ${balance:.2f}")

        elif choice == "2":
            try:
                amount = float(input("Enter withdrawal amount: $"))
                if amount <= 0:
                    print("Error: Withdrawal amount must be positive!")
                elif amount > balance:
                    print("Error: Insufficient funds!")
                else:
                    balance -= amount
                    print(f"Withdrawal successful. New balance: ${balance:.2f}")
            except ValueError:
                print("Error: Please enter a valid amount!")

        elif choice == "3":
            try:
                amount = float(input("Enter deposit amount: $"))
                if amount <= 0:
                    print("Error: Deposit amount must be positive!")
                else:
                    balance += amount
                    print(f"Deposit successful. New balance: ${balance:.2f}")
            except ValueError:
                print("Error: Please enter a valid amount!")

        elif choice == "4":
            print("Thank you for using our ATM. Goodbye!")
            break

        else:
            print("Error: Invalid choice. Please select 1, 2, 3, or 4.")

if __name__ == "__main__":
    main()

This program demonstrates:

For loop with range for PIN attempts
While loop for the main menu
If/elif/else for menu choices and validation
Try/except for input validation
Break and return for flow control
String formatting for currency display

Exercise 3: Grade Book Manager

Build a grade management system:

Add students and their grades
Calculate individual and class averages
Find highest and lowest scores
Display grade distribution (A, B, C, D, F counts)

Sample Solution

def get_letter_grade(score):
    """Convert numeric score to letter grade."""
    if score >= 90:
        return 'A'
    elif score >= 80:
        return 'B'
    elif score >= 70:
        return 'C'
    elif score >= 60:
        return 'D'
    else:
        return 'F'

def main():
    students = {}  # Dictionary to store student grades

    while True:
        print("\nGrade Book Manager")
        print("1. Add student and grade")
        print("2. View all students")
        print("3. Calculate class statistics")
        print("4. Exit")

        choice = input("Enter your choice (1-4): ").strip()

        if choice == "1":
            name = input("Enter student name: ").strip()
            if not name:
                print("Error: Name cannot be empty!")
                continue

            try:
                score = float(input("Enter grade (0-100): "))
                if score < 0 or score > 100:
                    print("Error: Grade must be between 0 and 100!")
                else:
                    students[name] = score
                    print(f"Added {name} with grade {score}")
            except ValueError:
                print("Error: Please enter a valid numeric grade!")

        elif choice == "2":
            if not students:
                print("No students in the grade book yet.")
            else:
                print("\nStudent Grades:")
                for name, score in students.items():
                    letter = get_letter_grade(score)
                    print(f"{name}: {score} ({letter})")

        elif choice == "3":
            if not students:
                print("No students in the grade book yet.")
            else:
                # Calculate statistics
                scores = list(students.values())
                class_average = sum(scores) / len(scores)
                highest = max(scores)
                lowest = min(scores)

                # Grade distribution
                distribution = {'A': 0, 'B': 0, 'C': 0, 'D': 0, 'F': 0}
                for score in scores:
                    letter = get_letter_grade(score)
                    distribution[letter] += 1

                print("
Class Statistics:")
                print(f"Number of students: {len(students)}")
                print(f"Class average: {class_average:.2f}")
                print(f"Highest score: {highest}")
                print(f"Lowest score: {lowest}")
                print("\nGrade Distribution:")
                for grade, count in distribution.items():
                    print(f"{grade}: {count} students")

        elif choice == "4":
            print("Goodbye!")
            break

        else:
            print("Error: Invalid choice. Please select 1, 2, 3, or 4.")

if __name__ == "__main__":
    main()

This program demonstrates:

Dictionary for storing student data
While loop for the main menu
If/elif/else for menu choices and grade validation
For loops for iterating through students and calculating statistics
List comprehensions and built-in functions (sum, max, min)
String formatting for output display
Input validation for names and scores

Common Mistakes to Avoid¶

Infinite loops: Always ensure loop conditions can become false

# BAD - infinite loop
count = 1
while count <= 5:
    print(count)
    # Forgot to increment count!

# GOOD - proper increment
count = 1
while count <= 5:
    print(count)
    count += 1

Off-by-one errors: Be careful with range boundaries

# BAD - misses last element
for i in range(len(my_list) - 1):
    print(my_list[i])

# GOOD - includes all elements
for i in range(len(my_list)):
    print(my_list[i])

Indentation errors: Python relies on proper indentation

# BAD - incorrect indentation
if score >= 90:
print("Excellent!")  # Should be indented

# GOOD - proper indentation
if score >= 90:
    print("Excellent!")

Summary¶

Control structures are the building blocks of program logic:

Sequence: Execute instructions in order
Selection: Make decisions with if/elif/else
Iteration: Repeat with for and while loops

Key points to remember:

Translate pseudocode step-by-step to Python
Use proper indentation for code blocks
Validate user input to prevent errors
Combine structures to solve complex problems
Test edge cases and boundary conditions

4.1 Control Structures in Python¶

From Algorithms to Code¶

Sequence (Sequential Execution)¶

What It Is¶

Grade Calculator Example¶

Selection (Conditional Statements)¶

Simple If Statements¶

If-Else Statements¶

If-Elif-Else (Multiple Conditions)¶

Nested If Statements¶

Logical Operators¶

Iteration (Loops)¶

For Loops (Definite Iteration)¶

Range Function Variations¶

While Loops (Indefinite Iteration)¶

Input Validation with While¶

Menu System Example¶

Mapping from Pseudocode to Python¶

Example 1: Number Guessing Game¶

Example 2: Grade Classifier¶

Combining Control Structures¶

Password Strength Checker¶

Shopping Cart Calculator¶

Common Patterns and Best Practices¶

Input Validation Pattern¶

Flag-Controlled Loop Pattern¶

Practice Exercises¶

Common Mistakes to Avoid¶

Summary¶