Control Structures in Python

Control structures are the building blocks that control the flow of execution in your Python programs. They allow you to make decisions, repeat actions, and create dynamic, interactive applications.

Conditional Statements

The if Statement

The if statement allows your program to make decisions:

age = 18

if age >= 18:
    print("You are eligible to vote!")

if-else Statement

Add an alternative action with else:

temperature = 25

if temperature > 30:
    print("It's hot outside!")
else:
    print("The weather is pleasant.")

if-elif-else Statement

Handle multiple conditions with elif:

score = 85

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

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

Nested if Statements

weather = "sunny"
temperature = 25

if weather == "sunny":
    if temperature > 20:
        print("Perfect day for a picnic!")
    else:
        print("Sunny but a bit cold.")
else:
    print("Maybe stay indoors today.")

Comparison Operators

x = 10
y = 5

print(x == y)  # Equal to: False
print(x != y)  # Not equal to: True
print(x > y)   # Greater than: True
print(x < y)   # Less than: False
print(x >= y)  # Greater than or equal: True
print(x <= y)  # Less than or equal: False

Logical Operators

and Operator

age = 25
has_license = True

if age >= 18 and has_license:
    print("You can drive!")

or Operator

day = "Saturday"

if day == "Saturday" or day == "Sunday":
    print("It's weekend!")

not Operator

is_raining = False

if not is_raining:
    print("Great day for outdoor activities!")

Loops

The for Loop

Use for loops to iterate over sequences:

# Loop through a list
fruits = ["apple", "banana", "orange"]
for fruit in fruits:
    print(f"I like {fruit}")

# Loop through a range
for i in range(5):
    print(f"Number: {i}")

# Loop with start, stop, step
for i in range(2, 10, 2):
    print(f"Even number: {i}")

The while Loop

Use while loops for conditional repetition:

count = 0
while count < 5:
    print(f"Count is: {count}")
    count += 1

# User input loop
password = ""
while password != "secret":
    password = input("Enter password: ")
    if password != "secret":
        print("Incorrect password. Try again.")
print("Access granted!")

Loop Control Statements

break Statement

# Exit the loop early
for i in range(10):
    if i == 5:
        break
    print(i)
# Prints: 0, 1, 2, 3, 4

continue Statement

# Skip the current iteration
for i in range(10):
    if i % 2 == 0:
        continue
    print(i)
# Prints: 1, 3, 5, 7, 9

else with Loops

# else executes if loop completes normally
for i in range(3):
    print(i)
else:
    print("Loop completed!")

# else doesn't execute if loop is broken
for i in range(5):
    if i == 3:
        break
    print(i)
else:
    print("This won't print")

Practical Examples

Example 1: Number Guessing Game

import random

secret_number = random.randint(1, 100)
attempts = 0
max_attempts = 7

print("Welcome to the Number Guessing Game!")
print(f"I'm thinking of a number between 1 and 100. You have {max_attempts} attempts.")

while attempts < max_attempts:
    guess = int(input("Enter your guess: "))
    attempts += 1
    
    if guess == secret_number:
        print(f"Congratulations! You guessed it in {attempts} attempts!")
        break
    elif guess < secret_number:
        print("Too low!")
    else:
        print("Too high!")
    
    remaining = max_attempts - attempts
    if remaining > 0:
        print(f"You have {remaining} attempts left.")
else:
    print(f"Game over! The number was {secret_number}")

Example 2: Grade Calculator

print("Grade Calculator")
print("Enter -1 to finish entering grades")

grades = []
total = 0
count = 0

while True:
    grade = float(input("Enter a grade: "))
    
    if grade == -1:
        break
    
    if 0 <= grade <= 100:
        grades.append(grade)
        total += grade
        count += 1
    else:
        print("Please enter a grade between 0 and 100")

if count > 0:
    average = total / count
    print(f"\nGrades entered: {grades}")
    print(f"Average grade: {average:.2f}")
    
    if average >= 90:
        letter_grade = "A"
    elif average >= 80:
        letter_grade = "B"
    elif average >= 70:
        letter_grade = "C"
    elif average >= 60:
        letter_grade = "D"
    else:
        letter_grade = "F"
    
    print(f"Letter grade: {letter_grade}")
else:
    print("No valid grades entered.")

Example 3: Password Validator

def validate_password(password):
    """Validate password based on security criteria"""
    
    if len(password) < 8:
        return False, "Password must be at least 8 characters long"
    
    has_upper = False
    has_lower = False
    has_digit = False
    has_special = False
    special_chars = "!@#$%^&*()_+-=[]{}|;:,.<>?"
    
    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 special_chars:
            has_special = True
    
    if not has_upper:
        return False, "Password must contain at least one uppercase letter"
    if not has_lower:
        return False, "Password must contain at least one lowercase letter"
    if not has_digit:
        return False, "Password must contain at least one digit"
    if not has_special:
        return False, "Password must contain at least one special character"
    
    return True, "Password is valid!"

# Test the validator
while True:
    user_password = input("Enter a password (or 'quit' to exit): ")
    
    if user_password.lower() == 'quit':
        break
    
    is_valid, message = validate_password(user_password)
    print(message)
    
    if is_valid:
        print("Password accepted!")
        break

Common Patterns

Input Validation

while True:
    try:
        age = int(input("Enter your age: "))
        if age < 0:
            print("Age cannot be negative.")
            continue
        break
    except ValueError:
        print("Please enter a valid number.")

Menu Systems

while True:
    print("\n--- Menu ---")
    print("1. Option 1")
    print("2. Option 2")
    print("3. Quit")
    
    choice = input("Enter your choice: ")
    
    if choice == "1":
        print("You selected Option 1")
    elif choice == "2":
        print("You selected Option 2")
    elif choice == "3":
        print("Goodbye!")
        break
    else:
        print("Invalid choice. Please try again.")

Best Practices

1. Use meaningful variable names
2. Keep conditions simple and readable
3. Avoid deeply nested statements
4. Use appropriate loop types for your needs
5. Include proper error handling
6. Comment complex logic

Practice Exercises

Exercise 1: FizzBuzz

Write a program that prints numbers 1 to 100, but:

• Print "Fizz" for multiples of 3
• Print "Buzz" for multiples of 5
• Print "FizzBuzz" for multiples of both 3 and 5

Exercise 2: Prime Number Checker

Create a program that checks if a number is prime.

Exercise 3: Simple ATM

Build a simple ATM system with:

• Balance inquiry
• Deposit money
• Withdraw money
• Exit option

Summary

Control structures are essential for creating dynamic programs:

• Conditional statements help make decisions
• Loops allow repetition of actions
• Break and continue control loop execution
• Proper validation makes programs robust

Master these concepts, and you'll be able to build interactive and intelligent Python applications!

Next, we'll explore Python's powerful data structures that will help you organize and manipulate data efficiently.