Control Flow: Making Decisions and Creating Loops

Welcome to the world of smart programming! So far you’ve learned how to store data and perform basic operations. Now it’s time to make your programs think and decide for themselves.

What is Control Flow?

Control flow is like being the director of a play. You decide:

What happens next based on current conditions
When to repeat actions
How to handle different situations

Imagine you’re cooking pasta:

If the water is boiling → add pasta
While the pasta is cooking → stir occasionally
For each minute of cooking → check if it’s done

That’s control flow in action!

Method 1: Conditional Statements (if-else) - Like Decision Making

The Restaurant Decision Analogy

Imagine you’re at a restaurant deciding what to order:

hungry = True
has_money = True
is_vegetarian = False

if hungry and has_money:
    if is_vegetarian:
        print("I'll order a vegetable stir-fry!")
    else:
        print("I'll order chicken curry!")
else:
    print("Maybe just coffee for now.")

Basic if Statement

The if statement is like asking “Should I do this?“

age = 18

if age >= 18:
    print("You can vote!")
    print("You can drive!")
    print("You can get a tattoo!")

print("This always prints")  # Not indented, so always runs

if-else Statement

The else is like saying “If not this, then that.”

temperature = 25

if temperature > 30:
    print("It's hot! Turn on the AC.")
    print("Drink cold water.")
else:
    print("Nice weather!")
    print("Perfect for a walk.")

print("Weather check complete.")

if-elif-else Chain

elif means “else if” - for multiple conditions.

score = 85

if score >= 90:
    print("Grade: A - Excellent!")
elif score >= 80:
    print("Grade: B - Good job!")
elif score >= 70:
    print("Grade: C - Satisfactory")
elif score >= 60:
    print("Grade: D - Needs improvement")
else:
    print("Grade: F - Try again")

print("Grading complete.")

Nested if Statements

You can put if statements inside other if statements.

has_ticket = True
age = 16
with_parent = True

if has_ticket:
    if age >= 18:
        print("Welcome! Full price ticket.")
    elif age >= 13:
        print("Welcome! Teen ticket.")
    else:
        if with_parent:
            print("Welcome! Child ticket with parent.")
        else:
            print("Sorry, children under 13 need a parent.")
else:
    print("Please buy a ticket first.")

Method 2: Loops (while and for) - Like Repeating Actions

The Exercise Routine Analogy

Imagine your morning exercise routine:

While loop: Keep running until you’re tired For loop: Do 10 push-ups, then 10 sit-ups, then 10 jumping jacks

while Loop - Repeat Until Condition is False

# Count down from 10
countdown = 10

while countdown > 0:
    print(f"Countdown: {countdown}")
    countdown = countdown - 1  # Or: countdown -= 1

print("Blast off! 🚀")

Infinite Loops (Careful!)

# This will run forever! (Don't run this)
# while True:
#     print("This never stops!")

# Safe version with break
attempts = 0
while True:
    password = input("Enter password: ")
    if password == "secret123":
        print("Access granted!")
        break  # Exit the loop
    else:
        attempts += 1
        if attempts >= 3:
            print("Too many attempts. Account locked.")
            break
        print("Wrong password. Try again.")

for Loop - Repeat for Each Item

# Loop through a list
fruits = ["apple", "banana", "orange", "grape"]

for fruit in fruits:
    print(f"I like {fruit}s!")

print("All fruits listed!")

range() Function with for Loops

# Count from 1 to 5
for number in range(1, 6):  # range(start, end) - end is exclusive
    print(f"Number: {number}")

print()

# Count by 2s
for even in range(0, 11, 2):  # range(start, end, step)
    print(f"Even number: {even}")

print()

# Count backwards
for count in range(10, 0, -1):
    print(f"Countdown: {count}")
print("Done!")

Method 3: Loop Control Statements

break - Exit the Loop Immediately

# Find the first even number
numbers = [1, 3, 5, 6, 8, 9, 10]

for num in numbers:
    if num % 2 == 0:  # If even
        print(f"Found first even number: {num}")
        break  # Exit the loop
    print(f"Checking: {num} (odd)")

print("Loop ended.")

# Print only odd numbers, skip even ones
for num in range(1, 11):
    if num % 2 == 0:  # If even
        continue  # Skip to next number
    print(f"Odd number: {num}")

print("All odd numbers printed.")

else with Loops

# Check if number is prime
number = 17
is_prime = True

for divisor in range(2, int(number ** 0.5) + 1):
    if number % divisor == 0:
        is_prime = False
        break

if is_prime:
    print(f"{number} is a prime number!")
else:
    print(f"{number} is not a prime number.")

# Using else with for loop
for divisor in range(2, int(number ** 0.5) + 1):
    if number % divisor == 0:
        print(f"{number} is divisible by {divisor}")
        break
else:  # This runs only if break was never called
    print(f"{number} is a prime number!")

Real-World Examples

Example 1: Simple Calculator

print("Simple Calculator")
print("Operations: +, -, *, /")
print("Type 'quit' to exit")

while True:
    operation = input("Enter operation (+, -, *, /) or 'quit': ")

    if operation.lower() == 'quit':
        print("Goodbye!")
        break

    if operation not in ['+', '-', '*', '/']:
        print("Invalid operation. Try again.")
        continue

    try:
        num1 = float(input("Enter first number: "))
        num2 = float(input("Enter second number: "))

        if operation == '+':
            result = num1 + num2
        elif operation == '-':
            result = num1 - num2
        elif operation == '*':
            result = num1 * num2
        elif operation == '/':
            if num2 == 0:
                print("Cannot divide by zero!")
                continue
            result = num1 / num2

        print(f"Result: {num1} {operation} {num2} = {result}")

    except ValueError:
        print("Please enter valid numbers.")
        continue

    print()  # Empty line for readability

Example 2: Number Guessing Game

import random

print("🎯 Number Guessing Game!")
print("I'm thinking of a number between 1 and 100.")

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

while attempts < max_attempts:
    try:
        guess = int(input(f"Attempt {attempts + 1}/{max_attempts}. Enter your guess: "))
        attempts += 1

        if guess < secret_number:
            print("Too low! Try higher. 📈")
        elif guess > secret_number:
            print("Too high! Try lower. 📉")
        else:
            print(f"🎉 Congratulations! You got it in {attempts} attempts!")
            break

    except ValueError:
        print("Please enter a valid number.")
        continue

else:  # This runs if the loop completed without break
    print(f"😅 Game over! The number was {secret_number}.")

print("Thanks for playing!")

def show_menu():
    print("\n🍕 Pizza Restaurant Menu")
    print("1. View Menu")
    print("2. Order Pizza")
    print("3. Check Order Status")
    print("4. Exit")
    print("-" * 30)

def show_pizza_menu():
    print("\n🍕 Available Pizzas:")
    print("1. Margherita - $12")
    print("2. Pepperoni - $15")
    print("3. Hawaiian - $16")
    print("4. Veggie - $14")

# Main program
order_placed = False
current_order = None

while True:
    show_menu()
    choice = input("Enter your choice (1-4): ")

    if choice == '1':
        show_pizza_menu()

    elif choice == '2':
        show_pizza_menu()
        pizza_choice = input("Enter pizza number (1-4): ")

        pizza_menu = {
            '1': ('Margherita', 12),
            '2': ('Pepperoni', 15),
            '3': ('Hawaiian', 16),
            '4': ('Veggie', 14)
        }

        if pizza_choice in pizza_menu:
            pizza_name, price = pizza_menu[pizza_choice]
            current_order = f"{pizza_name} Pizza - ${price}"
            order_placed = True
            print(f"✅ Order placed: {current_order}")
        else:
            print("❌ Invalid pizza choice.")

    elif choice == '3':
        if order_placed and current_order:
            print(f"📦 Your order: {current_order}")
            print("Status: Being prepared... (Ready in 20 minutes)")
        else:
            print("📦 No active orders.")

    elif choice == '4':
        if order_placed:
            print(f"Thanks for your order: {current_order}")
        print("👋 Goodbye! Have a great day!")
        break

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

Common Mistakes to Avoid

1. Infinite Loops

# ❌ Wrong - this will run forever
# while True:
#     print("Stuck!")

# ✅ Correct - proper exit condition
count = 0
while count < 5:
    print(f"Count: {count}")
    count += 1

2. Off-by-One Errors

# ❌ Wrong - range(1, 10) goes from 1 to 9
# for i in range(1, 10):
#     print(i)  # Prints 1, 2, 3, 4, 5, 6, 7, 8, 9

# ✅ Correct - range(1, 11) goes from 1 to 10
for i in range(1, 11):
    print(i)  # Prints 1, 2, 3, 4, 5, 6, 7, 8, 9, 10

3. Forgetting to Update Loop Variables

# ❌ Wrong - i never changes
# i = 0
# while i < 5:
#     print(i)

# ✅ Correct - update the variable
i = 0
while i < 5:
    print(i)
    i += 1

Practice Exercises

Exercise 1: Temperature Converter

Create a program that converts between Celsius and Fahrenheit with a menu.

Exercise 2: Simple ATM Simulator

Create a simple ATM that allows deposits, withdrawals, and balance checks.

Exercise 3: Multiplication Table Generator

Ask the user for a number and print its multiplication table (1-10).

Exercise 4: Password Validator

Create a password validator that checks:

At least 8 characters
Contains uppercase and lowercase letters
Contains at least one number

Exercise 5: Rock-Paper-Scissors Game

Create a rock-paper-scissors game against the computer.

Summary

Control flow gives your programs the power to:

Make decisions with if-elif-else
Repeat actions with while and for loops
Handle different situations gracefully
Create interactive programs that respond to user input

Remember:

if checks conditions
while repeats until a condition is false
for repeats for each item in a sequence
break exits loops immediately
continue skips to the next iteration

Next up: Functions - the building blocks of reusable code! 🚀

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