🔁 Lesson 6: Loops — Making Python Repeat Tasks

💡 Why Loops?

# Without loops (the painful way):
print("I will not throw paper airplanes")
print("I will not throw paper airplanes")
print("I will not throw paper airplanes")
# ... 97 more times?! No thanks!

# With loops (the smart way):
for i in range(100):
    print("I will not throw paper airplanes")

🔢 The For Loop — Counting Made Easy

A for loop is like a reliable friend who counts for you:

📐 Understanding range()

The range() function is like a number generator:

# range(stop) - counts from 0 to stop-1
for i in range(5):
    print(i)  # Prints: 0, 1, 2, 3, 4

# range(start, stop) - counts from start to stop-1
for i in range(2, 7):
    print(i)  # Prints: 2, 3, 4, 5, 6

# range(start, stop, step) - counts by step
for i in range(0, 10, 2):
    print(i)  # Prints: 0, 2, 4, 6, 8

# Counting backwards
for i in range(10, 0, -1):
    print(i)  # Prints: 10, 9, 8, 7, 6, 5, 4, 3, 2, 1
print("Blast off! 🚀")

🔤 Looping Through Strings and Lists

For loops can walk through text letter by letter, or through lists item by item:

# Looping through a string
name = "Python"
for letter in name:
    print(f"Give me a {letter}!")
print("What does that spell? PYTHON!")

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

# With index numbers
for i in range(len(fruits)):
    print(f"{i+1}. {fruits[i]}")

# enumerate() - get both index and value
fruits = ["apple", "banana", "cherry"]

# The Pythonic way
for index, fruit in enumerate(fruits):
    print(f"{index}: {fruit}")

# Start counting from 1 instead of 0
for number, fruit in enumerate(fruits, start=1):
    print(f"{number}. {fruit}")

# zip() - loop through multiple lists together
names = ["Alice", "Bob", "Carol"]
ages = [25, 30, 28]

for name, age in zip(names, ages):
    print(f"{name} is {age} years old")

⏳ The While Loop — Keep Going Until…

While loops are like saying "keep doing this as long as something is true":

# Basic while loop
countdown = 5
while countdown > 0:
    print(f"{countdown}...")
    countdown = countdown - 1  # or countdown -= 1
print("Happy New Year! 🎉")

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

🎮 Interactive Example: Guessing Game

Let's build a fun number guessing game using loops:

import random

# Computer picks a random number
secret_number = random.randint(1, 100)
guesses = 0
found = False

print("I'm thinking of a number between 1 and 100!")
print("Can you guess it?")

while not found:
    guess = int(input("Your guess: "))
    guesses += 1

    if guess < secret_number:
        print("Too low! Try higher.")
    elif guess > secret_number:
        print("Too high! Try lower.")
    else:
        found = True
        print(f"🎉 Correct! You got it in {guesses} tries!")

        if guesses <= 5:
            print("Amazing! You're a mind reader!")
        elif guesses <= 10:
            print("Great job!")
        else:
            print("Keep practicing!")

# Use FOR loops when:
# - You know the number of iterations
for i in range(100):
    process_item(i)

# - Iterating through collections
for item in my_list:
    print(item)

# Use WHILE loops when:
# - Condition-based repetition
while user_input != "quit":
    user_input = input("Enter command: ")

# - Unknown number of iterations
while not found:
    keep_searching()

🚦 Break and Continue

Sometimes you need an emergency exit from a loop, or want to skip certain steps:

# Using break - exit when found
items = ["sock", "shoe", "keys", "wallet", "phone"]
print("Looking for my keys...")

for item in items:
    print(f"Found: {item}")
    if item == "keys":
        print("Got them! No need to keep looking.")
        break

# Using continue - skip certain items
print("\nCounting to 10, but 7 is unlucky:")
for i in range(1, 11):
    if i == 7:
        continue  # Skip 7
    print(i, end=" ")  # end=" " keeps numbers on same line

🪆 Nested Loops and Patterns

You can put loops inside loops, like a clock (hours loop contains minutes loop):

# Drawing a rectangle with stars
width = 5
height = 3

for row in range(height):
    for col in range(width):
        print("*", end=" ")
    print()  # New line after each row

# Output:
# * * * * *
# * * * * *
# * * * * *

# Multiplication table
print("\nMultiplication Table:")
for i in range(1, 6):
    for j in range(1, 6):
        print(f"{i*j:3}", end=" ")  # :3 makes numbers align
    print()

🎨 Fun Project: ASCII Art Patterns

# Triangle pattern
print("Triangle:")
for i in range(1, 6):
    print("*" * i)

# Pyramid pattern
print("\nPyramid:")
for i in range(1, 6):
    spaces = " " * (5 - i)
    stars = "*" * (2 * i - 1)
    print(spaces + stars)

# Diamond pattern
print("\nDiamond:")
for i in range(1, 5):
    spaces = " " * (4 - i)
    stars = "*" * (2 * i - 1)
    print(spaces + stars)
for i in range(3, 0, -1):
    spaces = " " * (4 - i)
    stars = "*" * (2 * i - 1)
    print(spaces + stars)

Triangle:
*
**
***
****
*****

Pyramid:
    *
   ***
  *****
 *******
*********

# Traditional loop approach
squares = []
for i in range(10):
    squares.append(i ** 2)

# List comprehension (same result, one line!)
squares = [i ** 2 for i in range(10)]

# With conditions
evens = [x for x in range(20) if x % 2 == 0]

# Practical example: Convert temperatures
celsius = [0, 10, 20, 30, 40]
fahrenheit = [(c * 9/5) + 32 for c in celsius]

🧩 Common Loop Patterns

# Pattern 1: Accumulator (building up a result)
total = 0
for i in range(1, 11):
    total += i
print(f"Sum of 1-10: {total}")

# Pattern 2: Finding maximum/minimum
numbers = [45, 22, 88, 13, 99, 31]
highest = numbers[0]  # Start with first number
for num in numbers:
    if num > highest:
        highest = num
print(f"Highest number: {highest}")

# Pattern 3: Counting occurrences
text = "mississippi"
count = 0
for letter in text:
    if letter == "s":
        count += 1
print(f"The letter 's' appears {count} times")

# Pattern 4: Input validation
while True:
    age = input("Enter your age (0-150): ")
    if age.isdigit() and 0 <= int(age) <= 150:
        age = int(age)
        break
    print("Invalid age! Try again.")
print(f"Your age is {age}")

🐛 Common Mistakes to Avoid

# Wrong - this will run forever!
x = 1
while x > 0:
    print(x)
    # Forgot to change x!

# Right - always update the condition
x = 10
while x > 0:
    print(x)
    x -= 1  # This ensures loop will end

# Common mistake - range stops BEFORE the end
for i in range(10):
    print(i)  # Prints 0-9, not 1-10!

# If you want 1-10:
for i in range(1, 11):
    print(i)

🏋️ Practice Exercises

import time

seconds = int(input("Enter countdown seconds: "))

while seconds > 0:
    print(f"⏱️ {seconds}...")
    time.sleep(1)
    seconds -= 1

print("🎉 Time's up! Ding ding ding!")

password = input("Enter a password to check: ")

has_number = False
has_upper = False
has_special = False
specials = "!@#$%^&*()-_=+[]{}|;:',.<>?/"

for char in password:
    if char.isdigit():
        has_number = True
    elif char.isupper():
        has_upper = True
    elif char in specials:
        has_special = True

score = 0
if len(password) >= 8:
    score += 1
if has_number:
    score += 1
if has_upper:
    score += 1
if has_special:
    score += 1

print(f"\nPassword Strength: {score}/4")
if score <= 1:
    print("🔴 Weak — add more variety!")
elif score <= 2:
    print("🟡 Fair — getting better")
elif score <= 3:
    print("🟢 Strong — nice work!")
else:
    print("💪 Very Strong — excellent!")

shopping_list = []

while True:
    print(f"\n📝 List ({len(shopping_list)} items): {shopping_list}")
    action = input("(a)dd, (r)emove, or (d)one? ").lower()

    if action == "d":
        break
    elif action == "a":
        item = input("Item to add: ")
        shopping_list.append(item)
        print(f"✅ Added '{item}'")
    elif action == "r":
        item = input("Item to remove: ")
        if item in shopping_list:
            shopping_list.remove(item)
            print(f"❌ Removed '{item}'")
        else:
            print(f"'{item}' not in list!")

print(f"\nFinal list: {shopping_list}")

Summary

📌 Real-World Applications

Loops are everywhere: data processing (loop through thousands of records), games (game loop that runs until game over), animations (repeat drawing frames), servers (keep listening for requests), and automation (repeat tasks like file backups).

🚀 What's Next?

Now that you can make Python repeat tasks, we'll learn about lists — how to store and organize multiple pieces of data. Think of it as giving Python a better memory!