Loops: Repeating Actions Like a Pro

Welcome to the world of automation! Loops are your program’s way of doing repetitive tasks without writing the same code over and over.

What are Loops?

Loops are like washing machines:

You put in the clothes (data)
Set the cycle (loop type)
The machine does the work repeatedly
Stops when the cycle is complete

The while Loop - Repeat Until Condition Changes

Real-Life Example: Filling a Glass

glass_capacity = 250  # ml
current_water = 0
pour_amount = 50  # ml per pour

print("Filling glass with water...")

while current_water < glass_capacity:
    current_water += pour_amount
    print(f"Poured {pour_amount}ml. Total: {current_water}ml")

    if current_water > glass_capacity:
        overflow = current_water - glass_capacity
        print(f"Overflow: {overflow}ml")
        current_water = glass_capacity

print(f"Glass is now full: {current_water}ml")

Basic while Loop

# Count from 1 to 5
counter = 1

while counter <= 5:
    print(f"Count: {counter}")
    counter += 1  # Don't forget to update the condition!

print("Counting complete!")

Real-Life Example: ATM Withdrawal

balance = 1000
withdraw_amount = 100

print("ATM Withdrawal Simulator")
print(f"Initial balance: ${balance}")

while balance >= withdraw_amount:
    balance -= withdraw_amount
    print(f"Withdrew ${withdraw_amount}. Remaining: ${balance}")

print("Insufficient funds for another withdrawal.")
print(f"Final balance: ${balance}")

The for Loop - Repeat for Each Item

Real-Life Example: Grocery Shopping

shopping_list = ["apples", "bananas", "milk", "bread", "eggs"]

print("Shopping list:")
for item in shopping_list:
    print(f"☐ {item}")

print("\nStarting to shop...")
for item in shopping_list:
    print(f"✅ Bought {item}")

print("Shopping complete!")

Basic for Loop with Lists

fruits = ["apple", "banana", "orange", "grape", "kiwi"]

print("My favorite fruits:")
for fruit in fruits:
    print(f"I love {fruit}s!")

print(f"Total fruits: {len(fruits)}")

The range() Function - Creating Number Sequences

Basic range() Usage

# range(end) - from 0 to end-1
print("Counting from 0 to 4:")
for i in range(5):
    print(f"Number: {i}")

print()

# range(start, end) - from start to end-1
print("Counting from 1 to 5:")
for i in range(1, 6):
    print(f"Number: {i}")

print()

# range(start, end, step) - with custom step
print("Even numbers from 0 to 10:")
for i in range(0, 11, 2):
    print(f"Even: {i}")

print()

# Counting backwards
print("Countdown from 10 to 1:")
for i in range(10, 0, -1):
    print(f"T-minus {i}...")
print("Blast off! 🚀")

Real-Life Example: Multiplication Table

number = 7

print(f"Multiplication table for {number}:")
for i in range(1, 13):  # 1 to 12
    result = number * i
    print(f"{number} × {i} = {result}")

print("Table complete!")

Loop Control Statements

break - Exit the Loop Immediately

# Find the first even number
numbers = [1, 3, 5, 7, 8, 9, 10]

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

print("Search complete.")

# Print only odd numbers
print("Odd numbers from 1 to 10:")
for num in range(1, 11):
    if num % 2 == 0:  # If even, skip it
        continue
    print(f"Odd number: {num}")

print("All odd numbers printed.")

Real-Life Example: Quality Control

products = ["good", "defective", "good", "good", "defective", "good"]
passed_products = 0

print("Quality control check:")
for product in products:
    if product == "defective":
        print("❌ Defective product found - skipping")
        continue

    print(f"✅ {product} product passed")
    passed_products += 1

print(f"Total products passed: {passed_products}")

Nested Loops - Loops Inside Loops

Real-Life Example: Weekly Meal Plan

days = ["Monday", "Tuesday", "Wednesday"]
meals = ["Breakfast", "Lunch", "Dinner"]

print("Weekly Meal Plan:")
for day in days:
    print(f"\n{day}:")
    for meal in meals:
        print(f"  - {meal}: Plan your meal")

print("\nMeal planning complete!")

Multiplication Table Grid

print("Multiplication Table (1-5):")
print("   ", end="")
for i in range(1, 6):
    print("3d", end="")
print()

for i in range(1, 6):
    print("3d|", end="")
    for j in range(1, 6):
        print("3d", end="")
    print()

# Output:
#     1  2  3  4  5
#  1| 1  2  3  4  5
#  2| 2  4  6  8 10
#  3| 3  6  9 12 15
#  4| 4  8 12 16 20
#  5| 5 10 15 20 25

else with Loops

Real-Life Example: Job Search

job_requirements = ["Python", "SQL", "Excel"]
my_skills = ["Python", "JavaScript", "SQL", "Excel"]

print("Checking job requirements...")
for requirement in job_requirements:
    if requirement not in my_skills:
        print(f"❌ Missing skill: {requirement}")
        break
else:  # This runs only if no break occurred
    print("✅ You meet all requirements! Apply for the job!")

print("Skill check complete.")

Prime Number Checker

def is_prime(number):
    if number <= 1:
        return False

    for divisor in range(2, int(number ** 0.5) + 1):
        if number % divisor == 0:
            return False
    else:  # No divisor found
        return True

# Test the function
test_numbers = [2, 3, 4, 5, 17, 18, 23]

for num in test_numbers:
    if is_prime(num):
        print(f"{num} is prime")
    else:
        print(f"{num} is not prime")

Common Loop Patterns

Pattern 1: Accumulator Pattern

# Calculate sum of numbers
numbers = [1, 2, 3, 4, 5]
total = 0

for num in numbers:
    total += num  # Accumulate the sum

print(f"Sum of {numbers} = {total}")

# Count occurrences
text = "hello world"
letter_counts = {}

for letter in text:
    if letter.isalpha():  # Only count letters
        letter = letter.lower()
        if letter in letter_counts:
            letter_counts[letter] += 1
        else:
            letter_counts[letter] = 1

print("Letter counts:", letter_counts)

Pattern 2: Find Maximum/Minimum

scores = [85, 92, 78, 96, 88]
highest_score = scores[0]  # Start with first score

for score in scores[1:]:  # Check remaining scores
    if score > highest_score:
        highest_score = score

print(f"Highest score: {highest_score}")

# Find minimum
lowest_score = scores[0]
for score in scores[1:]:
    if score < lowest_score:
        lowest_score = score

print(f"Lowest score: {lowest_score}")

Pattern 3: Filter Items

numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
even_numbers = []

for num in numbers:
    if num % 2 == 0:
        even_numbers.append(num)

print(f"Even numbers: {even_numbers}")

# Filter with condition
grades = [85, 92, 78, 96, 88, 75, 89]
passing_grades = []

for grade in grades:
    if grade >= 80:
        passing_grades.append(grade)

print(f"Passing grades: {passing_grades}")

Infinite Loops (And How to Avoid Them)

Dangerous: Infinite while Loop

# ❌ This will run forever!
# counter = 1
# while counter <= 5:
#     print(counter)
#     # Forgot to increment counter!

# ✅ Correct version
counter = 1
while counter <= 5:
    print(counter)
    counter += 1  # Don't forget this!

Safe Infinite Loop with Break

# Menu system
while True:
    print("\nMenu:")
    print("1. Say hello")
    print("2. Say goodbye")
    print("3. Exit")

    choice = input("Enter choice (1-3): ")

    if choice == "1":
        print("Hello!")
    elif choice == "2":
        print("Goodbye!")
    elif choice == "3":
        print("Exiting...")
        break  # Exit the loop
    else:
        print("Invalid choice. Try again.")

Loop Best Practices

1. Use the Right Loop Type

# ✅ Use for loop when you know the number of iterations
for i in range(5):
    print(f"Iteration {i}")

# ✅ Use while loop when you don't know how many iterations
user_input = ""
while user_input != "quit":
    user_input = input("Enter something (or 'quit' to exit): ")

2. Avoid Modifying Lists While Iterating

# ❌ Dangerous - modifying list while iterating
numbers = [1, 2, 3, 4, 5]
for num in numbers:
    if num % 2 == 0:
        numbers.remove(num)  # This can cause problems!

print(numbers)  # Unpredictable result

# ✅ Safe - create new list
numbers = [1, 2, 3, 4, 5]
even_numbers = []
for num in numbers:
    if num % 2 == 0:
        even_numbers.append(num)

print(f"Original: {numbers}")
print(f"Evens: {even_numbers}")

3. Use enumerate() for Index and Value

fruits = ["apple", "banana", "orange"]

# ❌ Manual index tracking
index = 0
for fruit in fruits:
    print(f"{index}: {fruit}")
    index += 1

# ✅ Use enumerate()
for index, fruit in enumerate(fruits):
    print(f"{index}: {fruit}")

# Start index from 1
for index, fruit in enumerate(fruits, start=1):
    print(f"{index}. {fruit}")

Practice Exercises

Exercise 1: Number Pyramid

Create a program that prints a pyramid of numbers:

Exercise 2: Password Guessing Game

Create a game where the user has 3 attempts to guess a password.

Exercise 3: Fibonacci Sequence

Generate the first 10 numbers in the Fibonacci sequence.

Exercise 4: Shopping Cart Total

Calculate the total price of items in a shopping cart with tax.

Exercise 5: Number Guessing Game with Hints

Improve the number guessing game to give “hotter” or “colder” hints.

Summary

Loops make your code powerful by automating repetitive tasks:

while loops: Repeat until a condition becomes false
for loops: Repeat for each item in a sequence
range(): Generate number sequences for loops
break: Exit loop immediately
continue: Skip to next iteration
else with loops: Execute code when loop completes normally

Remember:

Always ensure your loop has an exit condition
Use for loops when you know the number of iterations
Use while loops for indefinite repetition
Be careful with infinite loops
Don’t modify lists while iterating over them

Next: Functions - reusable blocks of code! 🧱

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