Loops in Python

Loops are fundamental programming structures that allow you to repeat code multiple times. Whether you're processing items in a list, counting numbers, or running a program until a condition is met, loops make it possible. Python provides two main types of loops: for loops and while loops.

Why Use Loops?

Loops help you:

• Avoid repetition: Write code once, run it many times
• Process collections: Iterate through lists, strings, dictionaries
• Automate tasks: Repeat actions until a goal is achieved
• Handle unknown quantities: Process data when you don't know how much there is
• Make programs dynamic: Respond to user input or changing conditions

The for Loop

The for loop iterates over a sequence (list, string, range, etc.) and executes code for each item.

Basic for Loop Syntax

# Loop through a list
fruits = ['apple', 'banana', 'cherry', 'date']

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

Output:

I like apple
I like banana
I like cherry
I like date

Looping Through Different Types

# Loop through a string (iterates over characters)
word = "Python"
for letter in word:
    print(letter)

# Loop through a range of numbers
for number in range(5):
    print(f"Count: {number}")

# Loop with start and end
for number in range(1, 6):
    print(number)

# Loop with step
for number in range(0, 10, 2):  # Count by 2s
    print(number)

The range() Function

The range() function generates sequences of numbers.

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

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

# range(start, stop, step): with custom increment
for i in range(0, 20, 5):
    print(i)  # 0, 5, 10, 15

# Counting backwards
for i in range(10, 0, -1):
    print(i)  # 10, 9, 8, ..., 1

Practical range() Examples

# Print multiplication table
number = 7
for i in range(1, 11):
    print(f"{number} x {i} = {number * i}")

# Sum numbers from 1 to 100
total = 0
for i in range(1, 101):
    total += i
print(f"Sum of 1 to 100: {total}")

# Generate list of squares
squares = []
for i in range(1, 11):
    squares.append(i ** 2)
print(f"Squares: {squares}")

The while Loop

The while loop repeats as long as a condition is True.

Basic while Loop

# Count to 5
count = 1
while count <= 5:
    print(f"Count is: {count}")
    count += 1
print("Done counting!")

while Loop with User Input

# Keep asking until correct password
correct_password = "secret123"
password = ""

while password != correct_password:
    password = input("Enter password: ")
    if password != correct_password:
        print("Incorrect! Try again.")
print("Access granted!")

Infinite Loops (Use with Caution)

# Infinite loop with break condition
while True:
    user_input = input("Enter 'quit' to exit: ")
    if user_input.lower() == 'quit':
        print("Goodbye!")
        break
    print(f"You entered: {user_input}")

Loop Control Statements

break: Exit Loop Early

# Stop when we find what we're looking for
numbers = [1, 5, 8, 12, 15, 20, 25]

for num in numbers:
    if num > 15:
        print(f"Found a number greater than 15: {num}")
        break
    print(f"Checking {num}")

# Search for item in list
names = ['Alice', 'Bob', 'Charlie', 'Diana']
search_name = 'Charlie'

for name in names:
    if name == search_name:
        print(f"Found {search_name}!")
        break
else:
    print(f"{search_name} not found")

continue: Skip to Next Iteration

# Skip even numbers
for i in range(1, 11):
    if i % 2 == 0:
        continue  # Skip the rest of this iteration
    print(f"Odd number: {i}")

# Process only valid data
scores = [85, -1, 92, 0, 78, -5, 88]

for score in scores:
    if score < 0:
        print(f"Skipping invalid score: {score}")
        continue
    print(f"Valid score: {score}")

pass: Do Nothing (Placeholder)

# Placeholder for future code
for i in range(5):
    if i == 3:
        pass  # TODO: Add special handling later
    print(i)

# Empty loop body
for item in []:
    pass  # Does nothing but is syntactically required

Loop else Clause

Python loops can have an else clause that executes if the loop completes normally (without break).

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

for i in range(2, number):
    if number % i == 0:
        print(f"{number} is not prime (divisible by {i})")
        is_prime = False
        break
else:
    # This runs only if break was NOT called
    print(f"{number} is prime!")

# Search example
shopping_list = ['milk', 'bread', 'eggs', 'butter']
item_to_find = 'cheese'

for item in shopping_list:
    if item == item_to_find:
        print(f"Found {item_to_find}!")
        break
else:
    print(f"{item_to_find} not in shopping list")

Nested Loops

Loops can be placed inside other loops.

# Multiplication table
for i in range(1, 6):
    for j in range(1, 6):
        print(f"{i} x {j} = {i*j:2d}", end="  ")
    print()  # New line after each row

# Print a pattern
for i in range(5):
    for j in range(i + 1):
        print("*", end="")
    print()

# Process 2D data
matrix = [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9]
]

for row in matrix:
    for value in row:
        print(value, end=" ")
    print()

Breaking from Nested Loops

# Find first occurrence in 2D structure
found = False
data = [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9]
]

target = 5

for i, row in enumerate(data):
    for j, value in enumerate(row):
        if value == target:
            print(f"Found {target} at position ({i}, {j})")
            found = True
            break
    if found:
        break

Looping Through Collections

Enumerate: Get Index and Value

# Get both index and value
fruits = ['apple', 'banana', 'cherry']

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

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

Zip: Loop Through Multiple Lists

# Pair up elements from two lists
names = ['Alice', 'Bob', 'Charlie']
ages = [25, 30, 35]

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

# Zip works with any number of iterables
scores_math = [85, 90, 78]
scores_english = [88, 85, 92]
scores_science = [90, 87, 85]

for name, math, english, science in zip(names, scores_math, scores_english, scores_science):
    average = (math + english + science) / 3
    print(f"{name}: Math={math}, English={english}, Science={science}, Avg={average:.1f}")

Looping Through Dictionaries

# Dictionary loop basics
student = {
    'name': 'Alice',
    'age': 20,
    'grade': 'A',
    'gpa': 3.8
}

# Loop through keys
for key in student:
    print(key)

# Loop through values
for value in student.values():
    print(value)

# Loop through key-value pairs
for key, value in student.items():
    print(f"{key}: {value}")

# Real example: Calculate total
prices = {'apple': 0.5, 'banana': 0.3, 'cherry': 0.2}
quantities = {'apple': 10, 'banana': 15, 'cherry': 20}

total = 0
for item in prices:
    cost = prices[item] * quantities[item]
    print(f"{item}: ${cost:.2f}")
    total += cost
print(f"Total: ${total:.2f}")

List Comprehensions (Compact Loops)

List comprehensions provide a concise way to create lists.

# Traditional loop
squares = []
for i in range(1, 11):
    squares.append(i ** 2)

# List comprehension (more Pythonic)
squares = [i ** 2 for i in range(1, 11)]
print(squares)

# With condition
even_squares = [i ** 2 for i in range(1, 11) if i % 2 == 0]
print(even_squares)

# Transform strings
names = ['alice', 'bob', 'charlie']
capitalized = [name.capitalize() for name in names]
print(capitalized)

# Nested comprehension
matrix = [[i*j for j in range(1, 4)] for i in range(1, 4)]
print(matrix)

Practical Examples

Example 1: Validate User Input

# Keep asking until valid input
while True:
    age = input("Enter your age (1-120): ")
    
    if not age.isdigit():
        print("Please enter a number!")
        continue
    
    age = int(age)
    
    if age < 1 or age > 120:
        print("Age must be between 1 and 120!")
        continue
    
    print(f"Thank you! Your age is {age}")
    break

Example 2: Calculate Statistics

# Calculate statistics from a list of numbers
numbers = [15, 23, 8, 42, 16, 31, 27, 19]

# Find minimum and maximum
minimum = numbers[0]
maximum = numbers[0]

for num in numbers:
    if num < minimum:
        minimum = num
    if num > maximum:
        maximum = num

print(f"Min: {minimum}, Max: {maximum}")

# Calculate average
total = 0
for num in numbers:
    total += num
average = total / len(numbers)
print(f"Average: {average:.2f}")

# Count values above average
above_avg = 0
for num in numbers:
    if num > average:
        above_avg += 1
print(f"Numbers above average: {above_avg}")

Example 3: Generate Fibonacci Sequence

# Generate first n Fibonacci numbers
n = 10
fibonacci = []

a, b = 0, 1
for _ in range(n):
    fibonacci.append(a)
    a, b = b, a + b

print(f"First {n} Fibonacci numbers:")
print(fibonacci)

Example 4: Simple Menu System

# Interactive menu
while True:
    print("\n=== Main Menu ===")
    print("1. Calculate area")
    print("2. Calculate perimeter")
    print("3. Exit")
    
    choice = input("Enter choice (1-3): ")
    
    if choice == '1':
        length = float(input("Enter length: "))
        width = float(input("Enter width: "))
        area = length * width
        print(f"Area: {area:.2f}")
    
    elif choice == '2':
        length = float(input("Enter length: "))
        width = float(input("Enter width: "))
        perimeter = 2 * (length + width)
        print(f"Perimeter: {perimeter:.2f}")
    
    elif choice == '3':
        print("Goodbye!")
        break
    
    else:
        print("Invalid choice! Please try again.")

Example 5: Process Files or Data

# Process a list of transactions
transactions = [
    {'type': 'deposit', 'amount': 100},
    {'type': 'withdrawal', 'amount': 50},
    {'type': 'deposit', 'amount': 200},
    {'type': 'withdrawal', 'amount': 75},
    {'type': 'deposit', 'amount': 150}
]

balance = 0

for transaction in transactions:
    if transaction['type'] == 'deposit':
        balance += transaction['amount']
        print(f"Deposited ${transaction['amount']}, Balance: ${balance}")
    elif transaction['type'] == 'withdrawal':
        if balance >= transaction['amount']:
            balance -= transaction['amount']
            print(f"Withdrew ${transaction['amount']}, Balance: ${balance}")
        else:
            print(f"Insufficient funds for ${transaction['amount']} withdrawal")

print(f"\nFinal balance: ${balance}")

Common Loop Patterns

Pattern 1: Accumulator

# Sum all numbers
numbers = [1, 2, 3, 4, 5]
total = 0

for num in numbers:
    total += num
print(f"Sum: {total}")

Pattern 2: Counter

# Count specific items
grades = ['A', 'B', 'A', 'C', 'A', 'B', 'D', 'A']
a_count = 0

for grade in grades:
    if grade == 'A':
        a_count += 1
print(f"Number of A's: {a_count}")

Pattern 3: Filter

# Extract items that meet criteria
numbers = [1, 15, 8, 42, 16, 31, 27, 5]
large_numbers = []

for num in numbers:
    if num > 20:
        large_numbers.append(num)
print(f"Large numbers: {large_numbers}")

Pattern 4: Transform

# Convert all items
celsius = [0, 10, 20, 30, 40]
fahrenheit = []

for temp in celsius:
    f_temp = (temp * 9/5) + 32
    fahrenheit.append(f_temp)
print(f"Fahrenheit: {fahrenheit}")

Common Mistakes and Solutions

Mistake 1: Modifying List While Iterating

# WRONG: Modifying list during iteration
numbers = [1, 2, 3, 4, 5]
for num in numbers:
    if num % 2 == 0:
        numbers.remove(num)  # Dangerous!

# CORRECT: Create new list
numbers = [1, 2, 3, 4, 5]
odd_numbers = []
for num in numbers:
    if num % 2 != 0:
        odd_numbers.append(num)

# Or use list comprehension
odd_numbers = [num for num in numbers if num % 2 != 0]

Mistake 2: Forgetting to Increment in while Loop

# WRONG: Infinite loop
# count = 1
# while count <= 5:
#     print(count)
#     # Forgot to increment!

# CORRECT:
count = 1
while count <= 5:
    print(count)
    count += 1  # Don't forget this!

Mistake 3: Off-by-One Errors

# Be careful with range boundaries
# range(5) gives 0, 1, 2, 3, 4 (NOT 5)
for i in range(5):
    print(i)  # Prints 0-4

# To include 5:
for i in range(6):
    print(i)  # Prints 0-5

# Or use start and stop:
for i in range(1, 6):
    print(i)  # Prints 1-5

Practice Exercises

Exercise 1: FizzBuzz

Print numbers 1-100, but:

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

Exercise 2: Password Validator

Create a password validator that checks:

• Minimum 8 characters
• At least one uppercase letter
• At least one digit
• Keep prompting until valid

Exercise 3: Shopping Cart

Build a shopping cart that:

• Shows menu of items with prices
• Lets user add items
• Calculates total
• Allows checkout or continue shopping

Sample Solutions

Exercise 1:

for i in range(1, 101):
    if i % 3 == 0 and i % 5 == 0:
        print("FizzBuzz")
    elif i % 3 == 0:
        print("Fizz")
    elif i % 5 == 0:
        print("Buzz")
    else:
        print(i)

Exercise 2:

while True:
    password = input("Enter password: ")
    
    if len(password) < 8:
        print("Password must be at least 8 characters")
        continue
    
    has_upper = False
    has_digit = False
    
    for char in password:
        if char.isupper():
            has_upper = True
        if char.isdigit():
            has_digit = True
    
    if not has_upper:
        print("Password must contain an uppercase letter")
        continue
    
    if not has_digit:
        print("Password must contain a digit")
        continue
    
    print("Password is valid!")
    break

Exercise 3:

items = {
    'apple': 0.5,
    'banana': 0.3,
    'orange': 0.6,
    'milk': 2.5
}

cart = []

while True:
    print("\n=== Shop Menu ===")
    for item, price in items.items():
        print(f"{item}: ${price:.2f}")
    print("\nCommands: add <item>, checkout, quit")
    
    command = input("Enter command: ").lower().split()
    
    if command[0] == 'add' and len(command) > 1:
        item = command[1]
        if item in items:
            cart.append(item)
            print(f"Added {item}")
        else:
            print("Item not found")
    
    elif command[0] == 'checkout':
        total = sum(items[item] for item in cart)
        print(f"\nItems: {cart}")
        print(f"Total: ${total:.2f}")
        break
    
    elif command[0] == 'quit':
        break

Key Takeaways

• for loops iterate over sequences (lists, strings, ranges)
• while loops repeat while a condition is True
• range() generates number sequences
• break exits a loop immediately
• continue skips to the next iteration
• pass does nothing (placeholder)
• Loop else executes if loop completes normally
• enumerate() gives both index and value
• zip() pairs up multiple iterables
• List comprehensions provide compact syntax
• Always ensure while loops can exit (avoid infinite loops)

What's Next?

Now that you understand loops, you're ready for:

• Lists and Tuples - Collections that you'll loop through
• Dictionaries and Sets - More data structures
• Functions - Organize repeated code

Loops are the workhorses of programming. Practice with different types of loops and patterns to become fluent in controlling program flow!