Python Loops Complete Guide

# for loop - iterate over a sequence
fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(f"I like {fruit}")

# while loop - repeat while condition is true
count = 1
while count <= 5:
    print(f"Count: {count}")
    count += 1  # Increment counter

# Using range() function for iteration
for i in range(5):  # 0, 1, 2, 3, 4
    print(f"Number: {i}")

# Loop with else clause
for num in range(3):
    print(num)
else:
    print("Loop completed successfully!")

# for Loop Examples
print("=== for Loop Examples ===")

# Example 1: Iterating over a list
fruits = ["apple", "banana", "cherry", "date", "elderberry"]
print("My favorite fruits:")
for fruit in fruits:
    print(f"- {fruit.capitalize()}")

# Example 2: Using range() function
print("\nNumbers from 1 to 5:")
for i in range(1, 6):  # range(start, stop) stop is exclusive
    print(i, end=" ")  # Output: 1 2 3 4 5

print("\n\nEven numbers from 2 to 10:")
for i in range(2, 11, 2):  # range(start, stop, step)
    print(i, end=" ")  # Output: 2 4 6 8 10

# Example 3: Iterating over string
word = "Python"
print(f"\n\nCharacters in '{word}':")
for char in word:
    print(char, end="-")  # Output: P-y-t-h-o-n-

# Example 4: Iterating with enumerate()
colors = ["red", "green", "blue"]
print("\n\nColors with index:")
for index, color in enumerate(colors):
    print(f"{index}: {color}")
    # Output: 0: red, 1: green, 2: blue

# Example 5: Iterating over dictionary
student = {"name": "Alice", "age": 21, "grade": "A"}
print("\nStudent information:")
for key, value in student.items():
    print(f"{key}: {value}")

# while Loop Examples
print("=== while Loop Examples ===")

# Example 1: Basic counter
count = 1
print("Counting from 1 to 5:")
while count <= 5:
    print(f"Count: {count}")
    count += 1  # Increment to avoid infinite loop
print("Counting complete!\n")

# Example 2: Sum of numbers
total = 0
number = 1

print("Calculating sum of numbers from 1 to 10:")
while number <= 10:
    total += number
    print(f"Adding {number}, total: {total}")
    number += 1

print(f"Final sum: {total}")

# Example 3: while loop with else
counter = 0
print("\nWhile loop with else example:")
while counter < 3:
    print(f"Counter: {counter}")
    counter += 1
else:
    print("Loop completed normally")

# Example 4: Infinite loop with break
print("\nGuessing game (infinite loop with break):")
secret_number = 7

while True:  # Infinite loop
    guess = int(input("Guess a number between 1-10: "))
    
    if guess == secret_number:
        print("Congratulations! You guessed it!")
        break  # Exit the infinite loop
    elif guess < secret_number:
        print("Too low! Try again.")
    else:
        print("Too high! Try again.")

print("Game over!")

# Loop Control Statements Examples
print("=== Loop Control Statements ===")

# Example 1: break - Find first negative number
numbers = [10, 5, 8, -3, 7, 2]
print("Finding first negative number:")

for num in numbers:
    if num < 0:
        print(f"Found negative number: {num}")
        break
    print(f"Checking: {num}")
else:
    print("No negative numbers found")

# Example 2: continue - Skip even numbers
print("\nOdd numbers from 1 to 10:")
for i in range(1, 11):
    if i % 2 == 0:  # Even number
        continue  # Skip even numbers
    print(i, end=" ")
# Output: 1 3 5 7 9

# Example 3: pass - Placeholder for future code
print("\n\nProcessing numbers (with pass placeholder):")
for num in [1, 2, 3, 4, 5]:
    if num == 3:
        pass  # TODO: Add special handling for number 3
    else:
        print(f"Processing: {num}")

# Nested Loop Examples
print("=== Nested Loop Examples ===")

# Example 1: Multiplication table
print("Multiplication Table (1-5):")
print("   ", end="")
for i in range(1, 6):
    print(f"{i:4}", end="")  # Header row
print("\n" + "-" * 25)

for i in range(1, 6):  # Outer loop for rows
    print(f"{i:2} |", end="")
    for j in range(1, 6):  # Inner loop for columns
        print(f"{i*j:4}", end="")
    print()  # New line after each row

# Example 2: Pattern printing - Right triangle
print("\nRight Triangle Pattern:")
rows = 5
for i in range(1, rows + 1):
    for j in range(1, i + 1):
        print("*", end=" ")
    print()  # New line

# Example 3: Two-dimensional list processing
print("\nMatrix Operations:")
matrix = [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9]
]

print("Original Matrix:")
for row in matrix:  # Outer loop for rows
    for element in row:  # Inner loop for columns
        print(element, end=" ")
    print()

# Implementing loop flowchart logic
print("=== Loop Control Flow Example ===")

# Simulating a shopping cart checkout
cart_items = ["Apple", "Banana", "Orange", "Milk", "Bread"]
cart_total = 0

print("Processing shopping cart items:")
for item in cart_items:
    # Simulate getting item price (in reality would be from database)
    if item == "Apple":
        price = 1.50
    elif item == "Banana":
        price = 0.75
    elif item == "Orange":
        price = 1.20
    elif item == "Milk":
        price = 3.50
    elif item == "Bread":
        price = 2.25
    else:
        price = 0.00
    
    cart_total += price
    print(f"Added {item}: ${price:.2f}")
    
    # Flowchart decision point: continue or break?
    if cart_total > 5.00:
        print("Budget limit reached! Stopping checkout.")
        break

print(f"\nTotal: ${cart_total:.2f}")
print("Checkout process completed.")

# Python Loops Practice Exercises

print("=== Exercise 1: Factorial Calculator ===")
# Calculate factorial of a number using for loop
# n! = n × (n-1) × (n-2) × ... × 1
# Example: 5! = 5 × 4 × 3 × 2 × 1 = 120
n = 5
factorial = 1
for i in range(1, n + 1):
    factorial *= i
print(f"{n}! = {factorial}")

print("\n=== Exercise 2: Fibonacci Series ===")
# Generate first n Fibonacci numbers
# 0, 1, 1, 2, 3, 5, 8, 13, 21, ...
n = 10
fib_series = [0, 1]
for i in range(2, n):
    next_num = fib_series[-1] + fib_series[-2]
    fib_series.append(next_num)
print(f"First {n} Fibonacci numbers: {fib_series}")

print("\n=== Exercise 3: Prime Number Checker ===")
# Check if a number is prime (divisible only by 1 and itself)
num = 17
is_prime = True
if num > 1:
    for i in range(2, int(num ** 0.5) + 1):
        if num % i == 0:
            is_prime = False
            break
else:
    is_prime = False
print(f"{num} is prime: {is_prime}")

print("\n=== Exercise 4: Pattern Printing ===")
# Print pattern using nested loops
for i in range(1, 6):
    for j in range(i):
        print(i, end="")
    print()

print("\n=== Exercise 5: Multiplication Table ===")
# Print multiplication table 1-10
for i in range(1, 11):
    for j in range(1, 11):
        print(f"{i*j:4}", end="")
    print()

print("\n=== Exercise 6: Password Strength Checker ===")
# Check password has requirements
password = "Secure@123"
has_upper = has_lower = has_digit = has_special = False

for char in password:
    if char.isupper():
        has_upper = True
    elif char.islower():
        has_lower = True
    elif char.isdigit():
        has_digit = True
    elif not char.isalnum():
        has_special = True

if (len(password) >= 8 and has_upper and 
    has_lower and has_digit and has_special):
    print("Password is strong!")
else:
    print("Password needs improvement.")