Loops in Python (`for`, `while`, `range`, `else`)

🟢 Beginner

📖 Definition

Loops repeatedly execute a block of code as long as a condition holds true or for each item in a sequence (like a list, string, or range). Python supports for loops (sequence iteration), while loops (pre-test condition), break, continue, pass, and a unique for...else / while...else clause.

🇮🇳 Hindi

Repeated tasks ko automate karne ke liye for aur while loops ka use hota hai. Counter sequences ke liye range(start, stop, step) use karein. Python mein for...else block tabhi chalta hai jab loop bina break hit kiye naturally complete hota hai.

🚩 Marathi

Code punha punha chalavnyasathi for aani while loops vaparatat. Sequences sathi range() aani loop skip sathi continue vapara.

📝 1. Sequence Iteration & `range()`

The range(start, stop, step) function generates a sequence of numbers (the stop number is excluded!).

# Iterates 1, 2, 3, 4 (5 is excluded)
for i in range(1, 5):
    print("Count:", i)

# Step sequence: 0, 2, 4, 6, 8
for even in range(0, 10, 2):
    print("Even:", even)

📝 2. Loop Control Keywords

break: Exits the loop immediately.
continue: Skips the rest of the current iteration and jumps to the next cycle.
pass: Placeholder statement that does nothing (used when syntax requires a statement).

📝 3. Unique Python Feature: `for...else`

In Python, an else block attached to a loop executes ONLY if the loop finishes all iterations naturally without hitting a break statement!

numbers = [1, 3, 5, 7]
target = 4

for num in numbers:
    if num == target:
        print("Found target!")
        break
else:
    # Runs ONLY if target was not found and loop didn't break!
    print("Target not found in list.")

💡 Complete Example: Searching & Batch Processing

# Batch Processing & Inventory Search System
inventory = [
    {"name": "Laptop", "stock": 15, "price": 55000.00},
    {"name": "Mouse", "stock": 0, "price": 499.00},   # Out of stock
    {"name": "Keyboard", "stock": 25, "price": 1499.00},
    {"name": "Monitor", "stock": 8, "price": 12500.00}
]

print("=== INVENTORY BATCH SUMMARY ===")

# 1. Processing Inventory with for loop & continue
for item in inventory:
    if item["stock"] == 0:
        print(f"Skipping {item['name']}: Out of Stock!")
        continue  # Skip out-of-stock items
        
    print(f"Item: {item['name']:<10} | Stock: {item['stock']:<3} | Value: ₹{item['stock'] * item['price']:.2f}")

# 2. Search Target Item with for...else
search_name = "Keyboard"
print(f"\nSearching for '{search_name}'...")

for item in inventory:
    if item["name"].lower() == search_name.lower():
        print(f"FOUND: {item['name']} - ₹{item['price']} ({item['stock']} in stock)")
        break
else:
    print(f"NOT FOUND: '{search_name}' is not in inventory.")

👀 Output

=== INVENTORY BATCH SUMMARY ===
Item: Laptop     | Stock: 15  | Value: ₹825000.00
Skipping Mouse: Out of Stock!
Item: Keyboard   | Stock: 25  | Value: ₹37475.00
Item: Monitor    | Stock: 8   | Value: ₹100000.00

Searching for 'Keyboard'...
FOUND: Keyboard - ₹1499.0 (25 in stock)

⚠️ Common Mistakes

Infinite Loops: Forgetting to update counter variables inside a while loop.
Off-By-One Errors: Forgetting that range(1, 10) generates numbers from 1 to 9 (10 is excluded!).
Modifying a list structurally (adding or removing items) while iterating over it with a for loop.

🛡️ Safety / Important Notes

When you need both element indices and values during list iteration, use enumerate(sequence) instead of manual index counters:

fruits = ["apple", "banana", "cherry"]
for index, fruit in enumerate(fruits, start=1):
    print(f"{index}. {fruit}")

🌍 Real-World Usage

Traversing database query results, retrying network requests, reading text log files line-by-line, and rendering list UI templates.

🧪 Try It Yourself

Write a for loop using range() that prints all multiples of 5 from 5 to 50.
Write a while loop that calculates the sum of digits of 1234.

🎯 Mini Challenge

Write a program that uses for...else to check whether a given integer is a Prime Number (divisible only by 1 and itself).

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Loops in Python (for, while, range, else)