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🔄 While Loops

While loops are Python's most fundamental looping construct. They repeat a block of code as long as a specified condition remains true. Think of while loops as "keep doing this until something changes" - they're perfect for situations where you don't know exactly how many times you need to repeat an action.

While loops are essential for user input validation, waiting for events, processing data until a condition is met, and creating interactive programs that respond to changing conditions using decision-making concepts.

# Basic while loop
count = 1
while count <= 5:
    print(f"Count: {count}")
    count += 1  # Important: update the condition variable

print("Loop finished!")

📝 Basic While Loop Syntax

A while loop consists of the while keyword, followed by a condition, a colon, and an indented code block. The condition is checked before each iteration, and the loop continues as long as the condition evaluates to True.

The most critical aspect of while loops is ensuring the condition eventually becomes False. Otherwise, you'll create an infinite loop that runs forever.

# Basic countdown example
countdown = 5
while countdown > 0:
    print(f"T-minus {countdown}")
    countdown -= 1  # This change makes the condition eventually False
print("Blast off!")

🎛️ Loop Control Variables

While loops typically use control variables that change during each iteration. These variables determine when the loop should stop and help track the loop's progress.

Properly managing control variables is crucial for loop correctness and preventing infinite loops or premature termination.

# Different control variable patterns
print("=== COUNTER VARIABLE ===")
i = 1
while i <= 3:
    print(f"Iteration: {i}")
    i += 1

print("\n=== BOOLEAN FLAG ===")
found = False
numbers = [1, 3, 5, 8, 9]
index = 0

while not found and index < len(numbers):
    if numbers[index] % 2 == 0:
        print(f"Found even number: {numbers[index]}")
        found = True
    index += 1

print("\n=== ACCUMULATOR VARIABLE ===")
total = 0
i = 1
while i <= 5:
    total += i
    i += 1
print(f"Sum of 1-5: {total}")

🔄 Quick While Loop Practice

Let's practice writing while loops that calculate results!

Hands-on Exercise

Write a while loop to calculate the sum of numbers from 1 to 10.

python
# 10 While Loop Practice - Calculate sum of 1 to

total = 0
count = 1

# 10 TODO: Your code here: write a while loop that adds numbers 1 through
# The final result should be stored in 'total'

Solution and Explanation 💡

Click to see the complete solution
# While Loop Practice - Solution

total = 0
count = 1

while count <= 10:
    total += count
    count += 1

print(f"Sum of 1 to 10: {total}")

Key Learning Points:

  • 📌 Accumulator pattern: Use one variable to collect results (total)
  • 📌 Counter variable: Use another variable to track iterations (count)
  • 📌 Two updates: Update both accumulator and counter in each iteration
  • 📌 Final result: The loop builds up the answer step by step

🛡️ Input Validation with While Loops

One of the most common uses for while loops is input validation. They allow you to repeatedly ask users for input until they provide valid data, creating robust and user-friendly programs.

Input validation loops improve user experience by giving users multiple chances to provide correct input rather than crashing or accepting invalid data.

# Age validation with error handling
print("=== AGE VALIDATION ===")
valid_age = False
while not valid_age:
    try:
        age_input = "25"  # Simulated user input (normally input() function)
        age = int(age_input)
        if 0 <= age <= 120:
            valid_age = True
            print(f"Your age is: {age}")
        else:
            print("Age must be between 0 and 120")
            # In real code, this would continue asking for input
            break  # Breaking for demo purposes
    except ValueError:
        print("Please enter a valid number")
        # In real code, this would continue asking for input
        break  # Breaking for demo purposes

print("\n=== MENU VALIDATION ===")  
valid_choice = False
while not valid_choice:
    print("Choose an option:")
    print("1. Save")
    print("2. Load") 
    print("3. Quit")
    
    choice = "2"  # Simulated user input
    if choice in ["1", "2", "3"]:
        valid_choice = True
        options = {"1": "Save", "2": "Load", "3": "Quit"}
        print(f"You selected: {options[choice]}")
    else:
        print("Invalid choice. Please enter 1, 2, or 3.")
        break  # Breaking for demo purposes

📊 Common While Loop Patterns

There are several patterns you'll use frequently when working with while loops in different programming scenarios.

PatternExampleUse Case
Counter Loopwhile count < 10:Fixed number of iterations
Infinite Loop + Breakwhile True: ... breakInput validation
Flag-Controlledwhile not done:Event-driven processing
Sentinel Valuewhile value != -1:Process until special value
List Processingwhile items:Process until collection empty
# Common while loop patterns
print("=== COUNTER PATTERN ===")
count = 0
while count < 3:
    print(f"Count: {count}")
    count += 1

print("\n=== INFINITE LOOP + BREAK PATTERN ===")
attempt = 0
while True:
    attempt += 1
    password = "secret" if attempt == 2 else "wrong"  # Simulated input
    
    if password == "secret":
        print("Access granted!")
        break
    elif attempt >= 3:
        print("Too many attempts!")
        break
    else:
        print(f"Wrong password. Attempt {attempt}/3")

print("\n=== FLAG-CONTROLLED PATTERN ===")
processing = True
items_processed = 0
while processing:
    items_processed += 1
    print(f"Processing item {items_processed}")
    
    if items_processed >= 3:  # Stop condition
        processing = False

print("\n=== LIST PROCESSING PATTERN ===")
items = ["apple", "banana", "cherry"]
while items:
    item = items.pop(0)  # Remove first item
    print(f"Processing: {item}")

⚡ Loop Control with Break and Continue

While loops can be controlled using break and continue statements, providing fine-grained control over loop execution flow.

These control statements work with conditional logic to create sophisticated loop behavior.

# Loop control examples
print("=== BREAK EXAMPLE ===")
i = 0
while i < 10:
    i += 1
    if i == 2:
        continue  # Skip when i is 2
    if i == 5:
        break     # Exit when i is 5
    print(i)

print("\n=== WHILE-ELSE EXAMPLE ===")
# Using else with while loop
target = 7
num = 1
while num <= 5:
    if num == target:
        print(f"Found {target}!")
        break
    num += 1
else:
    print(f"{target} not found")  # Executes because loop completed normally

✅ While Loop Best Practices

Following best practices helps you write efficient, readable, and maintainable while loops.

📚 Key Takeaways

Test Your Knowledge

Test what you've learned about while loops in Python:

🚀 What's Next?

Congratulations! You now understand while loops and can create condition-based repetition in your Python programs. While loops are perfect for situations where you don't know exactly how many iterations you need.

Next, you'll learn about For Loops, which are designed for iterating through sequences and collections. For loops complement while loops by providing an elegant way to process lists, strings, and other iterable data structures.

While loops work hand-in-hand with input validation, decision making, and data processing scenarios.

Understanding while loops prepares you for advanced loop techniques, function development, and error handling.

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