While Loops

Beginner ~20 min read

The while loop executes code repeatedly as long as a condition is true. Unlike for loops that iterate a known number of times, while loops are perfect when you don't know in advance how many iterations you need!

Basic While Loop

A while loop checks its condition before each iteration. When the condition becomes False, the loop stops and execution continues after the loop.

while condition:
    # code to execute (repeat while condition is True)
    # IMPORTANT: Update something to eventually make condition False!

Output

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Key Difference: Use for loops when you know how many times to iterate (or are iterating over a sequence). Use while loops when the number of iterations depends on a condition that changes during execution.

The `while...else` Clause

Python's while loop has an optional else clause that runs when the condition becomes False normally. If the loop exits via break, the else is skipped.

# While Loop with Else Clause

# The else block runs when the condition becomes False
# It does NOT run if the loop is exited via 'break'

print("=== While-Else (Normal Exit) ===")
count = 1
while count <= 3:
    print(f"Count: {count}")
    count += 1
else:
    print("Loop completed normally!")  # This runs!

print()

# Else with break - else does NOT run
print("=== While-Else (Break Exit) ===")
count = 1
while count <= 5:
    print(f"Count: {count}")
    if count == 3:
        print("Breaking early!")
        break
    count += 1
else:
    print("This won't print because we used break")

print()

# Practical example: Search with else
print("=== Search Example ===")
numbers = [1, 3, 5, 7, 9]
target = 7
index = 0

while index < len(numbers):
    if numbers[index] == target:
        print(f"Found {target} at index {index}!")
        break
    index += 1
else:
    print(f"{target} not found in the list")

print()

# Another search - target not found
target = 4
index = 0

while index < len(numbers):
    if numbers[index] == target:
        print(f"Found {target} at index {index}!")
        break
    index += 1
else:
    print(f"{target} not found in the list")  # This runs!

Output

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Pro Tip: The while...else pattern is useful for search loops. The else block runs only if the item wasn't found (no break). Think of it as "if not found" code!

Practical Examples

While loops excel at input validation, processing queues, retry logic, and any situation where you repeat until a condition is met.

# Practical While Loop Examples

# 1. Input validation (simulated)
print("=== Input Validation ===")
# Simulating user input - in real code use input()
inputs = ["abc", "-5", "42"]  # Simulated inputs
input_index = 0

valid_number = None
while valid_number is None:
    user_input = inputs[input_index]
    input_index += 1
    print(f"Checking input: '{user_input}'")

if user_input.isdigit() and int(user_input) > 0:
        valid_number = int(user_input)
        print(f"Valid number: {valid_number}")
    else:
        print("Please enter a positive number.")

print()

# 2. Processing queue/list until empty
print("=== Processing Queue ===")
tasks = ["Send email", "Update database", "Generate report"]

while tasks:  # While list is not empty
    current_task = tasks.pop(0)  # Remove first item
    print(f"Processing: {current_task}")

print("All tasks completed!")

print()

# 3. Retry with maximum attempts
print("=== Retry Logic ===")
max_attempts = 3
attempts = 0
success = False

while attempts < max_attempts and not success:
    attempts += 1
    print(f"Attempt {attempts}...")

# Simulate: success on 3rd attempt
    if attempts == 3:
        success = True
        print("Operation succeeded!")

if not success:
    print("Max attempts reached. Operation failed.")

print()

# 4. Reading data in chunks (simulated)
print("=== Chunk Processing ===")
data = list(range(1, 11))  # [1, 2, ..., 10]
chunk_size = 3
start = 0

while start < len(data):
    chunk = data[start:start + chunk_size]
    print(f"Processing chunk: {chunk}")
    start += chunk_size

print("All data processed!")

Output

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Infinite Loops: Danger and Use Cases

An infinite loop occurs when the condition never becomes False. Usually this is a bug, but sometimes while True: with a break is a valid pattern.

# Infinite Loops and How to Avoid Them

# DANGEROUS: Infinite loop examples (DON'T run as-is!)
print("=== Common Infinite Loop Mistakes ===")
print()

# Mistake 1: Forgetting to update counter
print("Mistake 1: Forgetting to update counter")
print('''
# This runs FOREVER!
count = 1
while count <= 5:
    print(count)
    # Missing: count += 1
''')

# Mistake 2: Wrong comparison
print("Mistake 2: Wrong comparison direction")
print('''
# This runs FOREVER!
n = 10
while n > 0:
    print(n)
    n += 1  # Should be n -= 1
''')

# Mistake 3: Condition always True
print("Mistake 3: Condition never becomes False")
print('''
# This runs FOREVER!
x = 5
while x != 10:
    print(x)
    x += 2  # x is 5, 7, 9, 11, 13... never equals 10!
''')

print()
print("=== Intentional Infinite Loop with Break ===")

# Sometimes infinite loops with break are useful
print("(Simulated - runs only 3 times)")
iteration = 0
while True:  # Infinite loop!
    iteration += 1
    print(f"Iteration {iteration}")

# Condition to break out
    if iteration >= 3:
        print("Breaking out of infinite loop")
        break

print()

# Safe pattern: Always have a failsafe
print("=== Safe Infinite Loop Pattern ===")
max_iterations = 100  # Failsafe
iteration = 0

while True:
    iteration += 1

# Your actual exit condition
    if iteration >= 3:
        print("Normal exit condition met")
        break

# Failsafe to prevent truly infinite loop
    if iteration >= max_iterations:
        print("Failsafe triggered!")
        break

print(f"Working... iteration {iteration}")

Output

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Infinite Loop Prevention: Always ensure your while loop has: (1) a condition that can become False, (2) code inside the loop that changes the condition, or (3) a break statement that will eventually execute. Consider adding a failsafe maximum iteration count!

Common Mistakes

1. Forgetting to update the loop variable

# Wrong - infinite loop!
count = 1
while count <= 5:
    print(count)
    # Missing: count += 1

# Correct
count = 1
while count <= 5:
    print(count)
    count += 1

2. Off-by-one errors

# Wrong - prints 1-4, not 1-5
count = 1
while count < 5:  # Should be <= 5
    print(count)
    count += 1

# Correct - prints 1-5
count = 1
while count <= 5:
    print(count)
    count += 1

3. Updating in wrong direction

# Wrong - counting up when should count down!
n = 10
while n > 0:
    print(n)
    n += 1  # Should be n -= 1!

# Correct
n = 10
while n > 0:
    print(n)
    n -= 1

4. Condition never matches

# Wrong - 5 can never equal 10 with step of 2
x = 5
while x != 10:
    print(x)
    x += 2  # x: 5, 7, 9, 11, 13... never 10!

# Correct - use >= or <=
x = 5
while x < 10:
    print(x)
    x += 2

Exercise: Countdown Timer

Task: Create a countdown using a while loop.

Requirements:

Print numbers from 10 down to 1
Print "Blastoff!" after the loop

Output

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Show Solution

start = 10
current = start

while current > 0:
    print(current)
    current -= 1  # Decrease by 1 each iteration

print("Blastoff!")

Summary

while condition: Repeats while condition is True
Must update something to eventually make condition False
while...else: else runs only if loop completes normally (no break)
while True: Intentional infinite loop, must have break
Use cases: Input validation, retry logic, processing until done
Danger: Infinite loops when condition never becomes False

What's Next?

Now that you know both for and while loops, let's learn about loop control statements - break, continue, and pass. These give you fine-grained control over loop execution!

Previous For Loops

Next Loop Control

Basic While Loop

The while...else Clause

Practical Examples

Infinite Loops: Danger and Use Cases

Common Mistakes

1. Forgetting to update the loop variable

2. Off-by-one errors

3. Updating in wrong direction

4. Condition never matches

Exercise: Countdown Timer

Summary

What's Next?

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The `while...else` Clause