- Understand when to use while loops vs for loops
- Write while loops that run until a condition changes
- Use break to exit loops early
- Use continue to skip iterations
While Loops and Loop Control
Sometimes you don't know in advance how many times you need to repeat something. A for loop is great when you're iterating over a list of 10 items. But what if you're asking users for input until they type "quit"? Or waiting for a game to end? Or processing data until you find what you're looking for?
This is where while loops shine. They keep running as long as a condition is true – whether that's 3 times, 100 times, or until something specific happens. Combined with break and continue, they give you complete control over your program's flow.
The while Loop
A while loop repeats as long as its condition remains True.
Anatomy of a while Loop
while Loop Structure
while condition: ← Check condition BEFORE each run
# do something ← Loop body (indented)
# update something ← Important: change the condition!
next_code() ← Runs after loop ends
If condition never becomes False → Infinite loop!
Visual Flow
while Loop Flow
Check Condition
True False
Run Loop Exit Loop
Body
Simple Countdown Example
countdown = 5
while countdown > 0:
print(countdown)
countdown -= 1 # Don't forget this!
print(" Liftoff!")
Output:
5
4
3
2
1
Liftoff!
What happens:
countdown = 5: condition5 > 0is True → run body- Print 5, then
countdown = 4: condition4 > 0is True → run body - ... continues until
countdown = 0 0 > 0is False → exit loop
for vs while: When to Use Each
for vs while Decision Guide
Use FOR when:
Iterating over a sequence (list, string, range)
You know how many times to repeat
Processing each item in a collection
Use WHILE when:
You don't know how many iterations needed
Repeating until a condition changes
Waiting for user input or external events
Implementing game loops or continuous processes
Comparison Examples
# FOR: Known iterations
# "Process these 5 specific items"
numbers = [10, 20, 30, 40, 50]
for num in numbers:
print(num)
# WHILE: Unknown iterations
# "Keep asking until they say yes"
answer = ""
while answer != "yes":
answer = input("Do you agree? (yes/no): ")
print("Thank you for agreeing!")
The Infinite Loop Trap
The most common while loop mistake is forgetting to update the condition:
# DANGER: Infinite loop!
count = 0
while count < 5:
print("Hello!")
# Oops! We never change count!
# This prints "Hello!" forever...
# CORRECT: Update the condition
count = 0
while count < 5:
print("Hello!")
count += 1 # Now it will stop!
How to Stop an Infinite Loop
If you accidentally create one:
- In the terminal: Press
Ctrl + C - In an IDE: Click the stop button
Intentional Infinite Loops
Sometimes infinite loops are useful with break:
# Menu system - runs until user chooses to exit
while True:
print("\n1. Play Game")
print("2. View Score")
print("3. Exit")
choice = input("Choose: ")
if choice == "3":
print("Goodbye!")
break # Exit the loop
elif choice == "1":
print("Playing game...")
elif choice == "2":
print("Score: 100")
break: Emergency Exit
break immediately exits the loop, no matter what the condition says.
Real-World Analogy
Think of break like finding what you're looking for while shopping. You don't need to check every aisle once you've found your item – you go straight to checkout!
break Example
# Search for a number
numbers = [4, 8, 15, 16, 23, 42]
target = 16
for num in numbers:
print(f"Checking {num}...")
if num == target:
print(f"Found {target}!")
break # Stop searching
print("Search complete")
Output:
Checking 4...
Checking 8...
Checking 15...
Checking 16...
Found 16!
Search complete
Notice: 23 and 42 were never checked!
break with while
# ATM withdrawal - max 3 attempts
attempts = 0
correct_pin = "1234"
while True:
pin = input("Enter PIN: ")
attempts += 1
if pin == correct_pin:
print(" Access granted!")
break
if attempts >= 3:
print(" Too many attempts. Card blocked.")
break
print(f"Wrong PIN. {3 - attempts} attempts remaining.")
⏭ continue: Skip and Move On
continue skips the rest of the current iteration and moves to the next one.
Real-World Analogy
Imagine reviewing job applications. If an applicant doesn't meet the minimum requirements, you skip to the next application without reading the rest. That's continue!
continue Example
# Print only even numbers
for num in range(1, 11):
if num % 2 != 0: # If odd
continue # Skip to next number
print(num)
# Output: 2, 4, 6, 8, 10
Processing with Exceptions
scores = [85, -1, 92, 78, -1, 95, 88]
print(" Valid Scores:")
total = 0
count = 0
for score in scores:
if score < 0: # Invalid score
print(f" Skipping invalid score: {score}")
continue
print(f" {score}")
total += score
count += 1
if count > 0:
average = total / count
print(f"\nAverage of valid scores: {average:.1f}")
break vs continue Visual
break vs continue
BREAK: CONTINUE:
Iteration 1 Iteration 1
Iteration 2 Iteration 2
BREAK! CONTINUE!
Iteration 3 (skip) Iteration 3 (skipped)
Iteration 4 (skip) Iteration 4 (runs!)
Iteration 5 (skip) Iteration 5 (runs!)
↓
Exit loop
break = "I'm done with this loop entirely"
continue = "Skip this one, try the next"
Common while Loop Patterns
Pattern 1: Input Validation
# Keep asking until valid input
while True:
age_str = input("Enter your age: ")
if age_str.isdigit():
age = int(age_str)
if 0 < age < 150:
break
print("Please enter a valid age (1-149)")
print(f"Your age is {age}")
Pattern 2: Counter-Controlled
# Count until condition met
count = 0
total = 0
while total < 100:
total += 10
count += 1
print(f"Step {count}: Total = {total}")
print(f"Reached {total} in {count} steps")
Pattern 3: Sentinel Value
# Process until special value
print("Enter numbers (type 'done' to finish):")
numbers = []
while True:
value = input("> ")
if value.lower() == "done":
break
if value.isdigit():
numbers.append(int(value))
print(f"You entered: {numbers}")
if numbers:
print(f"Sum: {sum(numbers)}")
Practical Examples
Example 1: Guessing Game
import random
secret = random.randint(1, 100)
attempts = 0
print(" Guess the Number (1-100)")
print("-" * 30)
while True:
guess = input("Your guess: ")
if not guess.isdigit():
print("Please enter a number!")
continue
guess = int(guess)
attempts += 1
if guess < secret:
print(" Too low! Try higher.")
elif guess > secret:
print(" Too high! Try lower.")
else:
print(f" Correct! You got it in {attempts} attempts!")
break
Example 2: Simple Login System
users = {
"alice": "password123",
"bob": "secure456"
}
max_attempts = 3
attempts = 0
print(" Login System")
print("=" * 30)
while attempts < max_attempts:
username = input("Username: ")
password = input("Password: ")
if username in users and users[username] == password:
print(f"\n Welcome, {username}!")
break
attempts += 1
remaining = max_attempts - attempts
if remaining > 0:
print(f" Invalid credentials. {remaining} attempts remaining.\n")
else:
print("\n Account locked. Too many failed attempts.")
Example 3: Progress Tracker
goal = 10000 # Steps goal
current_steps = 0
day = 0
print(" 10,000 Steps Challenge")
print("-" * 30)
while current_steps < goal:
day += 1
# Simulate daily steps (in real app, this would be actual input)
daily_steps = int(input(f"Day {day} - Steps walked: "))
if daily_steps < 0:
print("Steps can't be negative!")
continue
current_steps += daily_steps
remaining = max(0, goal - current_steps)
progress = min(100, (current_steps / goal) * 100)
print(f" Total: {current_steps} | Progress: {progress:.1f}% | Remaining: {remaining}")
if current_steps >= goal:
print(f"\n Goal reached in {day} days! You walked {current_steps} steps!")
Key Takeaways
Remember These Points
while loops repeat while condition is True
while condition:
(body)
Always update something to avoid infinite loops!
break: Exit the loop immediately
⏭ continue: Skip to next iteration
Use for: known iterations (lists, ranges)
Use while: unknown iterations (until condition changes)
while True + break = common pattern for menus/input
What's Next?
Congratulations! You've completed the Control Flow module. You can now make decisions with if/elif/else and repeat actions with for and while loops. These are fundamental building blocks that you'll use in every program you write.
In the next module, we'll explore Data Structures – learning about lists, tuples, dictionaries, and sets. These let you organize and manage collections of data efficiently!