Lesson 5.2: while Loops

What Is a while Loop?

A while loop repeats a block of code as long as a condition is True. Unlike a for loop (which iterates a known number of times), a while loop keeps going until its condition becomes False.

# Count from 1 to 5
count = 1

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

print("Done!")

Here is how it works step by step:

1. Check: Is count <= 5? (1 <= 5 is True) → run the body
2. Print 1, then count becomes 2
3. Check: Is count <= 5? (2 <= 5 is True) → run the body
4. ...continues until count is 6
5. Check: Is count <= 5? (6 <= 5 is False) → exit the loop

while vs. for: When to Use Which

Use a for loop when you know how many times to repeat (or you are iterating over a collection). Use a while loop when you do not know in advance and the loop depends on a condition.

# Keep halving a number until it's less than 1
number = 100

while number >= 1:
    print(number)
    number /= 2

print(f"Final value: {number}")

Infinite Loops (and How to Avoid Them)

If the condition never becomes False, the loop runs forever. This is called an infinite loop, and it will freeze your program.

# INFINITE LOOP - DO NOT RUN THIS!
# count = 1
# while count <= 5:
#     print(count)
#     # Forgot to increment count!
#     # count never changes, so count <= 5 is always True

# FIXED version:
count = 1
while count <= 5:
    print(count)
    count += 1    # This makes count eventually > 5

Three common causes of infinite loops:

1. Forgetting to update the loop variable
2. Updating the variable in the wrong direction
3. A condition that can never become False

Input Validation with while Loops

One of the most practical uses of while loops is asking the user for input until they give a valid answer.

# Ask for a positive number
number = -1  # Start with an invalid value

while number < 0:
    number = int(input("Enter a positive number: "))
    if number < 0:
        print("That's negative! Try again.")

print(f"You entered: {number}")

correct_password = "python123"
attempts = 0
max_attempts = 3

while attempts < max_attempts:
    guess = input("Enter password: ")
    attempts += 1

    if guess == correct_password:
        print("Access granted!")
        break  # Exit the loop early
    else:
        remaining = max_attempts - attempts
        if remaining > 0:
            print(f"Wrong! {remaining} attempts left.")
        else:
            print("Account locked.")

Sentinel Values

A sentinel value is a special value that signals the loop to stop. It is commonly used when processing a series of user inputs.

# Add up numbers until the user types 'done'
total = 0
count = 0

print("Enter numbers to add up. Type 'done' to finish.")

while True:
    user_input = input("Number: ")

    if user_input.lower() == "done":
        break

    total += float(user_input)
    count += 1

if count > 0:
    print(f"Sum: {total}")
    print(f"Average: {total / count}")
else:
    print("No numbers entered.")

Practical Examples

seconds = 5

while seconds > 0:
    print(f"{seconds}...")
    seconds -= 1

print("Go!")

secret = 42
guess = 0
attempts = 0

while guess != secret:
    guess = int(input("Guess the number (1-100): "))
    attempts += 1

    if guess < secret:
        print("Too low!")
    elif guess > secret:
        print("Too high!")
    else:
        print(f"Correct! You got it in {attempts} attempts.")