While Loops

Introduction

A while loop runs a block of code repeatedly as long as a condition remains True. Unlike a for loop, it does not iterate over a fixed sequence — it keeps going until you tell it to stop.

Basic Syntax

while condition:
    # code to run while condition is True

Example

count = 0

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

Output

0
1
2
3
4

Always make sure something inside the loop will eventually make the condition False, or the loop will run forever.

Infinite Loops

An infinite loop runs indefinitely. Useful in some contexts (like waiting for user input), but usually a bug.

# Intentional: keep asking until a valid response
while True:
    answer = input("Type 'yes' to continue: ")
    if answer == "yes":
        break
    print("Please type 'yes'")

Use Ctrl+C in the terminal to kill an unintended infinite loop.

break and continue

Same as in for loops — break exits immediately, continue skips to the next iteration.

attempts = 0

while attempts < 5:
    password = input("Enter password: ")
    if password == "secret":
        print("Access granted")
        break
    attempts += 1
    print(f"Wrong. {5 - attempts} attempts remaining.")
else:
    print("Too many failed attempts.")

The else block on a while loop runs only if the loop finished without hitting a break.

When to Use while vs for

Use for when	Use while when
You know how many times to iterate	You don't know how many iterations are needed
Iterating over a list or range	Waiting for a condition to change
Reading items from a collection	Retry logic, polling, or user input

Practice Exercises

• Write a while loop that prints numbers from 10 down to 1.
• Write a loop that keeps asking the user to guess a number until they get it right (use a fixed number as the answer).
• Use a while loop to sum all integers from 1 to 100 and print the result.
• Write a loop that counts how many times you can halve 1000 before it goes below 1.