while loops

A while loop repeats a block of code as long as a condition remains true. Use it when you do not know in advance how many times the loop needs to run.

The basic while loop

A while loop checks a condition before each iteration:

count = 0

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

This prints 0 through 4. The loop:

1. checks if count < 5
2. if true, runs the body
3. increments count
4. repeats from step 1

When the condition becomes false, the loop stops and execution continues after the loop.

Infinite loops

A loop whose condition is always true runs forever — or until you stop it:

while True:
    command = input("> ")    # input() reads a line from the user
    if command == "quit":
        break
    print(f"You typed: {command}")

input() displays a prompt and waits for the user to type a line of text. It returns whatever the user types as a string.

This is a common pattern for interactive programs. The loop runs until the user types a specific command, which triggers a break.

break

The break statement exits the loop immediately:

attempts = 0

while attempts < 3:
    password = input("Enter password: ")
    if password == "secret":
        print("Access granted.")
        break
    attempts = attempts + 1
else:
    print("Too many attempts.")

When break executes, the loop ends and any else clause is skipped.

continue

The continue statement skips the rest of the current iteration and jumps back to the condition check:

total = 0

while True:
    line = input("Enter a number (or 'done'): ")
    if line == "done":
        break
    if not line:
        continue        # skip empty lines
    total = total + int(line)

print(f"Total: {total}")

continue does not exit the loop — it just skips to the next iteration.

The else clause on loops

Python allows an else block on loops. It runs when the loop finishes normally — that is, the condition became false — but not when the loop exits via break:

n = 2

while n < 10:
    if n == 5:
        print("Found 5.")
        break
    n = n + 1
else:
    print("Loop completed without break.")

This prints "Found 5." and the else block does not run. If you remove the break, the else block runs after the loop finishes.

In practice, else on loops is rarely used. break with a flag variable or a function with return is often clearer. It is worth learning because you will encounter it in other people’s code, and understanding it helps you read Python more broadly.

Common while loop patterns

Reading until a condition

line = input("Enter text (empty line to stop): ")

while line:
    print(f"Got: {line}")
    line = input("Enter text (empty line to stop): ")

This pattern duplicates the input() call — once before the loop and once inside it. The while True / break pattern you saw earlier avoids this duplication and is generally preferred for interactive input.

Retry with a limit

import random

max_retries = 3
attempt = 0

while attempt < max_retries:
    # Simulate an operation that might fail (50% success rate)
    success = random.random() > 0.5
    if success:
        print("Succeeded.")
        break
    attempt = attempt + 1
    print(f"Attempt {attempt} failed, retrying...")

Running total

total = 0

while True:
    value = int(input("Enter a number (0 to stop): "))
    if value == 0:
        break
    total = total + value

print(f"Sum: {total}")

What to carry forward

• while repeats a block as long as its condition is true
• the condition is checked before each iteration
• always ensure the loop can terminate — change the condition or use break
• break exits the loop immediately
• continue skips to the next iteration
• else on a loop runs only if the loop finishes without break
• while is best when you do not know the number of iterations in advance

When you do know how many times to iterate — or when you want to iterate over a collection — a for loop is the better tool. The next lesson covers for loops and the range() function.