while loops

The while loop repeats a block as long as a condition stays true. Use while when you don't know up front how many iterations you'll need: the number of rounds depends on something that changes inside the loop.

Syntax

n = 1
while n < 1000:
    n = n * 2
n  # 1024

The indented block is executed repeatedly. Before each iteration Python evaluates the condition: as soon as it becomes false, the loop terminates and execution continues after the block.

Common patterns

Manual counter — equivalent to a classic for in other languages:

i = 0
while i < 5:
    print(i)
    i = i + 1

Infinite loop + internal exit condition — useful when the condition is evaluated after doing something:

acc = 0
while True:
    acc = acc + 1
    if acc * acc > 100:
        break
acc  # 11   (11*11 = 121 is the first square > 100)

break exits immediately from the innermost loop (we'll see it in the next lesson).

The classic mistake: the infinite loop

If the variable in the condition does not change inside the loop, the program never terminates. It is the number-one bug with while:

n = 0
while n < 10:
    print(n)   # n is never modified → infinite loop!

In our exercises there is an execution timeout, so an infinite loop will fail with a clear message instead of freezing the browser. But in production an infinite loop is a serious bug.

Infinite loops risk and termination conditions

A while loop executes as long as its condition evaluates to True. If you forget to update the variables that affect the condition, your loop will run infinitely, freezing the runtime. Always ensure that the loop body makes progress towards the exit condition.

Try it yourself

Review exercise

Additional challenge