Repeat until something changes

while condition: repeats as long as the condition is true. Use it when you do not know in advance how many times.

Hone is a place to practise programming. This is one of its lessons, written out in full and free to read without an account.

What it is for

Reading until end of file, retrying until success, a game loop, walking a linked structure: all 'until', not 'this many times'.

How to think about it

When should this stop, and what moves it there? Write the stopping condition first, in English. Then make sure something inside the loop moves toward it every time, or you have written forever.

Worked example

n = 27; steps = 0

The Collatz walk: until n reaches 1.

while n != 1:

Stop condition.

    n = n // 2 if n % 2 == 0 else 3 * n + 1

Something changes each turn.

    steps += 1

Count.

print(steps)

111.

Your turn

Keep halving until below 1.

while x  1:
    x = x / 2

The trap

Forgetting to change the variable in the condition. The loop never ends; on a phone the tab hangs.

Practise while loops on HoneA question on it now, a coding challenge where there is one, and it is remembered for review. Free, no email needed.