while loops

A while loop repeats its block as long as its condition stays True:

n = 3
while n > 0:
    print(n)
    n -= 1     # without this the loop never ends!

Each pass through the loop is called an iteration.

Step through it: the highlight goes round and round while the n box drops 3, 2, 1, 0. When it reaches 0, n > 0 is False and the loop ends.

See it run

coins = 1
days = 0
while coins < 10:
    coins = coins * 2
    days = days + 1
print(days, coins)

A while loop re-checks its condition before every pass. Watch coins double until the test fails.

Practice while loops

Exercise 1

Count down from n = 5 with a while loop, printing 5, 4, 3, 2, 1 on separate lines, then print Liftoff!.

Exercise 2

A delivery robot has battery = 14. Each trip uses 4 units. Use a while loop to make as many complete trips as possible; store the count in trips and the remaining battery in battery. Print Trips: 3 then Battery: 2.

Exercise 3

A fundraiser starts at saved = 2 and doubles it each day. Use a while loop until savings first reach or exceed target = 25. Store days and saved, then print 4 days: 32. Do not stop a day early or run an extra day.

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Practice

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Visualizer Watch your code run line by line

Run your code, then choose Play to follow its execution.

Memory · names → values

Variables appear here as Python creates them.

Output & plots