While loops

Loop while a condition is true

• A while loop repeats as long as its condition is true.
• It checks the condition before each pass.
• Use it when you do not know the number of repeats in advance.

Always change the condition

• Something inside the loop must move toward making the condition false.
• If it never becomes false, the loop runs forever (an infinite loop).
• Press Stop if a loop ever gets stuck.

count = 3
while count > 0:
    print(count)
    count = count - 1

break and continue

• break stops the loop right away.
• continue skips the rest of this pass and goes to the next one.
• while True: loops forever until a break ends it.

n = 0
while True:
    n = n + 1
    if n == 4:
        break
    print(n)

Checking before or after

• A while loop checks first, so it may run zero times.
• A "repeat until" loop checks after, so it always runs at least once.
• Python has no repeat; use while True: with a break at the end.

In Cambridge pseudocode

• while is the pre-condition loop; REPEAT … UNTIL is the post-condition loop.

Count ← 3
WHILE Count > 0
    OUTPUT Count
    Count ← Count - 1
ENDWHILE

REPEAT
    OUTPUT Count
    Count ← Count + 1
UNTIL Count = 3

Common mistakes

• Change something inside the loop so the condition can become false, or it runs forever.
• A while checks its condition first, so it may run zero times.

Now you try

• Make sure your condition will eventually become false.
• Stuck on the loop? Do the short drill Finish the countdown first. It is one missing line.
• Press Check answer to test your code.