while loops

Repeating while something is true

A while loop repeats its body for as long as a condition holds:

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

Python checks the condition, runs the body, then checks again — over and over until the condition is false.

The line that matters most is count -= 1. Something inside the loop must move the condition towards being false. Forget it and the loop never ends; PyLearn will stop your program after five seconds and say so, which is a much gentler lesson than a frozen tab.

Your turn: add up every whole number from 1 to limit inclusive using a while loop, leaving the answer in total.

Counting with a condition

while suits problems where you do not know in advance how many steps are needed — you only know when to stop.

n = 100
steps = 0
while n > 1:
    n = n // 2
    steps += 1

That counts how many halvings it takes to get down to 1. You could not write that as a simple counting loop, because the number of steps depends on the arithmetic.

Your turn: write countdown_length(n) returning how many times you can halve n (using //) before it reaches 1 or below. For 8 that is 3.