while loops

Repeat as long as

Predict what this prints, line by line.

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

3
2
1
Go!

Read while aloud as "as long as": "as long as count is more than 0, print it and take one off". The condition isn't when to stop; it's when to keep going. The indented block is the body of the looploopCode that repeats. A while loop repeats its block as long as its condition is true., and each time through it is a round. Step through it:

3
2
1
Go!

count goes up or down by one each round. A name used like that is a counter, the first of several plans you'll learn to spot in loops. Watch the marked line jump back to line 2 after every round: that's where the decision happens.

The condition is checked at the top

Python checks the condition only at the start of each round, never in the middle. Here the goal is reached halfway through a round. Does the round stop?

Saved: 800
Saved: 1100
Goal reached

At step 6, savings is 1100 and the condition is already false, but line 4 still runs. The round always finishes; Python only notices at line 2. savings grows by adding to itself each round, a plan called a running total.

A loop that never stops

If nothing in the body moves the condition towards false, the loop never ends. Here the counter goes the wrong way. Run it and wait five seconds.

count = 3
total = 0
while count > 0:
    total = total + count
    count = count + 1
print(total)

You'll see "Stopped after 5 seconds. Is there an infinite loop?" count goes 3, 4, 5, … and is always more than 0. Change + 1 to - 1 on line 5 and it prints 6. When a loop won't stop, check that its body changes what the condition is about.

Ask until it's valid

Users type mistakes. A common plan is to keep asking until the answer is valid:

while True:
    score = int(input("Score (0-100): "))
    if 0 <= score <= 100:
        break
    print("Please enter a score from 0 to 100.")
print("Saved:", score)

while True: would run forever on its own. The way out is breakbreakLeaves the loop immediately. Python carries on with the first line after the loop., which leaves the loop at once. So: ask, and if the answer is good, break out; otherwise complain and go round again. score is a latest value: each round replaces it with the newest answer. You'll use this "ask until valid" plan in almost every program that takes input. Forget the int(), and the first comparison fails:

How many months?

Plans combine. Hana saves 700 birr a month towards a 5,000 birr phone. How many months until she has enough? savings is a running total and months is a counter:

goal = 5000
deposit = 700
savings = 0
months = 0
while savings < goal:
    savings = savings + deposit
    months = months + 1
print(f"{months} months: {savings} birr")

Try a deposit of 1,000: exactly 5 months. Then try 5,000 as the deposit.