while loops

A while loop repeats a block of code as long as a condition stays true.

count = 1
while count <= 5:
    print("Count is", count)
    count += 1
print("Done!")

Each time round the loop, Python checks the condition. When count reaches 6, count <= 5 is false and the loop ends.

The three parts of a counting loop

1. Start: set up a variable before the loop (count = 1).
2. Condition: decide when to keep going (count <= 5).
3. Change: move towards the end inside the loop (count += 1).

Forget the third part and the condition never becomes false.

Infinite loops

n = 1
while n > 0:
    n += 1

That loop never ends, because n only grows. CodeMIRAI stops a program that runs too long; on your own computer you would press Ctrl + C.

Accumulating a result

Loops often build up a total:

total = 0
n = 1
while n <= 100:
    total += n
    n += 1
print("1 + 2 + ... + 100 =", total)

Looping until a condition is met

while shines when you don't know in advance how many times to repeat:

savings = 1000
months = 0
while savings < 2000:
    savings *= 1.01      # 1% growth per month
    months += 1
print(f"Doubled after {months} months: {savings:.2f}")

Reading input until the user stops

total = 0
while True:
    entry = input("Number (or 'done'): ")
    if entry == "done":
        break
    total += int(entry)
print("Total:", total)

while True: loops forever on purpose, and break (more on it soon) leaves the loop when the user types done.