while Loops

A while loop repeats a block for as long as its condition stays true. It is the loop to reach for when you do not know in advance how many repetitions you need.

count = 3

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

print("Lift off")

Trace it once by hand: check 3 > 0 → print, decrement → check 2 > 0 → … → check 0 > 0 → false, so the loop ends and the last line runs.

Accumulating a result

The classic while shape is: set something up, loop until done, use the result.

total = 0
n = 1

while total < 100:
    total += n
    n += 1

print(f"Adding 1..{n - 1} first exceeds 100 at {total}")

Validating input

This is where while earns its place in everyday code: keep asking until the answer makes sense.

answer = ""

while not answer.isdigit():
    answer = input("Enter a positive whole number: ")
    if not answer.isdigit():
        print("  That is not a positive whole number.")

print(f"Thank you: {int(answer)}")

Three answers are pre-filled for this one — maybe, -4, then 7 — so you can watch it reject two and accept the third.

break and continue

break leaves the loop immediately. continue skips the rest of this pass and goes back to the condition.

n = 0
while True:                 # deliberately endless...
    n += 1
    if n % 2 == 0:
        continue            # skip even numbers
    if n > 9:
        break               # ...until this fires
    print(n)

print("Done at", n)

while True: with a break inside is a normal, readable Python idiom for “loop until something happens”. It is not a code smell, as long as the break is easy to find.

The loop else clause

Python loops can take an else, which runs only if the loop finished without hitting a break. It is unusual, and perfect for search loops.

target = 7
n = 1

while n <= 5:
    if n == target:
        print("Found it")
        break
    n += 1
else:
    print(f"{target} was not in range")

Without else you would need a found = False flag and an if not found: afterwards. The else says the same thing in one word.

Before you write your own

Predict. You save $30 a week towards $100. What does this print? Trace it by hand, one pass at a time, before you run it.

savings = 0
weeks = 0

while savings < 100:
    savings += 30
    weeks += 1

print(weeks, savings)

4 120

Fix. A countdown should print 5, 4, 3, 2, 1 and then Go!. One number is missing.

count = 5
while count > 1:
    print(count)
    count -= 1
print("Go!")

count = 5
while count > 0:
    print(count)
    count -= 1
print("Go!")

5
4
3
2
1
Go!

secret = 42
guesses = 0

# Your code here

secret = 42
guesses = 0

while True:
    guess = int(input("Guess: "))
    guesses += 1

    if guess == secret:
        print(f"Correct in {guesses} guesses")
        break
    elif guess < secret:
        print("  Higher")
    else:
        print("  Lower")

What you learned

• while condition: repeats until the condition goes false.
• Something in the body must move towards the exit, or the loop never ends.
• break exits now; continue skips to the next pass.
• while True: with a break is the idiom for “until something happens”.
• A loop’s else runs when no break fired.