Doing Things Again — while Loops

By the end of this lesson you can:

• Write a while loop with a clear stop condition.
• Use a counter variable to repeat a block a known number of times.
• Use a Boolean flag to stop a loop when the user is finished.

Yesterday's Rock-Paper-Scissors was fun — but to play three rounds, you had to run the program three times. That's the bit we'll fix today.

Quick predict-the-output puzzle. What does this print, and how many lines does it print in total?

n = 3
while n > 0:
    print("Round", n)
    n = n - 1
print("Game over")

Round 3
Round 2
Round 1
Game over

Part 1 · The shape of a while loop

A while loop looks almost identical to an if statement. The two big differences: it uses the word while, and Python runs the block again and again until the condition stops being true.

while CONDITION:
    do something
    do something else

Same colon, same 4-space indentation, same rule: every line that belongs inside the loop is indented by four spaces.

Part 2 · The counter pattern

The most common way to control a while loop is with a counter: a number you start at one value, check inside the condition, and change inside the loop body.

count = 1
while count <= 5:
    print("Hello number", count)
    count = count + 1

Read it like a recipe:

1. count starts at 1.
2. Is count <= 5 true? Yes — run the indented block.
3. The block prints, then nudges count up by one.
4. Back to step 2. Eventually count reaches 6, the condition becomes false, and the loop stops.

Part 3 · The dreaded infinite loop

If you forget to change the counter inside the loop, the condition stays true forever and your program never ends. This is the infinite loop bug.

count = 1
while count <= 5:
    print("Stuck!")

Part 4 · Stopping a loop with a Boolean flag

Counters work brilliantly when you know up front how many times to repeat. But sometimes you want to keep going until the user says stop — there's no number to count down. That's a job for a Boolean flag.

running = True
while running:
    answer = input("Type 'stop' to end, anything else to continue: ")
    if answer == "stop":
        running = False
    else:
        print("Still going…")
print("Loop is over.")

Two ideas at work: the loop runs while running is true, and somewhere inside the body, an if sets running to False when it's time to stop.

Part 5 · while True: + a flag

Some people prefer writing while True: and using if to flip the flag — same idea, slightly different shape. Either pattern is fine in Level 1. Stick with the flag version for now; it's easier to reason about.

Daniel Tan is saving for a new bike. Each week he wants to type in everything he spent and have Python total it up. We'll build a tiny spend tracker that keeps asking for amounts until he types done. Save this as spend_tracker.py.

Code

# spend_tracker.py — add spending amounts until the user is finished

total      = 0.0
item_count = 0
done       = False

while not done:
    answer = input("RM amount (or 'done' to finish)? ")
    if answer == "done":
        done = True
    else:
        amount = float(answer)
        total       = total + amount
        item_count  = item_count + 1
        print("  Added RM", amount, "· running total: RM", total)

print()
print("Spending done — you logged", item_count, "items.")
print("Final total: RM", total)

One sample run (lines you type are bold):

Output

RM amount (or 'done' to finish)? <strong>5.50</strong>
  Added RM 5.5 · running total: RM 5.5
RM amount (or 'done' to finish)? <strong>2.00</strong>
  Added RM 2.0 · running total: RM 7.5
RM amount (or 'done' to finish)? <strong>8.50</strong>
  Added RM 8.5 · running total: RM 16.0
RM amount (or 'done' to finish)? <strong>done</strong>

Spending done — you logged 3 items.
Final total: RM 16.0

Reading the loop slowly

• total and item_count are accumulators — they start at zero and grow inside the loop.
• done is a Boolean flag. The loop runs while it is not done — a really natural English reading thanks to not from PY-L1-10.
• Inside the loop, exactly one of two branches runs: either we set done = True to leave next time round, or we update the running total.
• When the loop finishes (because not done is now false), Python falls through to the final two print lines.

Run each task with at least two different inputs before moving on.

n = int(input("Count up to? "))
i = 1
while i <= n:
    print(i)
    i = i + 1

n = int(input("Sum up to? "))
i     = 1
total = 0
while i <= n:
    total = total + i
    i     = i + 1
print("Sum is", total)

biggest = 0
done    = False
while not done:
    answer = input("A number (or 'done')? ")
    if answer == "done":
        done = True
    else:
        value = int(answer)
        if value > biggest:
            biggest = value
print("Biggest was", biggest)

One small keyword unlocked a huge new ability. With while, a counter, and a stop condition, our programs can do a job ten times, or until the user says enough — without a single copy-pasted line. Watch the counter every time: forgetting to update it is how you build an infinite loop.