Repeating with while

Why this matters

Computers are good at repeating things: checking every message, adding up every price, asking again until the answer is valid. A loop repeats a block of code. while repeats it for as long as a condition stays true.

The idea

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

This prints 3, 2, 1, Liftoff!. Python follows these steps:

1. Check the condition (count > 0).
2. If it is true, run the indented block, then go back to step 1.
3. If it is false, skip the block and carry on below the loop.

Two patterns you will use constantly:

• A counter counts how many times something happened: weeks = weeks + 1 (or the shorthand weeks += 1).
• An accumulator builds up a total: total += price.

Both need a starting value before the loop.

The classic bug: the endless loop

If nothing inside the loop ever makes the condition false, it runs forever:

count = 3
while count > 0:
    print(count)      # forgot to change count!

In this site, press Stop ■ to stop a runaway program. Whenever you write a while, ask: what changes each time round, and will that eventually make the condition false?

Try it

Write a while loop that doubles a number (starting at 1) until it is bigger than 1000, printing each value. How many values are printed?

Your turn

You are saving for something that costs goal dollars by putting deposit dollars into a jar every week. The starter reads both numbers. Use a while loop to count the number of weeks until you have at least the goal, and print:

Weeks needed: 4

(for a goal of 100 and a deposit of 30: after 4 weeks you have 120). If the goal is 0, you need 0 weeks.

Recap

• while condition: repeats its block as long as the condition is true.
• Set up counters and accumulators before the loop and update them inside it.
• Make sure something in the loop moves towards ending it.

Common mistakes

• Forgetting to update the variable the condition depends on, which causes an endless loop.
• Initialising inside the loop, which resets the counter every time.
• Off-by-one errors: < stops when equal; <= keeps going. Test the boundaries.