Sequence: order is part of the logic

Worked example

A raffle kiosk, two orders, two totals

A school fun-fair raffle kiosk sells a $5 ticket bundle. Members get a 20% discount, and every sale adds a fixed $1 booking fee. The two lines that apply the discount and the fee are simple, correct code on their own, run them in a different order and the kiosk quietly charges the wrong amount.

Try it yourself

Run both orderings

The intended order is loaded below. Comment out those two lines and uncomment the swapped pair underneath to see the $4.80 total for yourself, then try a different discount or fee to see the gap change.

Worked example

A misplaced line turns a loop infinite

This code is shown, not run live: an infinite loop would freeze this page's Python engine exactly as it would freeze a real program running it, that is the whole point of the example. Trace it by hand instead.

stock = 3
while stock > 0:
    print("Ticket sold, stock now", stock)
    if stock > 5:
        stock = stock - 1

stock starts at 3 and the condition that would decrease it, stock > 5, can never be true while stock stays at 3 or below. The loop's own exit test, stock > 0, is therefore permanently true. The loop never ends.

stock = 3
while stock > 0:
    print("Ticket sold, stock now", stock)
    stock = stock - 1

Now every pass genuinely moves stock closer to 0: 3, 2, 1, 0, then the exit test finally becomes false and the loop stops. The fix wasn't a new line of code, it was moving one existing line out from inside the if.

Worked example

Two kiosks, each waiting on the other

The raffle stall shares one laptop (for typing in member numbers) and one printer (for the receipt) between two checkout points. Picture this: Chloe at Checkout A picks up the laptop first, then reaches for the printer. At that exact moment, Lachlan at Checkout B has already picked up the printer, and now reaches for the laptop. Chloe won't let go of the laptop until she's used the printer. Lachlan won't let go of the printer until he's used the laptop. Neither of them is ever going to get both.

# Checkout A (Chloe)
lock(laptop)
lock(printer)
issue_receipt()
unlock(printer)
unlock(laptop)

# Checkout B (Lachlan), running at the same time
lock(printer)
lock(laptop)
issue_receipt()
unlock(laptop)
unlock(printer)

If Chloe picks up the laptop right as Lachlan picks up the printer, Chloe is now stuck waiting for the printer, which Lachlan is holding, and Lachlan is stuck waiting for the laptop, which Chloe is holding. Each is waiting on the one thing the other refuses to put down. Neither can ever finish: a genuine deadlock.

# Both Checkout A and Checkout B, every time
lock(laptop)
lock(printer)
issue_receipt()
unlock(printer)
unlock(laptop)

Whichever checkout reaches the laptop first now genuinely goes first, the other one simply waits its turn for the laptop before it ever reaches for the printer. Chloe and Lachlan can no longer both be holding the one thing the other needs, so the standoff can't happen.