Countdown

The three parts of a for loop

A countdown is one line said over and over with a different number in it each time. You could write five separate println lines by hand. A loop is how you write the line once and tell Java exactly how the number should change.

for (int n = 0; n < 3; n++) {
    System.out.println(n);
}

Three parts, separated by semicolons, inside the round brackets:

• Start: int n = 0 runs once, before the loop takes a single turn. It creates the counter and gives it its first value.
• Condition: n < 3 is checked before every turn, including the first. As long as it is true, the loop takes another turn. The moment it is false, the loop stops — and the line that made it false never runs.
• Update: n++ runs at the end of every turn, after the braced block. It is what actually moves the counter — nothing about n < 3 on its own makes n go anywhere.

Nothing here is hidden the way range() hides it in some languages. All three parts are decisions you write yourself, which is exactly why a Java for loop can count downwards as easily as up: start high, keep the condition true while there is still counting to do, and subtract on each turn instead of adding.

A worked example

public class Main {
    public static void main(String[] args) {
        for (int step = 0; step < 3; step++) {
            System.out.println("step " + step);
        }
        System.out.println("done");
    }
}

Running that prints four lines:

step 0
step 1
step 2
done

Three turns, so the println inside the braces ran three times, holding a different value in step each time — 0 first, 2 last, and the loop stopped the moment step < 3 became false, which is why 3 itself is never printed. The final line sits outside the loop's closing brace, so it runs once, after the counting is over.

Your turn

The editor holds a launch sequence that already works. Press Run before changing anything and you can see the problem for yourself: it counts 0, 1, 2 and then announces liftoff. No rocket has ever left the ground that way.

One line needs changing — the for line. The banners, the rocket and the println inside the loop are all finished; leave them alone.

Rewrite the three parts so the count reads 5, 4, 3, 2, 1 before handing over to the liftoff banner: start at 5, keep going while the count has not dropped below 1, and move the counter downwards by one on every turn.

If something goes wrong

The likeliest result is a run that never finishes. If the countdown starts correctly but keeps climbing past 5 instead of stopping, the update is still adding one — n++ where n-- belongs. A loop whose condition can never become false is stopped automatically after a few seconds, as a timeout, rather than running forever in your browser.

The other likely result is a countdown one number short at one end — starting at 4, or finishing on 2. Both come from the condition: n >= 1 keeps going through 1 itself, while n > 1 stops one turn earlier than that.

Nothing you type here can break anything. The worst case is a countdown that reads oddly, or a run that gets stopped for taking too long — and then you change a number and run it again.