The Clapping Game

A question asked on every turn

You already have both halves of this. A for loop repeats a block of lines; an if chooses between two outcomes. Nest the if inside the loop's braces and its question is no longer asked once — it is asked again for every number the loop counts through, and the answer may come out differently each time.

Twenty turns, twenty decisions, one test.

for (int count = 1; count <= 20; count++) {
    if (count == 3) {
        System.out.println("clap");
    } else {
        System.out.println(count);
    }
}
System.out.println("finished");

Each pair of braces marks off the lines that belong to whatever sits in front of it. The loop's braces hold the whole if, so the question is asked once per number — twenty times over. The if braces and the else braces each hold one println, and only one of the two runs per turn. The last line sits after the loop's closing brace, outside everything, so it runs once, when the counting is over.

Catching every third one, not just the third one

count == 3 can only be true on one turn out of twenty. To catch 3, 6, 9, 12 and the rest, you need something all of them share: divide any one of them by three and nothing is left over.

% reports that leftover — the operator that told you how many pizza slices spilled past a whole pizza. 7 % 3 is 1, because two threes fit inside seven with one left behind. 9 % 3 is 0, because three threes fit exactly. A leftover of zero is the mark of every third number, so the test measures that leftover against 0 with ==.

A worked example

A row of lockers rather than a clapping game, so the rhythm in the editor stays yours to find:

public class Main {
    public static void main(String[] args) {
        for (int box = 1; box <= 6; box++) {
            if (box % 4 == 0) {
                System.out.println("Box " + box + " has the spare key");
            } else {
                System.out.println("Box " + box);
            }
        }
        System.out.println("Row checked.");
    }
}

Box 1
Box 2
Box 3
Box 4 has the spare key
Box 5
Box 6
Row checked.

Six numbers, six decisions. Only box 4 leaves nothing over when divided by four, so only box 4 takes the first branch. The last line sits outside the loop's braces, which is why it appears once at the bottom rather than after every box.

Your turn

Press Run before changing anything. The count from 1 to 20 arrives neatly, one clap lands at 3, and after that it is numbers all the way, because the test reads:

if (count == 3) {

Replace it with a test that is true for every third count: reach for %, and measure the leftover against 0. The loop, the printing lines, the braces and the banners are finished — the test inside the round brackets is the only thing to touch.

When it works, the claps land on 3, 6, 9, 12, 15 and 18, and the beat is readable from across the room.

If something goes wrong

The likeliest slip is the rhythm itself. If the claps come out on 1, 4, 7 and 10, the leftover is being measured against the wrong number — leftovers after threes run 1, 2, 0, 1, 2, 0, and it is the 0 that marks every third count.

If the output panel is completely empty, not even the top banner, a brace has gone missing somewhere in the edit. javac reads the whole program before running any of it, so an unclosed block stops the lot and reports reached end of file while parsing.

And a message about an unexpected type where a boolean was needed means the test was written as an assignment — one equals sign where two belong. None of this breaks anything: the program either runs with an odd rhythm or refuses to start, and the fix is one line away either way.