Workshop › Pool › Step 5 of 33

The rack

What we are doing

Ten numbered balls packed in a triangle, the rack: row 0 has 1 ball, row 1 has 2, up to 4 in the back row. Two loops place them, each with its own color and number.

```js
const COLORS = ['#facc15', '#2563eb', '#dc2626', '#7c3aed', '#f97316', '#16a34a', '#7f1d1d', '#111827', '#0891b2', '#db2777']

// Ten balls in a triangle pointing at the cue ball: 1, 2, 3, then 4 in the back row.
function rack() {
  cue = ball(CUE_START.x, CUE_START.y, '#f8fafc', 0)
  balls = [cue]
  let n = 0
  for (let row = 0; row < 4; row++) {
    for (let i = 0; i <= row; i++) {
      const x = 330 + row * (R * 2 * 0.87 + 0.5)
      const y = 160 + (i - row / 2) * (R * 2 + 0.5)
      balls.push(ball(x, y, COLORS[n], n + 1))
      n += 1
    }
  }
}
```

What it means

• row goes 0 to 3; in each row i goes 0 to row: 1 + 2 + 3 + 4 = 10 balls. n counts them for color and number.
• Inside a row balls are one diameter apart (2R), centered on the middle line with i - row / 2.
• Rows are a little less than a diameter apart: in a packed triangle the centres form equilateral triangles, whose height is 2R × 0.87 (√3 / 2). The extra 0.5 keeps the balls from exactly touching.

Your turn

1. Under the R line write COLORS.
2. Put the comment above function rack() {.
3. In rack, under balls = [cue], write the counter and the two loops. Press Run.

Skills: Loops, Arrays and objects

← Draw the balls Numbers on the balls →

All steps

1. The room
2. The table
3. The cue ball
4. Draw the balls
5. The rack
6. Numbers on the balls
7. An aim line
8. Turn the aim
9. A shot is a velocity
10. Roll
11. Friction
12. Cushions
13. Aiming and rolling
14. Hide the aim while rolling
15. Count the shots
16. Balls that touch
17. Swap the push
18. Six pockets
19. In the pocket
20. Scratch
21. Balls left
22. Clear the table
23. Choose the power
24. A power bar
25. A slingshot
26. Press to aim
27. Pull back and let go
28. Smaller steps
29. Predict the hit
30. Draw the ghost ball
31. Where the hit ball goes
32. Your best
33. Build it yourself: two players