Workshop › Conway's Game of Life › Step 23 of 32

Which cell?

What we are doing

From a point in canvas pixels to a cell: divide by the cell size and round down. Points outside the grid give null.

```js
function cellAt({ x, y }) {
  const r = Math.floor((y - TOP) / CELL)
  const c = Math.floor(x / CELL)
  return r >= 0 && r < ROWS && c >= 0 && c < COLS ? { r, c } : null
}
```

What it means

• { x, y } in the parentheses takes x and y out of the object passed in.
• Math.floor rounds down: x = 85 gives 85 / 8 = 10.6, so column 10. For the row, TOP is taken off first.
• { r, c } is short for { r: r, c: c }. Outside the grid it returns null, "nothing".

Your turn

Write cellAt under toCanvas, above function update() {, with empty lines around it.

Skills: Keyboard and mouse input

← Where is the pointer? Click a cell →

All steps

1. The board
2. Measure the grid
3. One living cell
4. A function that draws
5. The world in memory
6. Draw every living cell
7. A random soup
8. Count the neighbours
9. Four rules
10. Draw again and again
11. Next generation with N
12. Count the generations
13. Show the generation
14. A world without edges
15. Count the living
16. A generation every few frames
17. Let it live
18. Running or not
19. One place for the buttons
20. Space to play, N to step
21. Clear and shuffle
22. Where is the pointer?
23. Which cell?
24. Click a cell
25. Paint or erase?
26. Drag to paint
27. Patterns from text
28. The glider gun
29. Fire the gun
30. Buttons on the screen
31. Tap the buttons
32. Build it yourself: a world at rest