Workshop › Conway's Game of Life › Step 8 of 32

Count the neighbours

What we are doing

The rules of Life depend on one number: how many of a cell's 8 neighbours (sides and corners) are alive. We write a function that counts them.

```js
// Live neighbours among the 8 around (r, c), skipping those off the board.
function countNeighbors(r, c) {
  let count = 0
  for (let dr = -1; dr <= 1; dr++) {
    for (let dc = -1; dc <= 1; dc++) {
      if (dr === 0 && dc === 0) continue
      const nr = r + dr
      const nc = c + dc
      if (nr >= 0 && nr < ROWS && nc >= 0 && nc < COLS) count += grid[nr][nc]
    }
  }
  return count
}
```

What it means

r and c are parameters: the row and column we pass in, as in countNeighbors(10, 10).
The two loops run dr and dc over -1, 0, 1: the row above, the same and below, times the column left, same and right. That is 9 places.
continue skips (0, 0), the cell itself.
The long if counts a neighbour only when it is on the board. Cells are 1 or 0, so we can just add them.
return count hands the number back to whoever called the function.

Your turn

Write the function under reset, after an empty line.

Skills: Loops, Functions

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