Workshop › Conway's Game of Life › Step 14 of 32

A world without edges

What we are doing

At the edges a glider crashes and turns into a block. We make the world wrap around instead: the column after the last one is the first, the row above the top is the bottom. The % (remainder) operator does it.

```js
// Live neighbours among the 8 around (r, c). The edges wrap around, so the world has no border.
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
      count += grid[(r + dr + ROWS) % ROWS][(c + dc + COLS) % COLS]
    }
  }
  return count
}
```

What it means

a % b is the remainder of a / b: 60 % 60 is 0, so column 60 becomes column 0.
-1 % 60 is still -1 in JavaScript, so we add COLS first: (-1 + 60) % 60 is 59, the last column. When it is not needed it does no harm: (5 + 60) % 60 is 5.
Every neighbour now lands on the board, so the "is it on the board?" check goes away.

Your turn

In countNeighbors, replace the lines const nr, const nc and the long if with the single wrapping line, and update the comment above the function.

Skills: Arrays and objects

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