Workshop › Sudoku › Step 12 of 28

The three rules in one loop

What we are doing

canPlace answers the question at the heart of Sudoku: can this digit go here? One loop of 9 checks the row, the column and the box at the same time.

```js
// Can digit d go at (r, c)? Not if it is already in the row, the column or the 3 by 3 box.
function canPlace(board, r, c, d) {
  const br = r - (r % 3)
  const bc = c - (c % 3)
  for (let i = 0; i < 9; i++) {
    if (board[r][i] === d || board[i][c] === d) return false
    if (board[br + Math.floor(i / 3)][bc + (i % 3)] === d) return false
  }
  return true
}
```

What it means

br, bc are the top-left cell of the box: r - (r % 3) rounds down to 0, 3 or 6.
In one loop, i walks the row (board[r][i]), the column (board[i][c]) and the box, whose cell number i becomes a row Math.floor(i / 3) and a column i % 3 inside the box.
As soon as the digit is found, the answer is no.

Your turn

Under let selected, write the comment and canPlace.

Skills: Functions, Loops

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