Workshop › Connect Four › Step 22 of 38

Walking along a line

What we are doing

lineThrough(row, col, [dx, dy]) starts at a disc and walks in one direction while the discs are the same color, then the other way, collecting the cells. [1, 0] is across: one column right per step.

```js
// Count the discs in a row through (row, col) in one direction and its opposite.
function lineThrough(row, col, [dx, dy]) {
  const who = board[row][col]
  const cells = [[row, col]]
  for (const sign of [1, -1]) {
    let r = row + dy * sign
    let c = col + dx * sign
    while (inside(r, c) && board[r][c] === who) {
      cells.push([r, c])
      r += dy * sign
      c += dx * sign
    }
  }
  return cells
}
```

What it means

[dx, dy] in the parameters names the two items of the direction array.
for (const sign of [1, -1]) runs once forwards and once backwards.
while repeats as long as its condition is true: while still on the board and the same color, add the cell and step on.
The list starts with the disc itself, so three neighbours make four.

Your turn

Above function play( write the comment and lineThrough, followed by an empty line.

Skills: Loops, Arrays and objects

← Inside the board? Four in a row? →

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