# X and O marks alternation

After solving the problem of the previous lesson, you should get the following code:

`let cells = document.querySelectorAll('#field td'); start(cells); function start(cells) { for (let cell of cells) { cell.addEventListener('click', function() { this.textContent = 'X'; }); } }`

Let's now make alternation of crosses and zeros. To do this, I propose to introduce a counter of moves:

`function start(cells) { let i = 0; // the counter initial value for (let cell of cells) { cell.addEventListener('click', function() { this.textContent = 'X'; i++; // increases the counter value }); } }`

Having such a counter, we can easily implement the alternation of a cross and a zero: it is obvious that the X mark will appear at even counter values, and the O mark at odd ones.

Implement the described alternation of a cross and a zero.

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