Combining Logic

Now let's make makeMove accept a position parameter so players can choose where to play.

uint type:

uint means unsigned integer (positive whole numbers). Perfect for array indices!

Update the function to accept a position:

function makeMove(uint _position) public { }

Then use the parameter:

require(board[_position] == Cell.Empty, "Cell already taken");

board[_position] = Cell.X;

Add position parameter

Requirements:

Has uint _position parameter

Checks board[_position] in require

Sets board[_position] = Cell.X

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract TicTacToe {
    address public playerX;
    address public playerO;

    enum Cell { Empty, X, O }

    Cell[9] public board;

    function startGame(address _opponent) public {
        playerX = msg.sender;
        playerO = _opponent;
    }

    function makeMove() public {
        require(board[0] == Cell.Empty, "Cell already taken");
        board[0] = Cell.X;
    }
}