Validating Position

Players should only be able to choose positions 0-8. We need to validate the position.

Comparison Operators:

Used to compare values and return true or false.

• < less than

• > greater than

• == equal to

Example:

require(age >= 18, "Must be 18 or older");

For Our Game:

The board has positions 0-8 (9 total squares). Any position 9 or higher is invalid.

We can check: _position < 9

This is true for 0, 1, 2, 3, 4, 5, 6, 7, 8 (valid positions)

This is false for 9, 10, 11... (invalid positions)

Add a require statement at the start of makeMove to validate the position is less than 9.

Validate position range

Requirements:

Has position validation require

Has "Invalid position" error message

Uses less than operator (<)

Validation comes before empty check

// 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;

    bool public xTurn;

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

    function makeMove(uint _position) public {
        require(board[_position] == Cell.Empty, "Cell already taken");

        Cell piece = xTurn ? Cell.X : Cell.O;
        board[_position] = piece;

        xTurn = !xTurn;
    }
}