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# Lab 2 : A Tic-Tac-Toe Game

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ECE244 Programming Fundamentals
Lab 2 : A Tic-Tac-Toe Game
1 Objectives
The main objective of this assignment is to introduce you to the use of classes, objects, and methods,
thus applying the related concepts presented in the lectures. You will do so by building a simple
Tic-Tac-Toe game. You will first implement a class that represents the state of the game and a
function that implements the logic of the game. You will then write a main function that uses these
classes to run a game.
2 Tic-Tac-Toe
Tic-tac-toe is a simple game commonly played by children. Two players, X and O, take turns
marking the spaces in a 3×3 grid or board. The player who succeeds in placing three of their marks
in a horizontal, vertical, or diagonal line wins the game. Player X is always the first to place a
mark. The following example shows the progression of a game won by player X.
X
O X
O X
X
O
X
X O
X
O
X O
X
X move O move X move O move X move
Figure 1: An example Tic-tac-toe game
The simplicity of the game makes it possible for each player to make a perfect move. Thus, the
game often ends in a draw. A player wins only if the opponent makes a mistake (which is why the
game is played only by children).
3 Game Overview
The game consists of two main components: the game controller or simply the controller and
the game logic. This is shown pictorially in Figure 2. The controller is responsible for collecting
players’ input to determine the grid (or board) location a player wishes to place a mark in. It is
also responsible for displaying the game board in the text format described later in this handout.
The controller is implemented by the main function of the game. The game logic contains a single
function called playMove. This function determines, for each player input, if the move represented
by the selected board coordinates are valid or not, if the game is over or not and accordingly “plays”
the move.
The controller and the game logic interact using an object of the type GameState. This object
stores the state of the game, including the board coordinates selected by a player, the marks at
each board location (i.e., X or O) , whether the move is valid or not, whose turn it is, whether the
game is over or not, etc.
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Game Controller
Game Logic
GameState
Figure 2: The game components
The game operation is simple. The controller prompts a player to enter two integers, representing the coordinates of a board’s location. If the coordinates are legal (see below), it updates
the GameState object with these coordinates and invokes the playMove function of the game logic,
passing to it (by reference) the GameState object. This function updates the GameState object
based on the selected coordinates. When the function returns to the controller, it prints the updated
GameState. The process repeats until there is a win or draw.
4 Problem Statement
You will implement the methods of the class GameState, as defined in GameState.h. You will
also implement a function playMove, which “plays” the move indicated by a player’s input. The
remainder of this section describes: (1) the GameState class, (2) the methods of which you must implement, (3) the playMove function that implements the game logic, (3) the controller functionality
implemented in main and (4) other key files that make up the game.
4.1 The GameState Class
The state of a tic-tac-toe game is represented by an object of the GameState class. This object is
created, initialized, and eventually destroyed by the controller. The definition of this class appears
in the file GameState.h, which is released with the assignment. You may NOT modify this
file to add to or delete from its content. Modifying the file often results in a mark of zero for the
assignment. The class contains the following data members:
• gameBoard is a boardSize × boardSize (3 × 3 in this assignment) two-dimensional array
that represents the game board. It stores the marks of each player, and thus the state of the
game. Each element of the array can be one of Empty, X or O. The elements of the array are
initialized by the controller to Empty. The definitions of boardSize, Empty, X and O appear
in the file globals.h, which is also part of the assignment release. You may NOT modify
this file to add to or delete from its content. Modifying the file often results in a mark of
zero for the assignment.
In the array, gameBoard[0][0] represents the top-left corner cell of the game grid. Similarly, gameBoard[boardSize-1][boardSize-1] represents the bottom-right corner cell of
the board. This effectively defines the row and column coordinates of each cell in the grid.
• selectedRow and selectedColumn are two integers that store the grid coordinates that are
selected by the player.
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• moveValid is a Boolean variable that is set to true when the selected coordinates represent a
valid move for the current game. That is, it is set to true when the grid cell at selectedRow
and selectedColumn is empty and is set to false otherwise.
• gameOver is a Boolean variable that should be set to true if the game is over as a result of
the last selected coordinates (i.e., win or draw) and to false otherwise.
• turn is a Boolean variable that indicates whose turn it is, X (true) or O (false) for the
current selected coordinates. If the move is valid, then the value of turn should be changed
by the game logic from true to false or from false to true to reflect the change in turn.
The controller does not use this variable at all.
• winCode is integer variable is set to a code that indicates which cells on the board have marks
that form a line, as shown in Table 1. If gameOver is false, the code should be set to 0. If
gameOver is true and the game is a draw, then winCode should also be set to 0. if gameOver
is true and one of the players won, the code should be set to one of the integer values as
indicated in the table.
Code Sequence
0 No win
1 Row 0 of the grid, cell (0,0) to cell (0,2)
2 Row 1 of the grid, cell (1,0) to cell (1,2)
3 Row 2 of the grid, cell (2,0) to cell (2,2)
4 Column 0 of the grid, cell (0,0) to cell (2,0)
5 Column 1 of the grid, cell (0,1) to cell (2,1)
6 Column 2 of the grid, cell (0,2) to cell (2,2)
7 Left to right diagonal, cell (0,0) to cell (2,2)
8 Right to left diagonal, cell (2,0) to cell (0,2)
Table 1: winCode values
The GameState class provides accessors and mutators to the respective class data members.
4.2 The playMove Function
The game logic implements a single function:
void playMove(GameState& game_state)
This function is to be implemented in the file playMove.cpp and it is called every time a player
makes a move. Its goal is to “play” the move and update the GameState object that is passed
by reference to function. Upon completion, the function must update the game state object by
updating:
• The game board at the appropriate location by either X or O.
• The turn value to reflect that the turn has changed.
• The Boolean variable validMove, described earlier.
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• The Boolean variable gameOver to either true or false to reflect if the move ends the game
in either a win or a draw.
• The variable winCode to either 0 if the game is not over or to the winning code (as described
above) if the game is over.
The two game grid examples shown in Figure 3 are used to demonstrate the expected updates
to the game state object.
O
X
Game Grid A: X’s turn
O
X
Game Grid B: O’s turn
O X
X
Figure 3: Input game boards
For game grid A on the left, the turn variable is true, indicating that it is X’s turn to play. If
selectedRow = 1 and selectedColumn = 2 when the playMove function is called, then an X is
placed in the second row and third column of the game board. The move is valid (validMove is
set to true), but the game is not over (gameOver is set to false). The variable winCode is set to
0. The variable turn is changed from true to false to indicate that it is now O’s turn to play.
However, if selectedRow = 1 and selectedCol = 1 when the playMove function is called by
the controller, then the move is not valid (O’s mark already occupies the cell). The game board is
not updated. Further, validMove is set to false, gameOver is set to false and winCode is set to
0. The variable turn is not changed to indicate that it is remains X’s turn to play with a valid
move.
Similarly, for game grid B on the right, upon entry to playMove, turn is false (or 0) indicating
that it is O’s turn to play. Thus, if selectedRow = 0 and selectedCol = 1, the game state object
should be updated to have validMove as true, gameOver as true, the game board updated with
an O in row 0 and column 1, and winCode set to 5.
4.3 The Game Controller
The game controller is implemented in the main function of the game, contained in the file
tictactoe.cpp. Its purpose is to: (1) create and initialize the game board, (2) prompt the player
for a pair of integers representing the row and column coordinates of the grid cell the player wishes
to mark (where row 0 and column 0 represent the upper left corner of the grid and row 2 and
column 2 represent the lower right corner), (2) check that the player enters integers that are in the
range of 0–2, (3) set the selectedRow and selectedCol members of the GameState object to the
entered values, (4) call the playMove function, and (5) print the GameState object.
You may assume that a player always enters integer input (i.e., not float or string, for example).
Thus, you need not handle such errors in your code. The printing of the GameState object is in
the following format:
• The selected row and column values
• The text “Game state after playMove:” on a line by itself.
• The text “Board:” on a line by itself.
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• The grid cell values, printed as X, O or B (for blank). They are printed one row at a time
starting with the top row (i.e., row 0). Each row is indented from the left by exactly three
spaces and its values are separated by exactly a single space.
• The value of moveValid.
• The value of gameOver.
• The value of winCode.
• A blank line.
The following is a sample run showing the output produced in response to player inputs.
Enter row and column of a grid cell: 1 1
Selected row 1 and column 1
Game state after playMove:
Board:
B B B
B X B
B B B
moveValid: true
gameOver: false
winCode: 0
Enter row and column of a grid cell: 0 1
Selected row 0 and column 1
Game state after playMove:
Board:
B O B
B X B
B B B
moveValid: true
gameOver: false
winCode: 0
Enter row and column of a grid cell: 0 1
Selected row 0 and column 1
Game state after playMove:
Board:
B O B
B X B
B B B
moveValid: false
gameOver: false
winCode: 0
Enter row and column of a grid cell: 0 0
Selected row 0 and column 0
Game state after playMove:
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Board:
X O B
B X B
B B B
moveValid: true
gameOver: false
winCode: 0
Enter row and column of a grid cell: 7 0
Invalid board coordinates 7 0
Enter row and column of a grid cell: 2 0
Selected row 2 and column 0
Game state after playMove:
Board:
X O B
B X B
O B B
moveValid: true
gameOver: false
winCode: 0
Enter row and column of a grid cell: 2 2
Selected row 2 and column 2
Game state after playMove:
Board:
X O B
B X B
O B X
moveValid: true
gameOver: true
winCode: 7
4.4 Include Files
In addition to the GameState.h file described above, the file globals.h has global definitions for
X, O and boardSize. You should include these files in your tictactoe.cpp and playMove.cpp.
Again, you may NOT modify these files to add to or delete from their content. Modifying the
files commonly results in a mark of zero for the assignment.
4.5 Reference Executable
In order to help you observe the behaviour of the game, a reference executable called tictactoe-ref.exe
is released with the assignment. When in doubt about what the game should do or print to the
output, you can use this reference to play the game.
Please note that this reference executable works only on ECF machines and on ECE244VM.
It will not work in Window machines or Macs. Thus, to use it, you must connect to ECF, as is
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described in the “Remote Connection to ECF” handout, or must have ECE244VM installed on
5 Procedure
Create directory called lab2 in your ece244 directory. Make sure that the permissions of this new
directory are such that it is readable by none other than you (refer to lab assignment 1 for how
to do so). Download the zip file containing the assignment release files and place it in this lab2
directory. Unzip the file, which will create the assignment files in the directory. You can move the
zip file out of the directory or remove it after this step.
There are six assignment files. You will add your code in three files: GameState.cpp, playMove.cpp
and tictactoe.cpp, The first contains the implementation of the methods of the class GameState.
The second contains the “logic” of the game in the playMove function. The third contains the main
function that implements the game controller. All three files contain some skeletal code to get you
started, particularly in tictactoe.cpp.
In addition, there are two include files globals.h and GameState.h. Remember that you may
NOT modify these files to add to or delete from their content. Modifying the files commonly
results in a mark of zero for the assignment. Finally, there is a Makefile that is used by NetBeans
to separately compile your project. Do not modify this file either.
Use the assignment files to build a C++ project for NetBeans. Start NetBeans and create a new
C++ project with existing sources. In the resulting dialog window, browse to your ece244/lab2
directory and click ”Select”. Ignore the warning message and click ”Finish”. This creates and
builds your project in directory nbproject inside your ece244/lab2 directory. You can now start
(i.e., tictactoe.exe) is in your ece244/lab2 directory. Run exercise as follows:
~ece244i/public/exercise 2 tictactoe.exe
The exercise command will let you know if your code has errors by providing it with several
test cases. Please note that some of the exercise test cases will be used by the autotester during
marking of your assignment. However, we will not provide all the autotester test cases in exercise,
so you should create additional test cases yourself and ensure you fully meet the specification listed
above.
Please note that there is behaviour of the game that is not completely specified in the handout
(i.e., “corner cases”). You must think of test inputs for such cases. The reference solution can be
used to find out what the output is for these cases.
6 Marking and Deliverables
You must submit your code for autotesting. The autotester will be used to check the correctness of