Bonus exercise: Add a computer player
A possible solution is at the end of this page.
Try it yourself
This is an optional task to get yet more familiar with the BBj language and your new coding tools:
Make a copy of your Tic-Tac-Toe and try to move it into a one-player version where the computer plays the other player. Play a bit with the computer algorithm, what can you implement? Random moves, MinMax, what else?
Possible solution
The original course had no solution for this exercise, so this one was written for this book. It starts from the Tic-Tac-Toe solution TicTacToe.bbj and is a complete program of its own. The computer plays O, and the list button switches between the strategies Random and MinMax while you play. MinMax looks ahead to the end of the game, so it never loses. MinMax calls itself, and a BBj method keeps its variables local to each call, so both strategies live in the small class ComputerPlayer (the next section introduces classes). The program uses plain controls without CSS and also runs in the DWC. The solution is in TicTacToeComputer.bbj, and you find all files in the exercises.zip link.
TicTacToeComputer.bbj
rem TicTacToeComputer.bbj
rem Possible solution for the bonus exercise Add a computer player, written for the Introduction to BBj Development book
rem It starts from the two-player solution TicTacToe.bbj. You play X, the computer plays O.
rem The computer picks its move with a Random or a MinMax strategy behind one method, nextMove, and you switch the strategy while playing.
rem MinMax calls itself, so both strategies live in a small class whose methods keep their variables local to each call.
rem It uses plain BBj controls and callbacks, without CSS, and also runs in the DWC.
class public ComputerPlayer
method public BBjNumber nextMove(BBjString board$, BBjString strategy$)
rem the board is a string of nine characters: "X", "O" or a space for a free cell, cell 1 first
if strategy$ = "MinMax" then
methodret #minMaxMove(board$)
else
methodret #randomMove(board$)
endif
methodend
method private BBjNumber randomMove(BBjString board$)
rem Random: pick one of the free cells by chance
freeCount = 0
for i = 1 to 9
if board$(i,1) = " " then
freeCount = freeCount + 1
endif
next i
if freeCount = 0 then
methodret 0
endif
rem RND(n) returns a whole number from 0 to n-1, so add 1 to count free cells from 1
pick = rnd(freeCount) + 1
seen = 0
for i = 1 to 9
if board$(i,1) = " " then
seen = seen + 1
if seen = pick then
methodret i
endif
endif
next i
methodret 0
methodend
method private BBjNumber minMaxMove(BBjString board$)
rem MinMax: try every free cell in ascending order and keep the one with the best score.
rem A cell replaces the best one only when its score is strictly better, so equal scores go to the lowest cell.
best = -2
bestCell = 0
for i = 1 to 9
if board$(i,1) = " " then
newBoard$ = #placeMark(board$,i,"O")
score = #minimax(newBoard$,"X")
if score > best then
best = score
bestCell = i
endif
endif
next i
methodret bestCell
methodend
method private BBjNumber minimax(BBjString board$, BBjString turn$)
rem score a board from the computer's side: 1 if O wins, -1 if X wins, 0 for a draw.
rem turn$ says who moves next. O picks the highest score, X the lowest.
w$ = #winner(board$)
if w$ = "O" then
methodret 1
endif
if w$ = "X" then
methodret -1
endif
freeCount = 0
for i = 1 to 9
if board$(i,1) = " " then
freeCount = freeCount + 1
endif
next i
if freeCount = 0 then
methodret 0
endif
if turn$ = "O" then
best = -2
else
best = 2
endif
for i = 1 to 9
if board$(i,1) = " " then
if turn$ = "O" then
newBoard$ = #placeMark(board$,i,"O")
score = #minimax(newBoard$,"X")
if score > best then
best = score
endif
else
newBoard$ = #placeMark(board$,i,"X")
score = #minimax(newBoard$,"O")
if score < best then
best = score
endif
endif
endif
next i
methodret best
methodend
method public BBjString winner(BBjString board$)
rem "X" or "O" when that mark has three in a line, otherwise an empty string
winLines$ = "123456789147258369159357"
for k = 0 to 7
a = num(winLines$(k*3+1,1))
b = num(winLines$(k*3+2,1))
c = num(winLines$(k*3+3,1))
if (board$(a,1) <> " ") and (board$(a,1) = board$(b,1)) and (board$(a,1) = board$(c,1)) then
methodret board$(a,1)
endif
next k
methodret ""
methodend
method private BBjString placeMark(BBjString board$, BBjNumber cell, BBjString mark$)
rem build a new board string with the mark in one cell, one character at a time
result$ = ""
for i = 1 to 9
if i = cell then
result$ = result$ + mark$
else
result$ = result$ + board$(i,1)
endif
next i
methodret result$
methodend
classend
declare ComputerPlayer computer!
declare BBjAPI api!
declare BBjSysGui sysgui!
declare BBjTopLevelWindow wnd!
declare BBjButton btn!
declare BBjStaticText status!
declare BBjStaticText strategyLabel!
declare BBjListButton strategyList!
declare BBjVector cells!
computer! = new ComputerPlayer()
api! = BBjAPI()
sysgui! = api!.openSysGui("X0")
wnd! = sysgui!.addWindow(100,100,320,430,"Tic-Tac-Toe against the computer")
rem the nine cell buttons get the control IDs 101 to 109, laid out in three rows of three
cells! = api!.makeVector()
for i = 0 to 8
btn! = wnd!.addButton(101+i,10+mod(i,3)*100,10+int(i/3)*100,95,95,"")
btn!.setCallback(BBjAPI.ON_BUTTON_PUSH,"cellPushed")
cells!.addItem(btn!)
next i
status! = wnd!.addStaticText(110,10,315,300,25,"")
rem the list button holds the strategy names, the first one is selected at the start
strategyLabel! = wnd!.addStaticText(113,10,348,115,25,"Computer plays:")
strategyList! = wnd!.addListButton(112,130,345,180,25,"Random"+$0A$+"MinMax")
strategyList!.selectIndex(0)
btn! = wnd!.addButton(111,10,380,300,30,"New game")
btn!.setCallback(BBjAPI.ON_BUTTON_PUSH,"newGame")
wnd!.setCallback(BBjAPI.ON_CLOSE,"byebye")
rem the eight winning lines as cell numbers: three rows, three columns, two diagonals
winLines$ = "123456789147258369159357"
gosub resetGame
process_events
cellPushed:
rem the pushed button tells us its control ID, cell 1 has ID 101
ev! = sysgui!.getLastEvent()
ctrl! = ev!.getControl()
cell = ctrl!.getID() - 100
rem ignore a click on a taken cell and every click after the game has ended
if (gameOver <> 0) or (board$[cell] <> "") then
return
endif
player$ = "X"
gosub makeMove
if gameOver = 0 then
gosub computerMove
endif
return
computerMove:
rem turn the board into a string of nine characters, a space marks a free cell
boardStr$ = ""
for i = 1 to 9
if board$[i] = "" then
boardStr$ = boardStr$ + " "
else
boardStr$ = boardStr$ + board$[i]
endif
next i
rem the list button decides the strategy, you can change it at any time
strategy$ = strategyList!.getItemAt(strategyList!.getSelectedIndex())
cell = computer!.nextMove(boardStr$,strategy$)
player$ = "O"
gosub makeMove
return
makeMove:
rem place the mark player$ in cell. Human and computer both go through here.
board$[cell] = player$
cellBtn! = cells!.getItem(cell-1)
cellBtn!.setText(player$)
moves = moves + 1
rem the win check comes first, so a ninth move that completes a line is a win and not a draw
gosub checkWinner
if winner$ <> "" then
if winner$ = "X" then
status!.setText("You win!")
else
status!.setText("The computer wins!")
endif
gameOver = 1
else
if moves = 9 then
status!.setText("It's a draw.")
gameOver = 1
else
if player$ = "X" then
status!.setText("The computer plays O")
else
status!.setText("Your move, you play X")
endif
endif
endif
return
checkWinner:
rem look at each line in the table: three cell numbers, all holding the same mark
winner$ = ""
for k = 0 to 7
a = num(winLines$(k*3+1,1))
b = num(winLines$(k*3+2,1))
c = num(winLines$(k*3+3,1))
if (board$[a] <> "") and (board$[a] = board$[b]) and (board$[a] = board$[c]) then
winner$ = board$[a]
endif
next k
return
newGame:
gosub resetGame
return
resetGame:
rem board$[1] to board$[9] hold "X", "O" or an empty string for a free cell
dim board$[9]
player$ = "X"
moves = 0
gameOver = 0
for i = 1 to 9
cellBtn! = cells!.getItem(i-1)
cellBtn!.setText("")
next i
status!.setText("Your move, you play X")
return
byebye:
release