Project P009
Tic-tac-toe
Play tic-tac-toe against a friend or against the computer, which wins when it can and blocks when it must; wins and draws are found and a running score is kept - from a text menu.
3 modules · 2 recorded sessionstext-menu UI in the terminalupdated 2026-09-29
Every screen below was recorded under CPython. When this page was built, the EML interpreter replayed each session from the same input and printed the same bytes.
About
Tic-tac-toe for two players, or for one player (X) against the computer (O). X always starts; the board shows the free squares by number; a game ends in a win or a draw, or when a player leaves it with q. The score is kept while the program runs.
main.eml- the menu, one game, and the checks on each square typedboard.eml- the board as nine cells, how it is drawn, the eight winning lines, and whether the board is fullai.eml- the computer's move, in order of preference: win now if it can; otherwise block the square where the other player would win; otherwise the centre, then the first free corner, then the first free side. There is no randomness, so the same moves always get the same answer.
What is checked: a square must be one of 1 to 9 and still free; anything else asks again without losing the turn.
Sessions: sessions/basic.in plays a two-player game won by X; a game against the computer in which it takes the centre and a corner and blocks one of two threats, and X wins with the other; a game the computer wins because winning comes before blocking; a two-player draw; and the score; sessions/bad-input.in types an unknown choice, squares 0, 10, a letter and nothing, a taken square, and leaves a game, which does not count.
Built on the verified corpus case mvp-tic-tac-toe (a board, moves that refuse a taken square, and finding a line of three).
Recorded sessions
What the screen shows while someone uses the program. Each typed line appears after its prompt, the way a terminal shows it.
bad-input
interpreter: byte-equal
== Tic-tac-toe ==
1) two players 2) against the computer 3) score 4) quit
choice> 9
Pick a number from 1 to 4.
== Tic-tac-toe ==
1) two players 2) against the computer 3) score 4) quit
choice> 3
X wins: 0, O wins: 0, draws: 0
== Tic-tac-toe ==
1) two players 2) against the computer 3) score 4) quit
choice> 1
1 | 2 | 3
---+---+---
4 | 5 | 6
---+---+---
7 | 8 | 9
X to play, square 1-9 (q to leave)> 0
Pick a free square from 1 to 9.
X to play, square 1-9 (q to leave)> 10
Pick a free square from 1 to 9.
X to play, square 1-9 (q to leave)> x
Pick a free square from 1 to 9.
X to play, square 1-9 (q to leave)>
Pick a free square from 1 to 9.
X to play, square 1-9 (q to leave)> 5
1 | 2 | 3
---+---+---
4 | X | 6
---+---+---
7 | 8 | 9
O to play, square 1-9 (q to leave)> 5
That square is taken.
O to play, square 1-9 (q to leave)> q
Game left.
== Tic-tac-toe ==
1) two players 2) against the computer 3) score 4) quit
choice> 3
X wins: 0, O wins: 0, draws: 0
== Tic-tac-toe ==
1) two players 2) against the computer 3) score 4) quit
choice> 4
Bye.
What was typed (12 lines)
9
3
1
0
10
x
5
5
q
3
4
basic
interpreter: byte-equal
== Tic-tac-toe ==
1) two players 2) against the computer 3) score 4) quit
choice> 1
1 | 2 | 3
---+---+---
4 | 5 | 6
---+---+---
7 | 8 | 9
X to play, square 1-9 (q to leave)> 1
X | 2 | 3
---+---+---
4 | 5 | 6
---+---+---
7 | 8 | 9
O to play, square 1-9 (q to leave)> 4
X | 2 | 3
---+---+---
O | 5 | 6
---+---+---
7 | 8 | 9
X to play, square 1-9 (q to leave)> 2
X | X | 3
---+---+---
O | 5 | 6
---+---+---
7 | 8 | 9
O to play, square 1-9 (q to leave)> 5
X | X | 3
---+---+---
O | O | 6
---+---+---
7 | 8 | 9
X to play, square 1-9 (q to leave)> 3
X | X | X
---+---+---
O | O | 6
---+---+---
7 | 8 | 9
X wins.
== Tic-tac-toe ==
1) two players 2) against the computer 3) score 4) quit
choice> 2
1 | 2 | 3
---+---+---
4 | 5 | 6
---+---+---
7 | 8 | 9
X to play, square 1-9 (q to leave)> 1
Computer plays 5.
X | 2 | 3
---+---+---
4 | O | 6
---+---+---
7 | 8 | 9
X to play, square 1-9 (q to leave)> 9
Computer plays 3.
X | 2 | O
---+---+---
4 | O | 6
---+---+---
7 | 8 | X
X to play, square 1-9 (q to leave)> 7
Computer plays 8.
X | 2 | O
---+---+---
4 | O | 6
---+---+---
X | O | X
X to play, square 1-9 (q to leave)> 4
X | 2 | O
---+---+---
X | O | 6
---+---+---
X | O | X
You win.
== Tic-tac-toe ==
1) two players 2) against the computer 3) score 4) quit
choice> 2
1 | 2 | 3
---+---+---
4 | 5 | 6
---+---+---
7 | 8 | 9
X to play, square 1-9 (q to leave)> 2
Computer plays 5.
1 | X | 3
---+---+---
4 | O | 6
---+---+---
7 | 8 | 9
X to play, square 1-9 (q to leave)> 8
Computer plays 1.
O | X | 3
---+---+---
4 | O | 6
---+---+---
7 | X | 9
X to play, square 1-9 (q to leave)> 3
Computer plays 9.
O | X | X
---+---+---
4 | O | 6
---+---+---
7 | X | O
The computer wins.
== Tic-tac-toe ==
1) two players 2) against the computer 3) score 4) quit
choice> 1
1 | 2 | 3
---+---+---
4 | 5 | 6
---+---+---
7 | 8 | 9
X to play, square 1-9 (q to leave)> 1
X | 2 | 3
---+---+---
4 | 5 | 6
---+---+---
7 | 8 | 9
O to play, square 1-9 (q to leave)> 5
X | 2 | 3
---+---+---
4 | O | 6
---+---+---
7 | 8 | 9
X to play, square 1-9 (q to leave)> 9
X | 2 | 3
---+---+---
4 | O | 6
---+---+---
7 | 8 | X
O to play, square 1-9 (q to leave)> 2
X | O | 3
---+---+---
4 | O | 6
---+---+---
7 | 8 | X
X to play, square 1-9 (q to leave)> 8
X | O | 3
---+---+---
4 | O | 6
---+---+---
7 | X | X
O to play, square 1-9 (q to leave)> 7
X | O | 3
---+---+---
4 | O | 6
---+---+---
O | X | X
X to play, square 1-9 (q to leave)> 3
X | O | X
---+---+---
4 | O | 6
---+---+---
O | X | X
O to play, square 1-9 (q to leave)> 6
X | O | X
---+---+---
4 | O | O
---+---+---
O | X | X
X to play, square 1-9 (q to leave)> 4
X | O | X
---+---+---
X | O | O
---+---+---
O | X | X
Draw.
== Tic-tac-toe ==
1) two players 2) against the computer 3) score 4) quit
choice> 3
X wins: 2, O wins: 1, draws: 1
== Tic-tac-toe ==
1) two players 2) against the computer 3) score 4) quit
choice> 4
Bye.
What was typed (27 lines)
1
1
4
2
5
3
2
1
9
7
4
2
2
8
3
1
1
5
9
2
8
7
3
6
4
3
4
Modules
The program as written, entry module first. Each module transpiles to its own Python file, which is what eml project run executes.
main.eml(entry)
eml# P009 tic-tac-toe: two players, or you (X) against the computer (O). X
# always starts. The score is kept while the program runs.
import board
import ai
def trim(s):
0 => i
len(s) => j
while i < j and s[i] == " ":
i + 1 => i
while j > i and s[j - 1] == " ":
j - 1 => j
return s[i:j]
def ask_square(b, who):
# The cell index of a free square for who, or -1 when q leaves the game.
while True:
trim(input(who + " to play, square 1-9 (q to leave)> ")) => typed
if typed == "q":
return -1
if len(typed) == 1 and typed in "123456789":
int(typed) - 1 => i
if b[i] == "":
return i
"That square is taken." ^0
else:
"Pick a free square from 1 to 9." ^0
def play(vs_computer):
# One game. Returns "X", "O", "draw", or "" when it was left.
board.empty() => b
"X" => who
while True:
if vs_computer and who == "O":
ai.move(b, "O", "X") => i
("Computer plays " + str(i + 1) + ".") ^0
else:
board.show(b)
ask_square(b, who) => i
if i < 0:
"Game left." ^0
return ""
board.placed(b, i, who) => b
board.winner(b) => w
if w != "":
board.show(b)
if not vs_computer:
(w + " wins.") ^0
elif w == "X":
"You win." ^0
else:
"The computer wins." ^0
return w
if board.full(b):
board.show(b)
"Draw." ^0
return "draw"
if who == "X":
"O" => who
else:
"X" => who
[0, 0, 0] => score
True => running
while running:
"" ^0
"== Tic-tac-toe ==" ^0
"1) two players 2) against the computer 3) score 4) quit" ^0
trim(input("choice> ")) => choice
if choice == "1" or choice == "2":
play(choice == "2") => result
if result == "X":
score[0] + 1 => score[0]
elif result == "O":
score[1] + 1 => score[1]
elif result == "draw":
score[2] + 1 => score[2]
elif choice == "3":
("X wins: " + str(score[0]) + ", O wins: " + str(score[1]) + ", draws: " + str(score[2])) ^0
elif choice == "4":
False => running
else:
"Pick a number from 1 to 4." ^0
"Bye." ^0
Python projection (main.py)
import board
import ai
def trim(s):
i = 0
j = len(s)
while i < j and s[i] == " ":
i = i + 1
while j > i and s[j - 1] == " ":
j = j - 1
return s[i:j]
def ask_square(b, who):
while True:
typed = trim(input(who + " to play, square 1-9 (q to leave)> "))
if typed == "q":
return -1
if len(typed) == 1 and typed in "123456789":
i = int(typed) - 1
if b[i] == "":
return i
print("That square is taken.")
else:
print("Pick a free square from 1 to 9.")
def play(vs_computer):
b = board.empty()
who = "X"
while True:
if vs_computer and who == "O":
i = ai.move(b, "O", "X")
print("Computer plays " + str(i + 1) + ".")
else:
board.show(b)
i = ask_square(b, who)
if i < 0:
print("Game left.")
return ""
b = board.placed(b, i, who)
w = board.winner(b)
if w != "":
board.show(b)
if not vs_computer:
print(w + " wins.")
elif w == "X":
print("You win.")
else:
print("The computer wins.")
return w
if board.full(b):
board.show(b)
print("Draw.")
return "draw"
if who == "X":
who = "O"
else:
who = "X"
score = [0, 0, 0]
running = True
while running:
print("")
print("== Tic-tac-toe ==")
print("1) two players 2) against the computer 3) score 4) quit")
choice = trim(input("choice> "))
if choice == "1" or choice == "2":
result = play(choice == "2")
if result == "X":
score[0] = score[0] + 1
elif result == "O":
score[1] = score[1] + 1
elif result == "draw":
score[2] = score[2] + 1
elif choice == "3":
print("X wins: " + str(score[0]) + ", O wins: " + str(score[1]) + ", draws: " + str(score[2]))
elif choice == "4":
running = False
else:
print("Pick a number from 1 to 4.")
print("Bye.")
board.eml
eml# P009 tic-tac-toe - the board. It is a list of nine cells, index 0 to 8 for
# squares 1 to 9 in reading order; a cell is "X", "O" or "" when empty.
# The eight lines that win: three rows, three columns, two diagonals.
[
[0, 1, 2], [3, 4, 5], [6, 7, 8],
[0, 3, 6], [1, 4, 7], [2, 5, 8],
[0, 4, 8], [2, 4, 6],
] => lines
def empty():
return ["" for i in [0:8]]
def mark(cell, i):
# What a square shows: its mark, or its number while it is free.
if cell == "":
return str(i + 1)
return cell
def show(b):
"" ^0
for r in [0:2]:
(" " + mark(b[3 * r], 3 * r) + " | " + mark(b[3 * r + 1], 3 * r + 1) + " | " + mark(b[3 * r + 2], 3 * r + 2)) ^0
if r < 2:
"---+---+---" ^0
def winner(b):
# "X" or "O" if that player has a whole line, otherwise "".
for line in lines:
b[line[0]] => a
if a != "" and b[line[1]] == a and b[line[2]] == a:
return a
return ""
def full(b):
for cell in b:
if cell == "":
return False
return True
def placed(b, i, who):
# A new board with who's mark on cell i.
return b[0:i] + [who] + b[i + 1:9]
Python projection (board.py)
lines = [[0, 1, 2], [3, 4, 5], [6, 7, 8], [0, 3, 6], [1, 4, 7], [2, 5, 8], [0, 4, 8], [2, 4, 6]]
def empty():
return ["" for i in range(0, 9)]
def mark(cell, i):
if cell == "":
return str(i + 1)
return cell
def show(b):
print("")
for r in range(0, 3):
print(" " + mark(b[3 * r], 3 * r) + " | " + mark(b[3 * r + 1], 3 * r + 1) + " | " + mark(b[3 * r + 2], 3 * r + 2))
if r < 2:
print("---+---+---")
def winner(b):
for line in lines:
a = b[line[0]]
if a != "" and b[line[1]] == a and b[line[2]] == a:
return a
return ""
def full(b):
for cell in b:
if cell == "":
return False
return True
def placed(b, i, who):
return b[0:i] + [who] + b[i + 1:9]
ai.eml
eml# P009 tic-tac-toe - the computer's move, in order of preference: win now if
# it can; otherwise block the square where the other player would win;
# otherwise take the centre, then the first free corner, then the first free
# side. No randomness, so the same moves always get the same answer.
import board
def completing(b, who):
# The free cell that would give who a whole line, or -1. Lines are tried
# in order, so the first such cell is the answer.
for line in board.lines:
0 => mine
-1 => free
for i in line:
if b[i] == who:
mine + 1 => mine
elif b[i] == "":
i => free
if mine == 2 and free >= 0:
return free
return -1
def move(b, me, other):
completing(b, me) => i
if i >= 0:
return i
completing(b, other) => i
if i >= 0:
return i
for i in [4, 0, 2, 6, 8, 1, 3, 5, 7]:
if b[i] == "":
return i
return -1
Python projection (ai.py)
import board
def completing(b, who):
for line in board.lines:
mine = 0
free = -1
for i in line:
if b[i] == who:
mine = mine + 1
elif b[i] == "":
free = i
if mine == 2 and free >= 0:
return free
return -1
def move(b, me, other):
i = completing(b, me)
if i >= 0:
return i
i = completing(b, other)
if i >= 0:
return i
for i in [4, 0, 2, 6, 8, 1, 3, 5, 7]:
if b[i] == "":
return i
return -1