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Tic-tac-toe, also called '''noughts and crosses''', '''hugs and kisses''', and many other names, is a Pen-and-paper Game for two players, '''O''' and '''X''', who take turns to mark the spaces in a 3×3 grid. The player who succeeds in placing three of their own marks in a horizontal, vertical or diagonal row wins the game.

This game is won by the first player, X:

This is a "cat's game", that is, a draw:

Players soon discover that best play leads to a draw, regardless of where the first player plays. Hence, tic-tac-toe is most often played by very young children; when they have discovered an unbeatable strategy they move on to more sophisticated games such as Dots And Boxes . This reputation for ease has led to Casino s offering gamblers the chance to play tic-tac-toe against trained Chicken s.http://info.detnews.com/casino/newdetails.cfm?column=pilarski&myrec=298http://gamblingmagazine.com/articles/27/27-1531.htm

of the Game Tree for tic-tac-toe.]]

The simplicity of tic-tac-toe makes it ideal as a pedagogical tool for teaching the concepts of Combinatorial Game Theory and the branch of Artificial Intelligence that deals with the searching of Game Tree s. It is straightforward to write a Computer Program to play tic-tac-toe perfectly, to enumerate the 765 essentially different positions (the State Space Complexity ), or the 26,830 possible games up to rotations and reflections (the Game Tree Complexity ) on this space.

Without eliminating symmetries, there are 255,168 possible games. Assuming that X makes the first move every time,
  • 131,184 games are won by X,

  • 77,904 games are won by O,

  • 46,080 games are a draw.


The first known Computer Game , '' OXO '' (or ''Noughts and Crosses'', 1952 ) for the EDSAC computer played perfect games of tic-tac-toe against a human opponent.

One example of a Tic-Tac-Toe playing computer is the Tinkertoy computer, developed by MIT students, and made out of Tinker Toys . It only plays Tic-Tac-Toe, and has never lost a game. It is currently on display at the Museum Of Science, Boston .


STRATEGY


A player can play perfect tic-tac-toe if they choose the move with the highest priority in the following table1.

#Win: complete three in a row.
#Block: block their opponent from completing three in a row
#Fork: threaten a win with two possible completions in two ways
#Block Fork 1: if there is a configuration where the opponent can fork, create two in a row to force a block
#Block Fork 2: if there is a configuration where the opponent can fork, block that fork
#Centre: play the centre
#Opposite Corner: if the opponent is in the corner, play the opposite corner
#Empty Corner: play an empty corner
#Empty Side: play on an empty side

The first player, whom we shall designate "X," has 3 possible positions to mark during the first turn. Superficially, it might seem that there are 9 possible positions, corresponding to the 9 squares in the grid. However, by rotating the board, we will find that in the first turn, every corner mark is strategically equivalent to every other corner mark. The same is true of every edge mark. For strategy purposes, there are therefore only three possible first marks: corner, edge, or center. Player X can win or force a draw from any of these starting marks, however playing the corner gives the opponent the smallest choice of squares which must played to avoid losing2.

The second player, whom we shall designate "O," must respond to X's opening mark in such a way as to avoid the forced win. Player O must always respond to a corner opening with a center mark, and to a center opening with a corner mark. An edge opening must be answered either with a center mark, a corner mark next to the X, or an edge mark opposite the X. Any other responses will allow X to force the win. Once the opening is completed, O's task is to follow the above list of priorities in order to force the draw, or else to gain a win if X makes a weak play.


VARIATIONS

Many , Nine Men's Morris , Pente , Gomoku , Qubic , Connect Four , Quarto , Gobblet . The ''m'',''n'',''k''-games are a family of Generalized Game s based on tic-tac-toe.

  • 3-dimensional tic-tac-toe on a 3×3×3 board, though the first player has an easy win by playing in the centre. Another Variant is played on a 4×4×4 board though it was Solved by Victor Allis in 1994 (the first player can force a win). A more complex variant can be played on boards utilising higher dimensional space, most commonly 4 dimensions in a 3×3×3×3 board. In such games the aim is to fill up the board and get more rows of three in total than the other player.

  • 4 player 3-dimensional tic-tac-toe: Not as easy to win by the first player, the symbols are O's X's triangles and squares, otherwise the rules to 3-D tic-tac-toe apply.

  • In Misère tic-tac-toe you win if the ''other player'' gets ''n'' in a row. The 3×3 game is a draw.

  • In nine board tic-tac-toe nine tic-tac-toe boards are themselves arranged in a 3×3 grid. The first player's move may go on any board; all moves afterwards are placed in the empty spaces on the board corresponding to the square of the previous move (that is, if a move were in the upper-left square of a board, the next move would take place on the upper-left board). If a player can't move because the indicated board is full, the next move may go on any board. Victory is attained by getting 3 in a row on any board. This makes the game considerably longer and more involved than tic-tac-toe, with a definite opening, middle game and endgame.

  • In Tic-Tac-Chess, players play a game of Chess and tic-tac-toe simultaneously. When a player captures an opponent's piece, the player can make a play on the tic-tac-toe board regardless if the other player has not yet made a play. The first person to get 3 X's or O's in a row wins the game. This makes for a much more defensive game of chess.

  • There is a game which is Isomorphic to tic-tac-toe, but on the surface appears completely different. Players take it in turn to say a number between one and nine. A particular number may not be repeated. Both players aim to say three numbers which add up to 15. Plotting these numbers on a 3×3 Magic Square will reveal the exact correspondence with the game of tic-tac-toe, given that three numbers will be arranged in a straight line if and only if they add up to 15.

  • Two players fill out a 3×3 grid with numbers one through nine in order of priority. They then compare their grids and play tic-tac-toe by filling in the squares by the priority they listed before.

  • In the 1970s, there was a two player game made by Tri-ang Toys & Games called '''''Check Lines''''', in which the board consisted of eleven holes arranged in a Geometrical pattern of twelve straight lines each containing three of the holes. Each player had exactly five tokens and played in turn placing one token in any of the holes. The winner was the first player whose tokens were arranged in '''two lines of three''' (which by definition were Intersecting lines). If neither player had won by the tenth turn, subsequent turns consisted of moving one of one's own tokens to the remaining empty hole, with the constraint that this move could only be from an Adjacent hole.

  • Toss Across is a tic-tac-toe game where players throw Bean Bags at a large board to mark squares.

  • Various Game Show s have been based around the game:

  • ---On Hollywood Squares nine celebrities filled the cells of the tic-tac-toe grid.

  • ---In Secret "X" , a Pricing Game on ''' The Price Is Right ''', contestants must get three Xs in a row by correctly pricing items, and then find the one X in the middle column to complete a line.

  • ---In Tic-Tac-Dough players put symbols up on the board by answering questions in various categories.

  • ---In Beat The Teacher contestants answer questions to win a turn to influence a tic-tac-toe grid.

  • The object of the fictional D'ni game of Gemedet is to get six balls in-a-row in a 9×9×9 cube grid.

  • The object of the fictional game Squid-Tac-Toad is to get four (or five) pieces in-a-row on a 4×4 or 5×5 checkerboard grid.

  • Some children play where getting a Y formation also counts as a win.

  • Quantum Tic Tac Toe allows players to place a quantum superposition of numbers on the board

  • Another variation on tic-tac-toe is played on a larger grid (say 10x10) where the object is to get 5 in a row. The increased amount of space creates a greater complexity.



ALTERNATIVE NAMES

Around the world, the game has a number of alternative names.


Sometimes, the names of the games Tic-tac-toe (where players keep adding "pieces") and Three Men's Morris (where pieces start to move after a certain number have been placed) are confused.


IN FICTION


In the cult 1983 film '' WarGames '' tic-tac-toe is used as an allegory for Nuclear War . In the film a computer hacker David Lightman ( Matthew Broderick ) in the Cold War era breaks into the missile defense computer WOPR designed to orchestrate nuclear war against the Soviet Union . In the process, he inadvertently triggers the system into "DEFCON 1" mode, whereby the computer arms its missiles in preparation for launch. The hacker eventually influences the computer to play tic-tac-toe against itself, whereby the computer determines that neither side can win-–an analogy to full scale Nuclear War , which is made explicit when the computer then fails to find a winning stratagem for a nuclear strike.

"The only winning move is not to play."http://www.imdb.com/title/tt0086567/quotes


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