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The term zero-player game commonly refers to games that have no human players, but also to games that have no players at all.
Many computer games can be set up so that the computer plays against itself using artificial intelligence, the classical example of course being chess. Since such games already include artificial intelligence, the feature of letting the computer play alone is often added as a gimmick that does not take much additional programming time. In many cases, this can highlight problems with the AI, when all players employ similar strategies. In some computer games, such as the shareware classic Scorched Earth, AI-controlled players continue playing the game even after all human players have lost.
Entirely computer-controlled games are also frequently used to demonstrate graphics or gameplay, especially in arcade games.
Genetic programming can be used to develop more or less successful strategies against other players through natural selection. Most games used within game theory are zero-player games by the common definition, where many iterations of the actions of simulated players are used to determine an optimal strategy for a particular problem, such as the Prisoner's dilemma. The simulation does not need to occur on a computer, but this is common for complex problems with many iterations and strategies.
The movie WarGames popularized the idea of using computers to test strategies; in it, a computer which controlled the American defense arsenal played many iterations of a game of "global thermonuclear war" (as well as "tic tac toe") against itself using all known strategies, and came to the conclusion that "the only winning move is not to play."
Classifying programs as games which do not actually have any players is more problematic. Conway's Game of Life is perhaps the best known game meeting that definition, along with similar cellular automata, but even here it can be debated whether it is a game, and whether there are players (cells). Complex simulated worlds may generally be counted in this category, such as the computer games Creatures and SimEarth when there is no player intervention. The classification evolutionary games would perhaps be more proper here. Most screensavers are non-interactive, but is arguable for each one whether they are games. The parody RPG Progress Quest, which plays itself with no human intervention outside some initial choices with no influence in the actual "game", could be considered a zero-player game.
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