Evaluation function

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For the string evaluation function, see eval.

An evaluation function, also known as a heuristic evaluation function or static evaluation function, is a function used by game-playing programs to estimate the value or goodness of a position in the minimax and related algorithms. The evaluation function is typically designed to be fast and accuracy is not a concern (therefore heuristic); the function looks only at the current position and does not explore possible moves (therefore static).

One popular strategy for constructing evaluation functions is as a weighted sum of various factors that are thought to influence the value of a position. For instance, an evaluation function for chess might take the form

 c₁ * material + c₂ * mobility + c₃ * king safety + c₄ * center control + ...

Chess beginners, as well as the simplest of chess programs, evaluate the position taking only "material" into account, i.e they assign a numerical score for each piece (with pieces of opposite color having scores of opposite sign) and sum up the score over all the pieces on the board. On the whole, computer evaluation functions of even advanced programs tend to be more materialistic than human evaluations. This is compensated by the increased speed of evaluation, which allows more plies to be examined. As a result, some chess programs may rely too much on tactics at the expense of strategy.