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  • Dispatch[ -> , -> , ... ] generates an optimized dispatch table representation of a list of rules. The object produced by Dispatch can be used to give the rules in expr /. rules.
  • The use of Dispatch will never affect results that are obtained, but may make the application of long lists of rules much faster.
  • Lists of rules are usually scanned sequentially when you evaluate an expression like expr /. rules. Rules such as a[1]->a1 and a[2]->a2, which cannot simultaneously apply, need not both be scanned explicitly. Dispatch generates a dispatch table which uses hash codes to specify which sets of rules need actually be scanned for a particular input expression.
  • Lists of rules produced by assignments made with = and := are automatically optimized with dispatch tables when appropriate.
  • See the Mathematica book: Section 2.4.2.
  • See also: ReplaceAll, Compile.

    Further Examples

    Here is a list of rules for the first five factorials.



    This sets up dispatch tables that make the rules faster to use.



    You can apply the rules using the /. operator.