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Dispatch[{lhs1rhs1,lhs2rhs2,…}]

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.

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Dispatch

Dispatch[{lhs1rhs1,lhs2rhs2,…}]

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.

Details

  • 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.
  • Dispatch is treated as a raw object by functions like AtomQ and for purposes of pattern matching.
  • Normal converts a Dispatch to a list of rules.

Examples

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Basic Examples  (2)

Create a dispatch table from a list of rules:

Wolfram Language code: rules = {a -> b, b -> c, c -> a, d -> e, e -> d};
Wolfram Language code: dispatch = Dispatch[rules]

The result after replacements is the same:

Wolfram Language code: {a, b, c, d, e} /. rules
Wolfram Language code: {a, b, c, d, e} /. dispatch

Dispatch may be much faster for a long list of rules:

Wolfram Language code: Short[rules = Table[x[i] -> RandomInteger[{1, i}], {i, 10000}]]
Wolfram Language code: dispatch = Dispatch[rules]
Wolfram Language code: Timing[Table[x[i] /. rules, {i, 10000}];]
Wolfram Language code: Timing[Table[x[i] /. dispatch, {i, 10000}];]

Scope  (2)

Use Dispatch with pattern-based rules:

Wolfram Language code: disp = Dispatch[{1 -> a, 2 -> b, c_ ? NumericQ -> ∞}]
Wolfram Language code: {1, 2, 3, Pi, Sin[x], Cos[3]} /. disp

Create a dispatch from an association:

Wolfram Language code: Dispatch[<|a -> b, c -> d|>]

Properties & Relations  (2)

Dispatch is AtomQ:

Wolfram Language code: AtomQ[Dispatch[{a -> b, b -> c, c -> a, d -> e, e -> d}]]

Use Normal to convert a dispatch to a normal list of rules:

Wolfram Language code: Normal[Dispatch[{a -> b, b -> c, c -> a, d -> e, e -> d}]]
Wolfram Research (1991), Dispatch, Wolfram Language function, https://reference.wolfram.com/language/ref/Dispatch.html (updated 2014).

Text

Wolfram Research (1991), Dispatch, Wolfram Language function, https://reference.wolfram.com/language/ref/Dispatch.html (updated 2014).

CMS

Wolfram Language. 1991. "Dispatch." Wolfram Language & System Documentation Center. Wolfram Research. Last Modified 2014. https://reference.wolfram.com/language/ref/Dispatch.html.

APA

Wolfram Language. (1991). Dispatch. Wolfram Language & System Documentation Center. Retrieved from https://reference.wolfram.com/language/ref/Dispatch.html

BibTeX

@misc{reference.wolfram_2026_dispatch, author="Wolfram Research", title="{Dispatch}", year="2014", howpublished="\url{https://reference.wolfram.com/language/ref/Dispatch.html}", note=[Accessed: 13-June-2026]}

BibLaTeX

@online{reference.wolfram_2026_dispatch, organization={Wolfram Research}, title={Dispatch}, year={2014}, url={https://reference.wolfram.com/language/ref/Dispatch.html}, note=[Accessed: 13-June-2026]}