Function Repository Resource:

# MapAtEvaluate

A version of MapAt that evaluates even within held expressions

Contributed by: Nikolay Murzin
 ResourceFunction["MapAtEvaluate"][f,expr,pos] maps function f at position pos of the expression expr with evaluation. ResourceFunction["MapAtEvaluate"][f,expr,pos,patt] specifies a pattern to delete after evaluation. ResourceFunction["MapAtEvaluate"][f,pos] represents an operator form that can be applied to an expression.

## Details

In ResourceFunction["MapAtEvaluate"][f,expr,pos,patt], patt is taken to be Unevaluated by default.

## Examples

### Basic Examples (2)

Map a function at a single position of an expression and evaluate in-place:

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Map at multiple positions:

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### Scope (8)

Use All and Span to map over multiple subexpression parts:

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Prevent evaluation of the function output with Unevaluated:

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The Unevaluated is not present in the result:

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Specify a different subexpression to delete after evaluation:

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Prevent evaluation of subexpressions using a function with held arguments:

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Evaluate subexpressions:

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Map at a held Sequence:

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Map at a held Evaluate:

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Unevaluated function evaluates at each position:

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### Properties and Relations (4)

MapAt does not evaluate the function it is mapping inside held arguments:

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MapAtEvaluate does:

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MapAt fails if any position is invalid:

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MapAtEvaluate silently ignores invalid positions:

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The resource function ReplaceAt with a _/;True condition can achieve the same results:

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With MapAtEvaluate, one can control which subexpressions not to evaluate without producing an additional wrapper:

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The resource function MapCases with the option "EvaluateCases"True has similar behavior to MapAtEvaluate, except that it takes patterns instead of positions:

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## Version History

• 1.0.0 – 03 May 2022