Mathematica: TIP: How to change a value within an Association inline within a Module (like pass by reference)

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TIP

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INFORMATIVE

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If you are an advocate of pure functional you may not find the following so appealing, but it's useful to at least know how this works.

Consider the following simple Association:


assoc = <|"a" -> 1, "b" -> 2|>
<|"a" -> 1, "b" -> 2|>

In the global scope the value of, say, "a", can be directly changed thus:


assoc["a"] = 3
3

assoc
<|"b" -> 2, "a" -> 3|>

But this does not work within a Module:


f1[assoc_, aNew_] := assoc["a"] = aNew

f1[assoc, 4]
Association::setps: <|b->2,a->1|> in the part assignment is not a symbol.

To make it work like pass-by-reference, apply a HoldFirst to f1:


SetAttributes[f1, HoldFirst]

f1[assoc, 4]
4

assoc
<|"b" -> 2, "a" -> 4|>

AppendTo[] can likewise be used to replace an Association value inline:


AppendTo[assoc, "a" -> 5]
<|"b" -> 2, "a" -> 5|>

assoc
<|"b" -> 2, "a" -> 5|>

But if you use it within a Module, you'll hit a similar error unless you apply a HoldFirst to it:


f2[assoc_, aNew_] := AppendTo[assoc, "a" -> aNew]
f2[assoc, 6]
AppendTo::rvalue: <|b->2,a->6|> is not a variable with a value, so its value cannot be changed.

But with HoldFirst applied it works fine:


SetAttributes[f2, HoldFirst]

f2[assoc, 6]
<|"b" -> 2, "a" -> 6|>

assoc
<|"b" -> 2, "a" -> 6|>

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