Map a function to every given key of a nested association

Question

Consider for example the nested association:

myA = <|"d" -> <|"c" -> <|"a" -> 1, "b" -> 2|>|>|>

It is easy to map a function to a key on the first level:

MapAt["c" /. # &, myA, "d"]

but MapAt doesn't work with levels higher than 1

MapAt["a" /. # &, myA, "c"]

So I tried a workaround, trasforming all the associations to lists:

  Replace[
    myA /. Association -> ass
    ,
    Rule["c", x_] -> Rule["c", ("a" + "b") /. x], Infinity
  ] /. ass -> Association 

getting the correct result, but with some errors that I can't fix

I've tried with Hold, but it looks like it works at level 1 only.

Thank you for your help!

Answer 1

Here is a function that finds all keys of a given name anywhere in your data structure and maps a function onto those positions:

mapAtKeyEveryWhere[assoc_, fun_, key_] := MapAt[
  fun,
  assoc,
  Append[Key[key]] /@ Position[
    assoc,
    KeyValuePattern[key -> _]
  ]
];

Test it:

myA = <|"d" -> <|"c" -> <|"a" -> 1, "b" -> 2|>|>|>;
mapAtKeyEveryWhere[myA, ("a" + "b") /. # &, "c"]
(* <|"d" -> <|"c" -> 3|>|> *)

Is that what you have in mind?