Applying a function by Key pattern, preserving Association order

Question

Any convenient ways to apply a function to Keys that match a pattern in an Association? Method should handle:

1. Mixture of String, Symbol and composite Keys
2. Preserve the input Association order
3. Be efficient

Off the bat, KeyValuePattern only matches a single instance per Association, so normalizing an Association into singleton Associations is not likely to satisfy (3)

Illustrating current gap in functionality rather than a path to solution (Dataset is not strictly necessary but helps abbreviate Queries)

ds =  <|"b2" -> 2, "a1" -> 1, c3 -> 3|> // Dataset

Note c3 is a Symbol - Suppose the pattern is:

keyPattern = k_String /; StringMatchQ[k, "a*"]

This tentative query:

keyPatternQuery[patt_ -> f_][as_Association] := 
  Query[{KeySelect[MatchQ[patt]] /* Query[All, f], 
      KeySelect[MatchQ[patt ] /* Not]} /* Apply[Join]][as];

Fails (2) or would require resorting by input keys, likely failing (3)

ds[keyPatternQuery[keyPattern -> f]] // Normal

<|"a1" -> f[1], "b2" -> 2, c3 -> 3|>

Answer 1

This should do what you want:

ClearAll[KeyPatternQuery];
KeyPatternQuery[pat_ -> f_][asc_] := With[{
     g = Function[{key, val}, If[MatchQ[key, pat], key -> f[val], key -> val]]
   },
   Association@@KeyValueMap[g, asc]
]

For example with

asc = <|"b2" -> 2, "a1" -> 1, c3 -> 3|>

and

keyPattern = k_String /; StringMatchQ[k, "a*"];

we get

KeyPatternQuery[keyPattern -> f][asc]
(* <|"b2" -> 2, "a1" -> f[1], c3 -> 3|> *)

or, alternatively

Dataset[asc][KeyPatternQuery[keyPattern -> f]] // Normal
(* <|"b2" -> 2, "a1" -> f[1], c3 -> 3|> *)

Performance seems to be ok, too:

(* generate an association with 1000 entries *)
rasc = Association@@Table[
  RandomChoice[{ToExpression, Identity}]@RandomWord[] -> RandomInteger[{0, 100}], 
  10^3
];

First@RepeatedTiming[KeyPatternQuery[keyPattern -> f][rasc]]

(* 0.0074 *)

Answer 2

Here is a way that uses MapIndexed:

keyPatternQuery[patt_ -> f_] := MapIndexed[If[MatchQ[#2, {Key[patt]}], f[#], #]&]

So then:

ds[keyPatternQuery[keyPattern -> f]] // Normal

(* <|"a1" -> f[1], "b2" -> 2, c3 -> 3|> *)

or:

<|{1, "x"} -> "a", {2, "y"} -> "b", {3, "y"} -> "c"|> // keyPatternQuery[{_, "y"} -> f]

(* <|{1, "x"} -> "a", {2, "y"} -> f["b"], {3, "y"} -> f["c"]|> *)

Answer 3

Consider MapAt:

MapAt[f, ds, Position[Keys@ds, keyPattern]]