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Considering following nested structure:

ls = {"a" -> 1,
      "b" -> 2,
      "c" -> {"AA" -> <|"Aaaa" -> <|"yi" -> 512, "h"  -> 45           |>, 
                           "V" -> <|"e2" ->   5, "rr" -> <|"e"->3.14|>|>|>,
              "VV" -> <|  "e4" -> 123|>}}

I want to update "e4" key to have 124 value instead of 123.

I can easily extract the value by "VV" /. ("c" /. ls). I can even update the value as follows:

Insert["VV" /. ("c" /. ls), "e4" -> 124, -1]

This nohow affects the ls.

What is the correct way to update the value exactly in ls?

Thanks!

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  • 1
    $\begingroup$ Do you have to work with this mixed form? $\endgroup$ – Kuba Oct 20 '15 at 11:30
  • $\begingroup$ If you mean the mix of lists and associations, then yes $\endgroup$ – SuTron Oct 20 '15 at 11:36
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You need to use Part to be able to change list elements. For a list of rules you can use this index finder:

RulesIndex[ls_List, keys_List] := 
  Sequence @@ 
   Flatten@FoldPairList[
     With[{pos = 
         FirstPosition[#1, #2 -> _, Missing@"NotFound", 1]~Append~
          2}, {pos, Extract[#1, pos]}] &, ls, keys];

ls[[RulesIndex[ls, {"c", "VV"}]]][["e4"]] = 124

which is the same as

ls[[3, 2, 2, 2, "e4"]] = 124
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