I am trying to change the value of one key in an association based on the value of another key in that association. So if my association starts as:

x=<|"firstValue" -> True, "isFirstValueTrue" -> False|>

I want to change it to:

<|"firstValue" -> True, "isFirstValueTrue" -> True|>

So I wrote the function:

f[x_] := If[x[["firstValue"]], x[["isFirstValueTrue"]] = True, x[["isFirstValueTrue"]] = False]

I got an error, but based on this answer: http://stackoverflow.com/questions/12875913/setsetps-in-the-part-assignment-is-not-a-symbol I was to correct it by using:

SetAttributes[f, HoldAll].

But let's say I want to do this to a list of associations:

    x={<|"firstValue" -> True, "isFirstValueTrue" -> False|>,
     <|"firstValue" -> True, "isFirstValueTrue" -> True|>,
     <|"firstValue" -> False, "isFirstValueTrue" -> False|>,
     <|"firstValue" -> False, "isFirstValueTrue" -> True|>}

so I wrote:

Map[f[#] &, x]

However, the "in the part assignment is not a symbol." error appears again and you can't SetAttributes[Map, HoldAll]. I realize I can make a copy of the list and make changes to that, but the actual list I'm working with is much larger so I do not want to do that. The third solution in the link above is able to be mapped, but it doesn't seem like you can replace in an association. Any suggestions?