Hi George,

There are two ways to do this:

ClearAll[doubleAge]
doubleAge[age_?NumericQ] := age*2

titanic = ExampleData[{"Dataset", "Titanic"}];
a = titanic[All, <|#, "age" -> doubleAge[#age]|> &]; 
b = titanic[All, {"age" -> doubleAge}]; 
a === b
(* True *)

Hi George,

Using a Dataset

titanic = ExampleData[{"Dataset", "Titanic"}]

ClearAll@doubleAge;
doubleAge[age_?NumericQ] := age * 2;

titanic[All, <|#, "age" -> doubleAge[#age]|> &]

You can replace doubleAge with any function and replace age with any column name.