[12.4] How can I make a duplicate copy of a list?

Perhaps confusingly, a list "value" kept in a variable isn't the data contained in the list items themselves, its just a pointer telling Director where those items can be found. As a result, if you assign it to another variable, both variables contain the same value, pointing to the same set of items, rather than keeping separate copies of all the items. If you change the items in one copy, all the other copies will also be changed:

  set originalList = [ 1, 2, 3 ]
  set copyList = originalList
  add copy, 4
  put originalList
  -- [ 1, 2, 3, 4 ]

If you want to make a duplicate copy of a list containing only numbers, a quick way is to do this:

  set copy = originalList * 1

The copy thus created will point to a new set of items all its own rather than to the same set as originalList, and the two lists can then be modified independently.

Why does this work? Well, arithmetic operations applied to lists map across the whole list, and produce a new list as a result:

  put ( 2 * [ 1, 2, 3 ] )
  -- [ 2, 4, 6 ]

Multiplying by one is an identity operation that leaves the content values unchanged, but still generates a new "results" list.

However, if you apply this to items which *aren't* integers, they'll mostly be treated as if they are (since all data, more or less, is internally represented by numbers), and your data will get mangled:

  put 1 * ["abc", #blah, birth(script "blah") ] -- [11374992, 534, 0]

A more general way to duplicate a list would be to use a handler like:

-- handler returning a duplicate copy of a linear list
-- this version recursively calls itself to copy items in aList
-- that are themselves lists, so these too will be duplicates
-- child objects in the list will still be shared by both the source
-- and duplicate lists, however, since there's no sensible way to
-- duplicate objects of unknown provenance (unless you provide
-- all your parents with their own "duplicate" methods...)

on duplicate aList
  if not listP( aList ) return aList
  set retList = []
  repeat with x in aList
    if listP(x) then add retList, duplicate(x) else add retList, x
  end repeat
  return retList
end duplicate