Avoiding copies when modifying lists

Question

Consider the following approach to modifying a list:

updateList01[lst_List, test_] :=
 Module[{xs = lst}, If[test, xs[[1]] += 1]; xs]
lst = {1, 2, 3}
lst = updateList[lst, True]

The good: all arguments are passed explicitly. The bad: xs and lst are apparently both entirely in memory(?). If that is right, what is the Mathematica idiom to avoid building a (potentially large) second list? (Assume there are many lists that needed to be updated and that actual updating is more complex.) Looking at HoldFirst, it sounds like I might avoid a copy by setting HoldFirst. That is also how I am inclined to read How to modify function argument?. Is that a correct interpretation? But even if one can avoid a copy this way, I'm clearly failing to understand something. The following fails:

SetAttributes[updateList02, HoldFirst]
updateList02[lst_List, test_] := If[test, lst[[1]] += 1]
lst02 = {1, 2, 3}
updateList02[lst02, True]  (* Not evaluated? *)
lst02   (* Unchanged *)

How do I fix it?

Answer 1

You've actually got two problems. The first is the fact that the function isn't evaluating. This happens because you pass lst02 to your function, which is a Symbol not, a List, so the pattern doesn't match your definition. You can see this by doing:

SetAttributes[updateList02, HoldFirst]
updateList02[lst_List, test_] := If[test, lst[[1]] += 1]
lst02 = {1, 2, 3}
updateList02[Evaluate@lst02, True]  
lst02   (* still unchanged *)

This evaluates, but not in the way you wanted it to.

Since you're trying to change the stuff referred to as lst02, write your function to accept the Symbol referring to the stuff you're trying to modify. But since you lost your original pattern match you'll need to make sure you only evaluate the function for lists, which you can do with a conditional:

ClearAll[updateList02];
SetAttributes[updateList02, HoldFirst]
updateList02[lst_Symbol, test_] /; Head@lst == List := 
 If[test, lst[[1]] += 1];

Now doing

lst02={1,2,3};
updateList02[lst02,True];
lst02

gives {2,2,3}

You can also use the conditional to add other safety measures, make sure your list is long enough etc.