Dart: How to update value in Set

Sets elements only have indices incidentally because they are iterables. You can't "update the value at position x" because any update may change the order.

I assume you do want to keep the iteration order (and in that case, you really should be using a list!), so the following won't work:

 void update<T>(Set<T> elements, int index, T newValue) {
   set.remove(set.elementAt(index));
   set.add(newValue);
 }

The problem here is that you won't preserve iteration order, the new value that you added is likely at the end of the iteration (if you are using an insertion-ordered set), or maybe the order has changed completely (if you are not).

An approach which works for insertion-ordered sets:

void replace<T>(Set<T> set, int index, T newValue) {
  if (set.contains(newValue)) throw StateError("New value already in set");
  int counter = 0;
  while (counter < set.length) {
    var element = set.first;
    set.remove(element);
    if (counter == index) element = newValue;
    set.add(element);
    counter++;
  }
}

This repeatedly removes the first element in the set, then inserts it again, except for the indexth element, where it inserts newValue instead. This only works for insertion-ordered sets (like the default LinkedHashSet). (Notice that this will not work if newValue is already in the set, so I made it throw in that case).

For all other sets, there is no better solution that (set.toList()..[index] = newValue).toSet().

You can not change values in a set directly with index, but you can do it with map.

Set<int> set = {1, 2, 3};
Set<int> updated = set.map((item) => item == 1 ? 0 : item).toSet();

print(updated); // => {0, 2, 3}

You cannot change the value in a set but you could get a similar result by converting it to a list and changing it back to set.

Set<int> set = {1,2,3};
List<int> list = set.toList();
list[1] = 12;
var newset = list.toSet();

this may be a quick solution to your problem.