Create a new list of random items with a specific length in Flutter/Dart

Your method of getting random items out of the original list will result in duplicates, which are then eliminated when you call .toSet(), which is why you get lists of varying lengths.

Instead of calling .toSet() to prevent duplicates, we need to be sure that no duplicates are chosen in the first place.

One way of doing that would be to generate a list of numbers from 0 up to originalList.length - 1, then shuffle them, and then choose the first n numbers from that list, and map them to values from original list.

void main() {
  List<String> originalList = ["hello", "what", "goodbye", "Test", "Apple", "Banana"];

  List<int> indices = List<int>.generate(originalList.length, (i) => i);

  indices.shuffle();

  int newCount = 3;

  List<String> randomList = indices.take(newCount).map((i) => originalList[i]).toList();

  print(randomList);
}

This method is efficient, and has a worst case performance of O(n), assuming dart has an efficient shuffle() implementation (which I'm sure it does).

The problem here is the list generates duplicates nonetheless.

var randomList = new List.generate(5, (_) => originalList[Random().nextInt(originalList.length)])

The list could contain [dello, kello, hello, gello, hello] (for example, assume items are taken from the original list)

Now performing .toSet() on this will remove the duplicate hello from the list, this is what causes the list size to be random. In this instance it's two duplicates, but later on it could have more.

A simple solution for this would be

var randomList = [];

List.generate(5, (_) {
   var randomString = originalList[Random().nextInt(originalList.length)];

   while (randomList.contains(randomString)) {
      randomString = originalList[Random().nextInt(originalList.length)];
   }

   randomList.add(randomString);
});