Lists, maps, and transformations

The idea

A List is an ordered collection indexed from zero. A Map associates keys with values, and a missing-key lookup can produce null. Assignment of a collection variable copies its reference rather than duplicating the collection. If two names refer to one list, mutation through either name is visible through the other. Create a new list when independence is required.

where selects matching elements and map transforms elements, producing iterable views that commonly perform their work during iteration. Calling toList evaluates the pipeline into a stored list. This matters when the source changes after the pipeline is declared. Decide whether the result should reflect future source contents or represent a snapshot of the values now.

void main() {
  final values = [1, 4, 6];
  final selected = values.where((x) => x > 2).map((x) => x + 1).toList();
  values.add(8);
  print(selected.join(","));
  print(values.length);
}

5,7
4

toList stores the transformed values before eight is added. The original list later has four elements, while the snapshot still has two.