Dart Lesson 29 of 102 2 min read

Sets in Dart: Unique Collections and Set Operations

Learn Set in Dart: storing unique values, fast lookups, and set operations such as union, intersection and difference.

A Set is a collection where every value appears at most once. It is the right tool when you care about whether something is in the collection, not where or how many times.

Creating a set #

void main() {
  var colours = {'red', 'green', 'blue', 'red'};
  print(colours);
  print(colours.length);

  var empty = <String>{};
  print(empty.isEmpty);
}

{red, green, blue}
3
true

The duplicate 'red' is dropped silently. Dart’s default set remembers the order in which items were added.

Adding and removing #

add returns true if the value was new, and false if it was already there.

void main() {
  var tags = {'dart', 'flutter'};

  print(tags.add('mobile'));
  print(tags.add('dart'));
  tags.addAll(['web', 'flutter']);
  tags.remove('mobile');

  print(tags);
}

true
false
{dart, flutter, web}

Fast lookups #

Checking contains on a set takes the same short time whether it holds ten items or ten million. On a list, Dart has to check items one by one.

void main() {
  var blocked = {'spam@x.com', 'bot@y.com'};

  var sender = 'bot@y.com';
  if (blocked.contains(sender)) {
    print('Message rejected');
  }
}

Message rejected

Set operations #

void main() {
  var football = {'Aman', 'Bina', 'Chandra'};
  var cricket = {'Bina', 'Chandra', 'Dipak'};

  print(football.union(cricket));        // plays either
  print(football.intersection(cricket)); // plays both
  print(football.difference(cricket));   // football only
  print(football.containsAll({'Aman', 'Bina'}));
}

{Aman, Bina, Chandra, Dipak}
{Bina, Chandra}
{Aman}
true

Removing duplicates from a list #

The classic use: convert to a set and back.

void main() {
  var visits = ['home', 'about', 'home', 'contact', 'about'];
  var uniquePages = visits.toSet().toList();
  print(uniquePages);

  // Or in one literal with the spread operator:
  print({...visits}.length);
}

[home, about, contact]
3

No indexes #

A set has no [0]. Loop over it, or use first, last and elementAt. If you need positions, convert it to a list.

void main() {
  var primes = {2, 3, 5, 7};

  for (final p in primes) {
    print(p);
  }
  print(primes.elementAt(2));
}

2
3
5
7
5

Uniqueness of your own objects #

A set decides whether two values are the same using == and hashCode. For your own classes you need to define both, as shown in equality and hashCode.

Try it yourself #

You have two lists of student names: those who submitted assignment 1 and those who submitted assignment 2. Print who submitted both, who submitted only the first, and the total number of different students.