Dart Set

last modified January 28, 2024

In this article we show how to work with a Set collection in Dart language.

A set is a collection of data with no duplicate elements. A set supports operations like union, intersection, or difference, known from mathematics.

There are three types of set collections in Dart. A HashSet is unordered, which means that its iteration order is unspecified, LinkedHashSet iterates in the insertion order of its elements, and a sorted set SplayTreeSet iterates the elements in sorted order.

A set can be created with a pair of {} brackets. This syntax is called the collection literal notation.

Dart set example

A new item is inserted to a set with add. The size of a set is determined with length property.

void main() {
  var brands = <String>{};

  brands.add("Wilson");
  brands.add("Nike");
  brands.add("Volvo");
  brands.add("IBM");
  brands.add("IBM");

  int n = brands.length;

  print("The set contains $n elements");
  print(brands);
}

In the program, we have a set of brands.

var brands = <String>{};

A set is created with a pair of {}. In the <> brackets we provide the set's data type.

brands.add("Wilson");
brands.add("Nike");
brands.add("Volvo");
brands.add("IBM");
brands.add("IBM");

Elements are inserted with add. Even though we add IBM twice, the final set contains it only once.

int n = brands.length;

We get the length of the set.

$ dart main.dart 
The set contains 4 elements
{Wilson, Nike, Volvo, IBM}

Dart set remove elements

In the next example, we remove elements from a set.

void main() {
  var brands = <String>{};

  brands.add("Wilson");
  brands.add("Nike");
  brands.add("Volkswagen");
  brands.add("Volvo");
  brands.add("IBM");
  brands.add("Kia");
  brands.add("Lenovo");

  print(brands);

  brands.remove('Volvo');

  print(brands);

  brands.removeWhere((e) => e.length == 3);

  print(brands);

  brands.clear();

  print(brands);

  print("The set contains ${brands.length} elements");
}

The elements are removed with remove, removeWhere, and clear.

brands.remove('Volvo');

The specified element is removed with remove.

brands.removeWhere((e) => e.length == 3);

With removeWhere, we delete all elements that satisfy the given predicate function.

brands.clear();

The clear method removes all elements.

$ dart main.dart 
{Wilson, Nike, Volkswagen, Volvo, IBM, Kia, Lenovo}
{Wilson, Nike, Volkswagen, IBM, Kia, Lenovo}
{Wilson, Nike, Volkswagen, Lenovo}
{}
The set contains 0 elements

Dart iterate set

In the next example, we iterate over the elements of a set.

import 'dart:collection';

void main() {
  var nums = HashSet<int>();
  nums.add(1);
  nums.add(2);
  nums.add(3);
  nums.add(4);
  nums.add(5);

  nums.forEach((e) {
    print(e);
  });

  for (var e in nums) {
    print(e);
  }
}

We can loop over the elements with forEach function or the for loop.

var nums = HashSet<int>();
nums.add(1);
nums.add(2);
nums.add(3);
nums.add(4);
nums.add(5);

We create a instance of the HashSet and insert some integers with add.

nums.forEach((e) {
    print(e);
});

We iterate over the elements with the built-in forEach method.

for (var e in nums) {
    print(e);
}

Here we use the for loop.

Dart set insertion order

The HashSet does not keep the insertion order of the elements. The LinkedHashSet does keep.

import 'dart:collection';

void main() {
  var nums1 = HashSet<int>();
  nums1.add(4);
  nums1.add(3);
  nums1.add(2);
  nums1.add(1);

  print(nums1);
  print(nums1.runtimeType);

  nums1.forEach((e) {
    print(e);
  });

  print("-----------------------");

  var nums2 = LinkedHashSet<int>();
  nums2.add(4);
  nums2.add(3);
  nums2.add(2);
  nums2.add(1);

  print(nums2);
  print(nums2.runtimeType);

  nums2.forEach((e) {
    print(e);
  });
}

We compare the order of the elements created with HashSet and LinkedHashSet.

print(nums1.runtimeType);

The type of the object can be determined with runtimeType.

$ dart main.dart 
{1, 2, 3, 4}
_HashSet<int>
1
2
3
4
--------------
{4, 3, 2, 1}
_CompactLinkedHashSet<int>
4
3
2
1

Dart set functions

There are some functions that are specific to sets, such as union.

void main() {
  var vals1 = <int>{1, 2, 3, 4, 5, 6, 7};
  var vals2 = <int>{6, 7, 8, 9, 10};

  var res = vals1.union(vals2);
  print(res);

  var res2 = vals1.intersection(vals2);
  print(res2);

  var res3 = vals1.difference(vals2);
  print(res3);
}

In the program, we use union, intersetction, and difference functions.

var res = vals1.union(vals2);

The union function returns elements from both sets.

var res2 = vals1.intersection(vals2);

The intersection returns only elements that are present in both sets.

var res3 = vals1.difference(vals2);

The difference function carries out the difference operation, which returns elements that are in the vals1 but not in vals2.

$ dart main.dart 
{1, 2, 3, 4, 5, 6, 7, 8, 9, 10}
{6, 7}
{1, 2, 3, 4, 5}

Source

Dart Set - language reference

In this article we have covered Dart sets.

Author

My name is Jan Bodnar and I am a passionate programmer with many years of programming experience. I have been writing programming articles since 2007. So far, I have written over 1400 articles and 8 e-books. I have over eight years of experience in teaching programming.

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