Sets in Dart

What is a Set?

A Set is an unordered collection of unique values. Duplicate items are automatically ignored. Sets are ideal when you need uniqueness or fast membership checks.

Creating Sets

Set<int> primes = {2, 3, 5, 7, 11};
Set<String> colors = {'red', 'green', 'blue'};

// Adding a duplicate — silently ignored
colors.add('red');
print(colors.length);  // 3 (still)

// Empty set — must use Set(), not {} (that creates a Map)
var empty = <String>{};  // or: Set<String>()

// From list (removes duplicates)
var nums = [1, 2, 2, 3, 3, 3];
var unique = nums.toSet();  // {1, 2, 3}

Set Operations

var a = {1, 2, 3, 4, 5};
var b = {3, 4, 5, 6, 7};

// Union — all elements from both
print(a.union(b));         // {1, 2, 3, 4, 5, 6, 7}

// Intersection — elements in both
print(a.intersection(b)); // {3, 4, 5}

// Difference — in a but not b
print(a.difference(b));   // {1, 2}

Example

void main() {
  Set<String> enrolled = {'Alice', 'Bob', 'Carol', 'Dave'};
  Set<String> paid = {'Bob', 'Carol', 'Eve'};

  // Who enrolled AND paid?
  var confirmed = enrolled.intersection(paid);
  print('Confirmed: $confirmed');  // {Bob, Carol}

  // Who enrolled but NOT paid?
  var unpaid = enrolled.difference(paid);
  print('Unpaid: $unpaid');  // {Alice, Dave}

  // Everyone involved
  var all = enrolled.union(paid);
  print('All: $all');  // {Alice, Bob, Carol, Dave, Eve}

  // Membership check — O(1)
  print(enrolled.contains('Alice'));  // true
}

Confirmed: {Bob, Carol}
Unpaid: {Alice, Dave}
All: {Alice, Bob, Carol, Dave, Eve}
true

🧠 Quiz

Summary

Sets are unordered collections of unique values. Duplicates are ignored. Key operations: union(), intersection(), difference(). Membership checks are O(1). Use <Type>{} to create an empty set (not {} which creates a Map).