One value is rarely enough.

A list of students. A map of grades. A set of unique tags.

Dart has three collection types for grouping values. You will use all of them.

List

A List holds values in a specific order. Think of a numbered shopping list: first item, second item, third item.

void main() {
  var names = ['Ali', 'Sara', 'John'];
  print(names[0]); // Ali
  print(names[1]); // Sara
  print(names[2]); // John
}

The values go inside [...], separated by commas. Each value has a position called an index.

But indexing does not start at 1. It starts at 0.

names[0] is the first item. names[1] is the second. names[2] is the third. This catches everyone at first. But it becomes natural quickly.

To replace a value, assign to its index:

void main() {
  var names = ['Ali', 'Sara', 'John'];
  names[0] = 'Ahmed'; // replace the first item
  print(names[0]); // Ahmed
}

The index stays the same. The value at that position changes.

Map

A Map holds key-value pairs. Think of a phone book: you look up a name (the key) and get back a number (the value).

void main() {
  var grades = {'Math': 90, 'English': 85};
  print(grades['Math']); // 90
}

The pairs go inside {...}. Each pair is written as key: value, separated by commas. To read a value, use its key inside [...].

Adding a new key and updating an existing key use the same syntax:

void main() {
  var grades = {'Math': 90};
  grades['Science'] = 78; // add a new key
  grades['Math'] = 95;    // update an existing key
  print(grades['Math']); // 95
}

Dart checks whether the key exists. If it does, the value is updated. If it does not, a new pair is added.

What if you read a key that does not exist?

void main() {
  var grades = {'Math': 90};
  print(grades['Science']); // null
}

A missing key returns null. We cover null fully in Lesson 9.

Set

A Set holds unique values. Order does not matter, and duplicates are not allowed.

void main() {
  var fruits = {'apple', 'banana', 'orange'};
  print(fruits); // {apple, banana, orange}
}

The values go inside {...}, separated by commas. No key-value pairs. Just values.

What if a value appears more than once?

void main() {
  var fruits = {'apple', 'banana', 'apple'}; // apple appears twice
  print(fruits); // {apple, banana}
}

Dart silently keeps only one copy. No error. No warning. The second 'apple' is dropped.

When to use which

There is a lot more you can do with collections: adding items, removing them, checking their length. But those require method calls, and method calls require classes. We cover them in Lesson 13.

Summary