Dart Map

Summary: in this tutorial, you’ll learn how to use the Dart Map type to manage collections of key/value pairs.

Introduction to the Dart Map type

Map type allows you to manage a collection of key/value pairs. The Map type is similar to the Dictionary type in other programming languages.

In a map, keys are unique. Each key has an associated value. Unlike keys, values can be duplicated. Generally, you cannot add or remove keys while performing an operation on the map.

Creating a map

The following creates a map where keys are strings and values are integers:

void main() {
  var fruits = {'apple': 1, 'banana': 2, 'orange': 3};
  print(fruits);
}Code language: Dart (dart)

In this example, we define a map called fruits which maps a string (key) to a value (integer). Since we use var keyword, Dart will infer the type of the map as:

Map<String, int>Code language: Dart (dart)

In this syntax, the String denotes the type of the key and the int represents the type of the value.

The following statement is equivalent to the one above but uses the Map<String, int> type explicitly:

Map<String, int> fruits = {'apple': 1, 'banana': 1, 'orange': 2};Code language: Dart (dart)

To define an empty map, you use curly braces without any elements like this:

Map<String, int> fruits = {};Code language: Dart (dart)

A shorter way to do this is to move the type of the map to the right:

var fruits = <String, int>{};Code language: Dart (dart)

Notice that if you don’t specify the type explicitly, Dart will infer the type as <dynamic, dynamic>. For example:

var fruits = {};Code language: Dart (dart)

Printing the elements of a map

To print elements of a map, you pass it to the print() function. For example:

void main() {
  var fruits = {'apple': 1, 'banana': 2, 'orange': 3};
  print(fruits);
}Code language: Dart (dart)

Output:

{apple: 1, banana: 2, orange: 3}Code language: Dart (dart)

Accessing elements from a map

To access an element of a map, you use the subscript notation with a key. For example:

void main() {
  var fruits = {'apple': 1, 'banana': 2, 'orange': 3};
  print(fruits['banana']);
}Code language: Dart (dart)

Output:

2Code language: Dart (dart)

Adding elements to a map

To add a new element to a map, you assign the element with the key that is not available on the map:

void main() {
  var fruits = {'apple': 1, 'banana': 2, 'orange': 3};
  fruits['straberry'] = 4;
  print(fruits);
}Code language: Dart (dart)

Output:

{apple: 1, banana: 2, orange: 3, straberry: 4}Code language: Dart (dart)

Updating elements

If you assign a value to an element with the key that already exists, you’ll update the value of the element. For example:

void main() {
  var fruits = {'apple': 1, 'banana': 2, 'orange': 3};
  fruits['apple'] = 0;
  print(fruits);
}Code language: Dart (dart)

Output:

{apple: 0, banana: 2, orange: 3}Code language: Dart (dart)

Removing elements

To remove an element by a key, you use the remove() method. For example:

void main() {
  var fruits = {'apple': 1, 'banana': 2, 'orange': 3};
  fruits.remove('apple');
  print(fruits);
}Code language: Dart (dart)

Output:

{banana: 2, orange: 3}Code language: Dart (dart)

Map properties

The Map type provides some useful properties:

• map.isEmpty – return true if the map has no element.
• map.isNotEmpty – return true if the map has at least one element.
• map.length – return the number of elements of the map.
• map.keys – return a list of keys.
• maps.values – return a list of values.

For example:

void main() {
  var fruits = {'apple': 1, 'banana': 2, 'orange': 3};
  print(fruits.isEmpty); // false
  print(fruits.isNotEmpty); // true
  print(fruits.length); // 3
  print(fruits.keys); // (apple, banana, orange)
  print(fruits.values); // (1, 2, 3)
}Code language: Dart (dart)

Output:

false
true
3Code language: Dart (dart)

Checking for the existence of keys or values

To check if a key exists in a map, you use the containsKey() method. For example:

void main() {
  var fruits = {'apple': 1, 'banana': 2, 'orange': 3};

  print(fruits.containsKey('apple')); // true
  print(fruits.containsKey('strawberry')); //false
}Code language: Dart (dart)

Output:

true
falseCode language: Dart (dart)

Similarly, to check if a value exists in a map, you use the containsValue() method:

void main() {
  var fruits = {'apple': 1, 'banana': 2, 'orange': 3};

  print(fruits.containsValue(1)); // true
  print(fruits.containsValue(2)); // false
}Code language: Dart (dart)

Iterating over elements of a map

To iterate over elements of the map, you use the for-in statement. However, you cannot use it directly with the map elements but keys like this:

void main() {
  var fruits = {'apple': 1, 'banana': 2, 'orange': 3};
  for (var name in fruits.keys) {
    print('$name: ${fruits[name]}');
  }
}Code language: Dart (dart)

Output:

apple: 1
banana: 2
orange: 3Code language: Dart (dart)

Also, you can use the entries property of a map. To iterate over elements of the map, you can iterate over the map.entries. Each MapEntry object has two properties including key and value. For example:

void main() {
  var fruits = {'apple': 1, 'banana': 2, 'orange': 3};
  for (var fruit in fruits.entries) {
    print('${fruit.key}: ${fruit.value}');
  }
}Code language: Dart (dart)

Output:

apple: 1
banana: 2
orange: 3Code language: Dart (dart)

Like a List, you can use the forEach() method:

void main() {
  var fruits = {'apple': 1, 'banana': 2, 'orange': 3};
  fruits.forEach((key, value) => print('$key: $value'));
}Code language: Dart (dart)

Output:

apple: 1
banana: 2
orange: 3Code language: Dart (dart)

Summary

• A map is a collection of key/value pairs.