Dart Map
last modified January 28, 2024
Dart Map tutorial shows how to work with a map collection in Dart language.
A map is a collection of key/value pairs. The value is retrieved from a map with its associated key. Maps are also called dictionaries, associative arrays, or hash tables.
Depending on the iteration order, there are three types of maps in Dart:
- HashMap - unordered
- LinkedHashMap - ordered by insertion order
- SplayTreeMap - ordered by sorted keys
By default, creating an instance using Map constructor (Map,
Map.from, or Map.of) creates a
LinkedHashMap.
A map is created with a map literal or with a map constructor. To work with
maps, we import the dart:collection library.
Dart Map literal
A map literal consists of a pair of curly brackes, in which we specify the key/value pairs. The pairs are separated by comma. The key is separated from the value by colon.
void main() {
var data = {'name': 'John Doe', 'occupation': 'gardener'};
print(data);
print(data.runtimeType);
var words = <int, String>{1: 'sky', 2: 'falcon', 3: 'rock'};
print(words);
print(words.runtimeType);
}
In the example, we create two maps with map literals.
var data = {'name': 'John Doe', 'occupation': 'gardener'};
Here we have a simple map with two pairs. The 'name' and 'occupation' are keys, the 'John Doe' and 'gardener' are values.
var words = <int, String>{1: 'sky', 2: 'falcon', 3: 'rock'};
In the second map, we also specify the data types for the keys and values. The data types precede the curly brackets.
$ dart main.dart
{name: John Doe, occupation: gardener}
_InternalLinkedHashMap<String, String>
{1: sky, 2: falcon, 3: rock}
_InternalLinkedHashMap<int, String>
Dart Map size
With the length property, we can determine the size of the map.
void main() {
var fruit = {1: 'Apple', 2: 'Orange'};
print(fruit.length);
print('There are ${fruit.length} elements in the map');
}
The example creates a simple map collection and prints its size.
$ dart main.dart 2 There are 2 elements in the map
Dart empty map
With isEmpty and isNotEmpty, we can determine whether
the map is empty or not empty.
void main() {
var words = {
1: 'sky',
2: 'fly',
3: 'ribbon',
4: 'falcon',
5: 'rock',
6: 'ocean',
7: 'cloud'
};
print(words.isEmpty);
print(words.isNotEmpty);
print('---------------');
words.clear();
print(words.isEmpty);
print(words.isNotEmpty);
}
In the example, we use the isEmpty and isNotEmpty
methods.
$ dart main.dart false true --------------- true false
Dart add pair
A new pair can be added to a map with the assignment operator or the
putIfAbsent method. The putIfAbsent adds a new value
if it is not in the map. It returns the existing or the new value.
void main() {
var fruit = {1: 'Apple', 2: 'Orange'};
fruit[3] = 'Banana';
print(fruit);
var val = fruit.putIfAbsent(3, () => 'Mango');
print(fruit);
print(val);
var val2 = fruit.putIfAbsent(4, () => 'Cherry');
print(fruit);
print(val2);
}
We add new pairs to the map in the example.
fruit[3] = 'Banana';
We add a new pair 3: 'Banana'.
var val = fruit.putIfAbsent(3, () => 'Mango');
Since there is already a pair with key 3, the new pair is not added to the map. The method returns Banana.
var val2 = fruit.putIfAbsent(4, () => 'Cherry');
The 4: 'Cherry' pair is added to the map. The method returns
Cherry.
$ dart main.dart
{1: Apple, 2: Orange, 3: Banana}
{1: Apple, 2: Orange, 3: Banana}
Banana
{1: Apple, 2: Orange, 3: Banana, 4: Cherry}
Cherry
Dart map removing pairs
We can remove pairs from the map with remove,
removeWhere, or clear.
void main() {
var words = {
1: 'sky',
2: 'fly',
3: 'ribbon',
4: 'falcon',
5: 'rock',
6: 'ocean',
7: 'cloud'
};
words.remove(1);
print(words);
words.removeWhere((key, value) => value.startsWith('f'));
print(words);
words.clear();
print(words);
}
The example removes pairs from the map of words.
words.remove(1);
With remove, we delete the pair having key 1.
words.removeWhere((key, value) => value.startsWith('f'));
The removeWhere deletes all pairs whose values (words) begin
with letter f.
words.clear();
The clear method removes all pairs from the map.
$ dart main.dart
{2: fly, 3: ribbon, 4: falcon, 5: rock, 6: ocean, 7: cloud}
{3: ribbon, 5: rock, 6: ocean, 7: cloud}
{}
Dart Map fromIterables
The fromIterables method creates a new map from the provided
iterables.
void main() {
var letters = ['I', 'II', 'V', 'X', 'L'];
var numbers = [1, 2, 5, 10, 50];
var data = Map<String, int>.fromIterables(letters, numbers);
print(data);
}
In the example, we create a map from two lists.
$ dart main.dart
{I: 1, II: 2, V: 5, X: 10, L: 50}
Dart Map collection if & for forms
With the collection if and for forms, we can dynamically create maps in Dart. The syntax is close to list comprehensions known from Haskell or Python.
void main() {
var words = ['sky', 'cloud', 'sod', 'worm', 'put', 'water', 'cup'];
var i = 0;
var data = {
for (var e in words)
if (e.length == 3) i++: e
};
print(data);
}
The example creates a map with a collection if/for form. We create a map from a list of words, where we include only words that have three characters. To each word, we assign a numeric key.
$ dart main.dart
{0: sky, 1: sod, 2: put, 3: cup}
Dart merge maps
We can merge maps with the addAll method or the spread (...)
operator.
void main() {
var f1 = {1: 'Apple', 2: 'Orange'};
var f2 = {3: 'Banana'};
var f3 = {4: 'Mango'};
var fruit = {}..addAll(f1)..addAll(f2)..addAll(f3);
print(fruit);
var fruit2 = Map.from(f1)..addAll(f2)..addAll(f3);
print(fruit2);
var fruit3 = {...f1, ...f2, ...f3};
print(fruit3);
}
In the example, we have three maps. We merge them with addAll and
the spread operator.
$ dart main.dart
{1: Apple, 2: Orange, 3: Banana, 4: Mango}
{1: Apple, 2: Orange, 3: Banana, 4: Mango}
{1: Apple, 2: Orange, 3: Banana, 4: Mango}
Dart containsKey/containsValue
With containsKey and containsValue, we can
determine if a map contains a specific key and value.
void main() {
var myMap = {1: 'Apple', 2: 'Orange', 3: 'Banana'};
print(myMap.containsKey(1));
print(myMap.containsKey(3));
print(myMap.containsValue('Apple'));
print(myMap.containsValue('Cherry'));
}
The example uses the containsKey and containsValue
methods for a small map.
$ dart main.dart true true true false
Dart map iteration
We can iterate over map pairs with forEach method or
for statement.
void main() {
var fruit = {1: 'Apple', 2: 'Banana', 3: 'Cherry', 4: 'Orange'};
fruit.forEach((key, val) {
print('{ key: $key, value: $val}');
});
print('---------------------------');
fruit.entries.forEach((e) {
print('{ key: ${e.key}, value: ${e.value} }');
});
print('---------------------------');
for (var key in fruit.keys) print(key);
for (var value in fruit.values) print(value);
}
In the example, we loop over a map of fruit.
fruit.forEach((key, val) {
print('{ key: $key, value: $val}');
});
With forEach method, we print the key/value pairs of the
fruit map.
fruit.entries.forEach((e) {
print('{ key: ${e.key}, value: ${e.value} }');
});
In a similar fashion, we loop over the entry objects of the fruit
map.
for (var key in fruit.keys) print(key); for (var value in fruit.values) print(value);
Finally, with for statements, we go through the keys and values
separately.
$ dart main.dart
{ key: 1, value: Apple}
{ key: 2, value: Banana}
{ key: 3, value: Cherry}
{ key: 4, value: Orange}
---------------------------
{ key: 1, value: Apple }
{ key: 2, value: Banana }
{ key: 3, value: Cherry }
{ key: 4, value: Orange }
---------------------------
1
2
3
4
Apple
Banana
Cherry
Orange
Dart sort map
When we need to sort maps, we can use the SplayTreeMap.
The SplayTreeMap is ordered by sorted keys.
import 'dart:collection';
void main() {
var fruit = new SplayTreeMap<int, String>();
fruit[0] = 'Banana';
fruit[5] = 'Plum';
fruit[6] = 'Strawberry';
fruit[2] = 'Orange';
fruit[3] = 'Mango';
fruit[4] = 'Blueberry';
fruit[1] = 'Apple';
print(fruit);
fruit.forEach((key, val) {
print('{ key: $key, value: $val}');
});
var sortedByValue = new SplayTreeMap<int, String>.from(
fruit, (key1, key2) => fruit[key1].compareTo(fruit[key2]));
print(sortedByValue);
}
We have a map of fruit. By default, the pairs are ordered by the keys -- numerically, in ascending order.
var sortedByValue = new SplayTreeMap<int, String>.from(
fruit, (key1, key2) => fruit[key1].compareTo(fruit[key2]));
We create a sorted map by values from the original map. We pass the
from function a comparison function, which compares the values of
the pairs.
$ dart main.dart
{0: Banana, 1: Apple, 2: Orange, 3: Mango, 4: Blueberry, 5: Plum, 6: Strawberry}
{ key: 0, value: Banana}
{ key: 1, value: Apple}
{ key: 2, value: Orange}
{ key: 3, value: Mango}
{ key: 4, value: Blueberry}
{ key: 5, value: Plum}
{ key: 6, value: Strawberry}
{1: Apple, 0: Banana, 4: Blueberry, 3: Mango, 2: Orange, 5: Plum, 6: Strawberry}
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