What is Map in dart & What is the Map.map() method in Dart?

In Dart programming, Maps are dictionary-like data types that exist in key-value form (known as lock-key). There is no restriction on the type of data that goes in a map data type. Maps are very flexible and can mutate their size based on the requirements. However, it is important to note that all locks (keys) need to be unique inside a map data type.

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 (MapMap.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.

Declaring a Dart Map

Dart Map can be defined in two methods.

  • Using Map Literal
  • Using Map Constructor

The syntax of declaring Dart Map is given below.

Map Literals:

Map can be declared using map literals as shown below:  

Syntax: 
// Creating the Map using Map Literals
var map_name = { key1 : value1, key2 : value2, ..., key n : value n }

Example:

void main() {
// Creating Map using is literals
var f = {'position1' : 'flutter', 'position2' : 'for', 'position3' : 'fever'};

// Printing Its content
print(f);

// Printing Specific Content
// Key is defined
print(f['position1']);

// Key is not defined
print(f[0]);
}

Map Constructors:

Syntax: 
// Creating the Map using Map Constructor
var map_name = new Map();
// Assigning value and key inside Map
map_name [ key ] = value;

Example :

Creating Map using Map Constructors 

void main() {
// Creating Map using Constructors
var f = new Map();

// Inserting values into Map
f [0] = 'flutter';
f [1] = 'for';
f [2] = 'fever';

// Printing Its content
print(f);

// Printing Specific Content
// Key is defined
print(f[0]);
}

output:

{0: flutter, 1: for, 2: flutter}
flutter

Map Properties in dart

PropertiesExplanation
KeysIt is used to get all keys as an iterable object.
valuesIt is used to get all values as an iterable object.
LengthIt returns the length of the Map object.
isEmptyIf the Map object contains no value, it returns true.
isNotEmptyIf the Map object contains at least one value, it returns true.

Example:

void main() {   
   var student = new Map();   
   student['name'] = 'Tom';   
   student['age'] = 23;   
   student['course'] = 'B.tech';   
   student['Branch'] = 'Computer Science';  
   print(student);   
  
  // Get all Keys  
  print("The keys are : ${student.keys}");  
  
 // Get all values  
 print("The values are : ${student.values}");  
   
 // Length of Map  
 print("The length is : ${student.length}");  
  
//isEmpty function  
print(student.isEmpty);  
  
//isNotEmpty function  
print(student.isNotEmpty);  
}  

Output:

{name: Tom, age: 23, course: B.tech, Branch: Computer Science}
The keys are : (name, age, course, Branch)
The values are : (Tom, 23, B.tech, Computer Science)
The length is : 4
false
true 

Map Methods in dart

The commonly used methods are given below.

addAll() – It adds multiple key-value pairs of other. The syntax is given below.

Syntax –

Map.addAll(Map<Key, Value> other)  

Parameter:

  • other – It denotes a key-value pair. It returns a void type.

Let’s understand the following example.

void main() {   
   Map student = {'name':'Tom','age': 23};   
   print('Map :${student}');   
     
   student.addAll({'dept':'Civil','email':'tom@xyz.com'});   
   print('Map after adding  key-values :${student}');   
}  

Dart map removing pairs

We can remove pairs from the map with removeremoveWhere, or clear.

main.dart

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.

main.dart

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 merge maps

We can merge maps with the addAll method or the spread (…) operator.

main.dart

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.

main.dart

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.

main.dart

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.

main.dart

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}

Map.map() method in Dart

The Map.map() method can be used to create a new map with all entries modified or transformed.

Syntax

map_name.map((k, v))

map_name is the name of the map.

Parameters

The map() method requires a key-value pair of a specific map to transform to a new map.

Return type

Map.map() returns a new key-value pair.

Code

The following code shows how to use the Map.map() method in Dart:

void main() {

  // Initialize map
  Map map1 = {'one': 2, 'two': 6,'three': 9};
  
  //Use .map() method
  var transformedMap = map1.map((k, v) {
    return MapEntry(k.toUpperCase(), v*v);
  });

  print(map1);

  print('The transformed map: ${transformedMap}');
}

output:

{one: 2, two: 6, three: 9} The transformed map: {ONE: 4, TWO: 36, THREE: 81}

The code above changes keys and values of all entries of map1 based on the condition to create the transformedMap. This changes the keys to uppercase and multiplies the values by themselves.

In this article, we have covered Dart maps.

dart map

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