Map in Dart/Flutter

In general, a map is an object that associates keys and values. Both keys and values can be any type of object. Each key occurs only once, but you can use the same value multiple times. Dart support for maps is provided by map literals and the Map type.

Initialize a Map with values

The examples show you how to:

• initialize Map in simple way using { } (curly braces).
• create a Map with all key/value pairs of other Map using from(),for() constructor.
• create a new Map from the given keys and values using fromIterables().
• create Map in which keys and values are computed using fromIterable()
• create a ‘const’ Map using unmodifiable() constructor.

Map<String, int> map1 = {'zero': 0, 'one': 1, 'two': 2};
print(map1);

// not specify key-value type
Map map2 = {'zero': 0, 'I': 'one', 10: 'X'};
print(map2);

var map3 = {'zero': 0, 'I': 'one', 10: 'X'};
print(map3);


Output:
{zero: 0, one: 1, two: 2}
{zero: 0, I: one, 10: X}
{zero: 0, I: one, 10: X}

Remember that when we use Map or var, the type will always be LinkedHashMap.

Map<String, int> map1 = {'zero': 0, 'one': 1, 'two': 2};

Map map2 = Map.from(map1);
print(map2);

Map map3 = Map.of(map1);
print(map3);

To initialize some values in map3 while using Map.of() to create a copy of map1, you can follow these steps:

Map<String, dynamic> map1 = {'key1': 'value1', 'key2': 'value2'};
Map<String, dynamic> map3 = Map.of(map1);

// Initialize values in map3
map3['key3'] = 'value3';
map3['key4'] = 'value4';

print(map3);

// output

{key1: value1, key2: value2, key3: value3, key4: value4};

Create a new Map from the given keys and values using fromIterables().

List<String> letters = ['I', 'V', 'X'];
List<int> numbers = [1, 5, 10];

Map<String, int> map = Map.fromIterables(letters, numbers);
print(map);

Output:

{I: 1, V: 5, X: 10}

Create a new Map from the given keys and values using fromIterable().

List<int> numbers = [1, 2, 3];

Map<String, int> map = Map.fromIterable(
  numbers,
  key: (k) => 'double ' + k.toString(),
  value: (v) => v * 2);

print(map);

Output:

{double 1: 2, double 2: 4, double 3: 6}

Create a ‘const’ Map using unmodifiable() constructor.

Map map = Map.unmodifiable({1: 'one', 2: 'two'});
print(map);
// {1: one, 2: two}

map[3] = 'three';

If you try to modify/add object to the unmodifiable Map, it will throw an error.

Unhandled exception:
Unsupported operation: Cannot modify unmodifiable map

Add item to a Map

There are 2 way to add an item (key-value pair) to a Map:

• using square brackets []
• calling putIfAbsent() method

Map map = {1: 'one', 2: 'two'};

map[3] = 'three';
print(map);

map.putIfAbsent(4, () => 'four');
print(map);

var threeValue = map.putIfAbsent(3, () => 'THREE');
print(map);
print(threeValue); // the value associated to key, if there is one

Output:

{1: one, 2: two, 3: three}
{1: one, 2: two, 3: three}
{1: one, 2: two, 3: three, 4: four}
three

You can add all key/value pairs of another Map to current Map using addAll() method.

Map update value by key

The examples show you how to update a Map value using:

• square brackets []
• update() method
• update() method with ifAbsent to add a new object/entry if the input key is absent

Map map = {1: 'one', 2: 'two'};

map[1] = 'ONE';
print(map);
map.update(2, (v) {
  print('old value before update: ' + v);
  return 'TWO';
});
print(map);
map.update(3, (v) => 'THREE', ifAbsent: () => 'addedThree');
print(map);

Output:

{1: ONE, 2: two}
old value before update: two
{1: ONE, 2: TWO}
{1: ONE, 2: TWO, 3: addedThree}

Map map = {1: 'one', 2: 'two'};
map.addAll({3: 'three', 4: 'four', 5: 'five'});
print(map);

Output:

{1: one, 2: two, 3: three, 4: four, 5: five}

Map update value by key

The examples show you how to update a Map value using:

• square brackets []
• update() method
• update() method with ifAbsent to add a new object/entry if the input key is absent

Map map = {1: 'one', 2: 'two'};

Map map = {1: 'one', 2: 'two'};

map[1] = 'ONE';
print(map);

map.update(2, (v) {
  print('old value before update: ' + v);
  return 'TWO';
});
print(map);

map.update(3, (v) => 'THREE', ifAbsent: () => 'addedThree');
print(map);

Output:

{1: ONE, 2: two}
old value before update: two
{1: ONE, 2: TWO}
{1: ONE, 2: TWO, 3: addedThree}

Access items from Map

The examples show you way to:

• find the number of key/value pairs using .length getter.
• check if a Map is empty or not using .isEmpty or .isNotEmpty.
• get all keys or values with keys & values property.
• get value of specified key in a Map or operator [].

Map map = {1: 'one', 2: 'two'};

print(map.length);      // 2

print(map.isEmpty);     // false
print(map.isNotEmpty);  // true

print(map.keys);        // (1, 2)
print(map.values);      // (one, two)

print(map[2]);          // two
print(map[3]);          // null

Remove objects from Map

The examples show you how to:

• remove key-value pair by key using remove() method.
• remove all entries with condition using removeWhere() method.
• remove all entries from a Map using clear() method.

Map map = {1: 'one', 2: 'two', 3: 'three', 4: 'four', 5: 'five'};

Map map = {1: 'one', 2: 'two', 3: 'three', 4: 'four', 5: 'five'};

map.remove(2);
print(map);

map.removeWhere((k, v) => v.startsWith('f'));
print(map);

map.clear();
print(map);

Output:

{1: one, 3: three, 4: four, 5: five}
{1: one, 3: three}
{}

Sort a Map

The example shows you how to sort a Map by values using Map fromEntries() method and List sort() method.

Map map = {1: 'one', 2: 'two', 3: 'three', 4: 'four', 5: 'five'};

var sortedMap = Map.fromEntries(
    map.entries.toList()
    ..sort((e1, e2) => e1.value.compareTo(e2.value)));
    
print(sortedMap);

Output:

{5: five, 4: four, 1: one, 3: three, 2: two}

Map get key by value

Dart Map supports getting value by key, how about the opposite?
We can get key by a specified value using keys property and List firstWhere() method.

Map map = {1: 'one', 2: 'two', 3: 'three'};

var key = map.keys.firstWhere((k) => map[k] == 'two', orElse: () => null);
print(key);
// 2

In conclusion, this tutorial provides a comprehensive overview of working with maps in Dart/Flutter. Maps are essential for associating keys and values in your code, and Dart offers various methods to initialize, add, update, access, remove, and sort map data. Understanding these fundamental map operations is crucial for efficient data manipulation in your Dart and Flutter applications.

For more https://bit.ly/3UoJFYa

Happy Coding! See you again.