Dart Maps
Map is an object-based data structure that associates keys and values that can be of any data type.
Every key can occur only once, while the values can be used multiple times.
var maps1 = { 1 : ‘one’, 2 : ‘two’, 3 : ‘three’ };
.length is used to get the number of key-value pairs in the map.
It is simple to add a new key-value pair to an existing map just like JavaScript,
Maps1[ 4 ] = ‘four’ ;
If you look for a key that isn’t in a map, then it returns null.
Methods of declaring a Dart Map
- Using Map literal
- Using Map constructor
Using Map Literals
In this method, the key - value pairs are enclosed within the curly braces ‘{ }’, separated by the commas.
Syntax :
var map_name = {key1 : val1, key2 : val2, key3 : val3, ….., keyn : valn} ;Example :
Consider the following code in Dart to understand the maps implementation using map literals,
void main( )
{
// declare and define a map named maps1
var maps1 = { 1 : 'one' , 2 : 'two' , 3 : 'three' } ;
// prints the values of map
print ( maps1 ) ;
// add new key-value pair to a map
maps1[ 4 ] = 'four' ;
print( maps1 ) ;
// prints the length of map
print( maps1.length ) ;
// trying to print the value of key that exists
print( maps1[ 3 ] ) ;
// trying to print the value of key that doesn't exist
print( maps1[ 5 ] ) ;
}
Output :
{1: one, 2: two, 3: three}
{1: one, 2: two, 3: three, 4: four}
4
three
null
Using Map( ) constructor
In this method, a map is declared using the map( ) constructor and then initialized with some values.
Syntax :
var map_name = new map( ) ; // declaring a map using a map( ) constructor
map_name[ key ] = value ; // initializing the keys with value
Example :
Consider the following code in Dart to understand the maps implementation using Map( ) constructor,
// using map constructor
void main( )
{
// declaring a map using a map( ) constructor
var maps1 = new Map( ) ;
// initializing keys with values
maps1[ 1 ] = 'one' ;
maps1[ 2 ] = 'two' ;
maps1[ 3 ] = 'three' ;
print( maps1 ) ;
}
Output :
{1: one, 2: two, 3: three}Maps Properties
In order to use map properties in the code, one must include dart : core package library that has a class named Map containing the properties.
Here is the list of some of the most used properties :
| Properties | Explanation |
| Keys | It is used to get all the keys as an iterable object. |
| values | It is used to get a value against the key as an iterable object in a map. |
| Length | Calculates the number of key-value pairs in a map and returns the same. |
| isEmpty | Checks if there are any key-value pairs in the map and accordingly returns true or false. |
| isNotEmpty | Checks if there is at least one key-value pair in a map and accordingly returns true or false. |
Consider the following code that lists all the properties of a Map.
Program
// using map constructor
void main( )
{
// declaring a map using a map( ) constructor
var maps1 = new Map( ) ;
// initializing keys with values
maps1[ 1 ] = 'one' ;
maps1[ 2 ] = 'two' ;
maps1[ 3 ] = 'three' ;
print( maps1 );
// using property keys. It prints all the keys of the given map
print( "\n The keys of the map are : ${ maps1.keys } " ) ;
// using property values. It prints all the values of the given map
print( "\n The values of the map are : ${ maps1.values } " ) ;
// using property length. It prints the number of key – value pairs in the map
print( "\n The length of the map is : ${ maps1.length } \n " ) ;
/// using property isEmpty. It returns true or false depending upon the
/// availability of key-value pair in the map
print( maps1.isEmpty ) ;
print( " \n " ) ;
/// using property isNotEmpty. It returns true or false depending upon the
/// availability of key - value pair in the map
print( maps1.isNotEmpty ) ;
}
Output
{1: one, 2: two, 3: three}
The keys of the map are : (1, 2, 3)
The values of the map are : (one, two, three)
The length of the map is : 3
false
true
Maps Functions
1. addAll( )
The addAll( ) function is used to add multiple key – value pairs to the list separated by a comma and enclosed within angular brackets ( < > ).
Implementation = Map.addAll( Map < Key, Value > other ) ;
Here, other denotes a key – value pair that returns a void type.
Program
void main( )
{
// assigning values to map
Map student = { 'name' : 'Yukta', 'Language' : 'Dart' } ;
// printing values of the map using string interpolation
print( 'Map Values : ${ student }' ) ;
// using addAll( ) function to add the entire set of keys - value pairs to map
student.addAll( { 'Age' : '19', 'College' : 'IIT - D' } ) ;
// printing the values of map after adding the new key - value pairs
print( ' Map after adding keys - values : ${ student } ' ) ;
}
Output :
Map Values : {name: Yukta, Language: Dart}
Map after adding keys - values : {name: Yukta, Language: Dart, Age: 19, College: IIT - D}
2. clear( )
The clear( ) function is used to remove all key – value pairs from the map.
Implementation = Map.clear( ) ;
Program
void main( )
{
// assigning values to the map student
Map student = { 'name' : 'Yukta', 'Language' : 'Dart' } ;
// printing values of the map using string interpolation
print( ' Map Values : ${ student } ' ) ;
// using addAll( ) function to add the entire set of keys - value pairs to map
student.addAll( { 'Age' : '19', 'College' : 'IIT - D' } ) ;
// printing the values of the map after adding the new key - value pairs
print( ' Map after adding keys - values : ${ student } ' ) ;
// using clear( ) function to clear the entire map
student.clear( ) ;
// printing the values of the map after clearing it
print( ' Map after clearing keys - values : ${ student } ' ) ;
}
Output :
Map Values : {name: Yukta, Language: Dart}
Map after adding keys - values : {name: Yukta, Language: Dart, Age: 19, College: IIT - D}
Map after clearing keys - values : {}
As you can see clearly, nothing gets printed after clearing the map. This is because the clear( ) function cleared all the key - value pairs in the map.
3. remove ( )
The remove ( ) function is used to remove a single key and its associated value in the map.
Implementation = Map.remove( Object key ) ;
Here, Key refers to the specific entry to be deleted. It returns the value associated with that key.
Program
void main( )
{
// assigning values to the map student
Map student = { 'name' : 'Yukta ', 'Language' : 'Dart' } ;
// printing values of map using string interpolation
print( ' Map Values : ${ student } ' ) ;
// using addAll( ) function to add the entire set of key - value pairs to map
student.addAll( { 'Age' : '19', 'College' : 'IIT - D' } ) ;
// printing the values of the map after adding the new key - value pairs
print( ' Map after adding keys - values : ${ student } ' ) ;
// using remove( ) function to remove the ‘ College ’ key from the map
student.remove( 'College' );
// printing the values of the map after removing the key ‘ College ’
print( ' Map after removing College key : ${ student } ' ) ;
}
Output:
Map Values : {name: Yukta , Language: Dart}
Map after adding keys - values : {name: Yukta , Language: Dart, Age: 19, College: IIT - D}
Map after removing College key : {name: Yukta , Language: Dart, Age: 19}
4. forEach( )
The forEach( ) function is used to iterate over the entries of a map.
Implementation = Map.forEach( void f ( K key, V value) ) ;
Here, f (K key, V value) applies f to each key-value pair of the map. Calling f must not add or remove keys from the map