[fluent] dart data type map type (create map set | initialize map set | traverse map set)

One 、 Dart data type Map type

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Dart Medium Map Data type and Java similar , It consists of key value pairs , key Key , value Value ;

among Key The value of the Map It has to be the only one , Value The value of can be repeated ;

Two 、 Map Type initialization and assignment

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1、 establish Map Objects are initialized at the same time

establish Map Objects are initialized at the same time : adopt {} initialization Map object , Each element has the form Key : Value , Of each element key ( Key ) And value ( Value ) Used between The colon " : " Division , Use between elements comma " , " Division ;

Code example :

    //  adopt  {}  initialization  Map  object ,  Each element has the form  Key : Value    //  key ( Key )  And   value  ( Value )  Use colons between  " : "  Division     //  Use commas between elements  " , "  Division     Map student = { 
    1 : "Tom", 2 : "Jerry", 3 : "Trump"};    //  Print  Map  aggregate     print(student);

Execution results :

{ 
    1: Tom, 2: Jerry, 3: Trump}

2、 First create Map Object is assigned again

First create Map Object is assigned again : First create an empty Map aggregate , Use Subscript By Map Set assignment , Usage is as follows :

    // II .  First create the empty  Map  aggregate  ,  Then initialize     Map president = { 
    };    //  by  Map  Set add elements     president[1] = "Bush";    president[2] = "Obama";    president[3] = "Trump";    //  Print  Map  aggregate     print(president);

Print the results :

{ 
    1: Bush, 2: Obama, 3: Trump}

3、 ... and 、 Map A collection of traverse

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1、 Use forEach Traverse Map aggregate

Use forEach Traverse Map aggregate :

    // 1 .  Use  forEach  Traversal     president.forEach( (key, value){ 
          print("forEach  Traverse  : $key : $value");    } );

Print the results :

forEach  Traverse  : 1 : BushforEach  Traverse  : 2 : ObamaforEach  Traverse  : 3 : Trump

2、 Using normal for Loop traversal Map aggregate

Using normal for Loop traversal Map aggregate :

    // 2 .  adopt  for  Loop traversal  Map  aggregate     //  call  Map  Object's  keys  member  ,  Returns a   key  Key  Array of components     for (var key in president.keys){ 
          print("for  Loop traversal  : Key : $key , Value : ${president[key]}");    }

Print the results :

for  Loop traversal  : Key : 1 , Value : Bushfor  Loop traversal  : Key : 2 , Value : Obamafor  Loop traversal  : Key : 3 , Value : Trump

3、 Use map Method to generate a new Map aggregate

Use map Method to generate a new Map aggregate : Use map Method Traversal , During traversal Generate a new Map aggregate , After traversing , Will return a new Map aggregate , Pass in a callback function , Parameter is Map Of each element in the collection Key value pair key and value , The return value is new Map aggregate ;

The following example will primary Map Key value pairs in the set are swapped , Make a new one Map aggregate , And print new Map What's in the collection ;

    // 3 .  Use  map  Method     //  Generate new  Map  aggregate     //  After traversing  ,  Will return a new  Map  aggregate     //  Pass in a callback function  ,  Parameter is  key value ,  The return value is new  Map  aggregate     Map president2 = president.map(            (key, value){ 
              //  There will be  Map  In the collection  Key  and  Value  Reverse  ,  Make a new one  Map  aggregate           return MapEntry(value, key);        }    );    //  Print new  Map  aggregate     print(president2);

Print the results :

{ 
    Bush: 1, Obama: 2, Trump: 3}

Four 、 Complete code example

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import 'package:flutter/material.dart';class DartType_Map extends StatefulWidget { 
      @override  _DartType_ListState createState() => _DartType_ListState();}class _DartType_ListState extends State<DartType_Map> { 
      @override  Widget build(BuildContext context) { 
        mapDemo();    return Container(child: Text('Map  data type '));  }  /** * Map  Example  */  mapDemo(){ 
          // I .  Definition  Map  Collect and initialize     //  adopt  {}  initialization  Map  data ,  Each element has the form  Key : Value    //  key ( Key )  And   value  ( Value )  Use colons between  " : "  Division     //  Use commas between elements  " , "  Division     Map student = { 
    1 : "Tom", 2 : "Jerry", 3 : "Trump"};    //  Print  Map  aggregate     print(student);    // II .  First create the empty  Map  aggregate  ,  Then initialize     Map president = { 
    };    //  by  Map  Set add elements     president[1] = "Bush";    president[2] = "Obama";    president[3] = "Trump";    //  Print  Map  aggregate     print(president);    // III . Map  A collection of traverse     // 1 .  Use  forEach  Traversal     president.forEach( (key, value){ 
          print("forEach  Traverse  : $key : $value");    } );    // 2 .  adopt  for  Loop traversal  Map  aggregate     //  call  Map  Object's  keys  member  ,  Returns a   key  Key  Array of components     for (var key in president.keys){ 
          print("for  Loop traversal  : Key : $key , Value : ${president[key]}");    }    // 3 .  Use  map  Method     //  Generate new  Map  aggregate     //  After traversing  ,  Will return a new  Map  aggregate     //  Pass in a callback function  ,  Parameter is  key value ,  The return value is new  Map  aggregate     Map president2 = president.map(            (key, value){ 
              //  There will be  Map  In the collection  Key  and  Value  Reverse  ,  Make a new one  Map  aggregate           return MapEntry(value, key);        }    );    //  Print new  Map  aggregate     print(president2);  }}

Execution results :

{ 
    1: Tom, 2: Jerry, 3: Trump}{ 
    1: Bush, 2: Obama, 3: Trump}forEach  Traverse  : 1 : BushforEach  Traverse  : 2 : ObamaforEach  Traverse  : 3 : Trumpfor  Loop traversal  : Key : 1 , Value : Bushfor  Loop traversal  : Key : 2 , Value : Obamafor  Loop traversal  : Key : 3 , Value : Trump{ 
    Bush: 1, Obama: 2, Trump: 3}

5、 ... and 、 Related resources

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Reference material :

• Dart Developer website : https://api.dart.dev/
• Flutter Chinese net ( unofficial , The translation is very good ) : https://flutterchina.club/ , http://flutter.axuer.com/docs/
• Flutter Official website : https://flutter.dev/ ( Quilt wall )
• official GitHub Address : https://github.com/flutter
• Flutter Related issues : https://flutterchina.club/faq/ ( It is recommended to watch it at the introductory stage )

Blog source download :

• GitHub Address : https://github.com/han1202012/flutter_app_hello ( Keep updating with the progress of the blog , There may not be the source code of this blog )

• Blog source snapshot : https://download.csdn.net/download/han1202012/15087696 ( The source code snapshot of this blog , You can find the source code of this blog )