dart Quick Start Tutorial (2)

2. Variables and Data Types

2.1. Variables and constants

Variable popular to say that amount may change, the role is used to store data, you can put a variable considered as a fruit basket, which can be installed apples, pears, bananas, etc., the constant is a fixed value, and variables are relatively the variables can change, constant change is not

1. declare variables

In the dart in the var to declare a variable, and this variable is declared as js

void main() {
  var num = 10;
  print(num);  // 10
}

Note: There is no assignment to a variable, the default is null

void main() {
  var num;
  print(num);  // null
}

Variables can be used to declare a final keyword can only be assigned once

void main() {
  final num = 10;
  print(num);
  // num = 30; 
  // print(num);  // 把这两行注释打开会报错
}

2. constant declaration

Constant declaration requires the use of the const keyword, this const and es6 in similar

void main() {
  const num = 10;
  print(num);
  num = 30; // 报错
}

Const and the difference 3.final

2.2. Data Types

Data type dart provided are the following:

1. 数字类型-numbber  2. 字符串类型-string
3. 布尔类型-Boolean 4. 列表-list
5. Map      6. Runes
7、Symbols

1. Digital type (number)

It comprises a numeric integer (int) and float (double), when the defined variables can be predetermined type

void main() {
  // num是整型和浮点型的总称, 这个时候变量a里面既可以存整型也可以存浮点型
  num a = 20;
  print(a);
  a = 20.5;
  print(a);
}

void main() {
  // int表示定义整型，这个时候里面不能存浮点型
  int a = 20;
  print(a);
  a = 20.5; // 报错
  print(a);
}

void main() {
  double a = 20.5;
  print(a);
  a = 20;  // 没报错
  print(a);  // 20.0
}

2. Type String (String)

String can be defined in various ways, including single and double quotation marks, three marks, like the original string

void main() {
  // 单引号形式
  String str1 = 'hello nodeing!!';
  // 双引号形式
  String str2 = "hello nodeing！！！";
  // 三引号形式，可以创建多行字符串
  String str3 = '''
    <div>
      <h1>hello nodeing!!</h1>
    </div>
  ''';
  // 定义原始字符串
  String str4 = r'hello \n nodeing!!!';
  // 没有使用原始字符串定义
  String str5 = 'hello \n nodeing!!!';
 print(str1);
 print(str2);
 print(str3);
 print(str4);
 print(str5);
}

The above code should be noted that, because no str5 original string is defined, it \ n will be treated as a carriage return line

3. Boolean type (Boolean)

Boolean type is very simple, only two values, true and false

void main() {
  bool a = true;
  bool b = false;
  print(a);
  print(b);
}

4.list list

js list and in an array of similar, looks all the same

void main() {
  // 创建一个list
  var lis = [1, 2, 3];
  print(lis);
  // 通过类的方式创建
  var lis2 = new List();
  print(lis2);
  // 创建一个不可变的列表
  var lis3 = const [1, 2, 3];
  lis3[0] = 2; // 报错
}

5.Map type

Map is that some type of key-value pairs

void main() {
  // 创建一个list
  var mp = {'name': 'xiaoqiang', 'age': 18};
  print(mp);
  // 通过类的方式创建
  var mp2 = new Map();
  print(mp2);
  // 创建一个不可变的Map
  var mp3 = const {'name': 'xiaosan', 'age': 30};
  // 获取值
  print(mp3['name']);
  mp3['name'] = 'xiaosi'; // 报错
}

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