第三天 dart基础知识 字符串

1.使用相邻字符串或者+号创建字符串

void main() {
//相邻字符串创建方式
  var str1 = 'hello'',world'",OK";
  print(str1);
//+号连接符创建方式
  var str2 = "hello" + 'world';
  var str3 = "li" + "huai";
  var str4 = '归海' + '一刀';

  print("str2=$str2,\nstr3=$str3,\nstr4=$str4");
}
hello,world,OK
str2=helloworld,
str3=lihuai,
str4=归海一刀

2.创建多行字符串—— ''' ... '''

void main() {
  var str = '''
  锄禾日当午
      汗滴禾下土
      谁知
     盘中
     餐
     
  粒粒皆辛苦
  ''';

  print(str);
}

  锄禾日当午
      汗滴禾下土
      谁知
     盘中
     餐
     
  粒粒皆辛苦
  

Process finished with exit code 0

3.在字符串前面加上字母小写的r
加r前

 var s1 = 'In a raw string, not even \n gets \t special treatment.';
  print(s1);

In a raw string, not even 
 gets    special treatment.

字符串中有转义字符可以识别

加r后

  var s = r'In a raw string, not even \n gets \t special treatment.';
  print(s);

In a raw string, not even \n gets \t special treatment.

加了r,转义符号 \n 换行 \t （按一次tab键的水平制表符）不起作用了

Booleans
bool 类型，只有两个值 true false
结果可以通过布尔表达式计算

void main() {
  var fullName = '';
//  assert(fullName.isEmpty);
  if(fullName.isEmpty){
    print("fullName is empty");
  }

// Check for zero.
  var hitPoints = 0;
//  assert(hitPoints <= 0);
  if(hitPoints <= 0){
    print("hitPoints <= 0");
  }

// Check for null.
  var unicorn;
//  assert(unicorn == null);
  if(unicorn == null){
    print("uicorn = null");
  }

// Check for NaN.
  var iMeantToDoThis = 0 / 0;
//  assert(iMeantToDoThis.isNaN);
  if(iMeantToDoThis.isNaN){
    print("0 / 0 不是一个数");
  }
}

fullName is empty
hitPoints <= 0
uicorn = null
0 / 0 不是一个数

Process finished with exit code 0

Lists

void main() {
  var list = [1,2,3];
  var list2 = ["hello","world",1,2,true];
//  list2 = list;


  bool h = true;
  list2[0] = h;
  list[0] = 3;
  for (var o in list2) {
    print(o);
  }
}

Maps
map使用key 和 value 进行关联，key和value可以是任意类型的值。
key不能重复，value可以重复。

void main() {
  //Map<String,String>
  var gifts = {
    // Key:    Value
    'first': 'partridge',
    'second': 'turtledoves',
    'fifth': 'golden rings'
  };
//Map<int, String>
  var nobleGases = {
    2: 'helium',
    10: 'neon',
    18: 'argon',
  };

  //1.gifts['first'] 中存的是字符串类型，这里我们存入数字类型就会报错
//  gifts['first'] = 2;

  //2.增加一个新的key-value
  gifts['five'] = 'bird';
  if(gifts.containsKey('five')){
    print(gifts['five']);//bird
  }
  //3.获取一个不存在的key-value,结果就是null
  var x = gifts['seven'];
  print("x=$x");//x=null
  //4.获取map中有几对 key-vlaue
  var y = nobleGases.length;
  print("nobleGases's length = $y");//3

  //5.使用Map() 代替 new Map() 构造器来创建map
  var fans = Map();
  fans[1] = '大米';
  fans[2] = '小米';
  fans[3] = '黑米';

  //6.lambda 表达式，向forEach传入一个回调函数
  fans.forEach((x,value) => print(fans[x]));

  //7.编译时常量的map
  final shui = const {
    '1':'农夫山泉',
    '3':'百岁山',
    '5':'怡宝'

  };
  //Error
//  shui = fans;//
  //shui['1'] = 'hello';//运行异常
}

bird
x=null
nobleGases's length = 3
大米
小米
黑米

Process finished with exit code 0