Set and mapping of collections in Scala

1. Set

• Unordered, unique and similar to Java
• Variable and immutable

Immutable set:

语法:
1) 创建一个空的不可变集，语法格式：
val/var 变量名 = Set[类型]()

2) 给定元素来创建一个不可变集，语法格式：
val/var 变量名 = Set(元素1, 元素2, 元素3...)

• Basic operations of immutable sets:

1. Get the size of the set (size)
2. Traverse set (consistent with traverse array)
3. Add an element, generate a Set (+)
4. Join two sets to generate a Set (++)
5. Combine sets and lists to generate a Set (++)

//定义
scala> val a = Set(15000,18000,20000,25000)
a: scala.collection.immutable.Set[Int] = Set(15000, 18000, 20000, 25000)

//获取长度
scala> a.size
res19: Int = 4

//遍历
scala> for(i<-a)println(i)
15000
18000
20000

// 拼接两个集
scala> a ++ Set(6,7,8)
res2: scala.collection.immutable.Set[Int] = Set(5, 1, 6, 2, 7, 3, 8, 4)
​
// 拼接集和列表
scala> a ++ List(6,7,8,9)
res6: scala.collection.immutable.Set[Int] = Set(5, 1, 6, 9, 2, 7, 3, 8, 4)

Mutable set: The operation is the same as the variable set, that is, you need to import the package import scala.collection.mutable.Set

2. Mapping

• Map can be called a mapping. It is a collection of key-value pairs. In scala, Map is also divided into immutable Map and mutable Map.

Immutable Map:

语法:
val/var map = Map(键->值, 键->值, 键->值...)  // 推荐，可读性更好

val/var map = Map((键, 值), (键, 值), (键, 值), (键, 值)...)

看例子:
scala> val map = Map("liuafu"->23,"hurong"->23)
map: scala.collection.immutable.Map[String,Int] = Map(liuafu -> 23, hurong -> 23)

//根据键获取值
scala> map("liuafu")
res24: Int = 23

Variable Map: The definition syntax is consistent with immutable Map. But to define a variable Map, you need to manually import import scala.collection.mutable.Map

About the basic operations of Map:

1. Get value (map(key))
2. Get all keys (map.keys)
3. Get all values ​​(map.values)
4. Traverse the map collection
5. getOrElse
6. Increase key, value pair +
7. Delete key-

scala> val map = Map("liuafu"->23,"hurong"->23)
map: scala.collection.immutable.Map[String,Int] = Map(liuafu -> 23, hurong -> 23)

//获取值
scala> map("liuafu")
res24: Int = 23

//获取所有key
scala> map.keys
res25: Iterable[String] = Set(liuafu, hurong)

//获取所有值
scala> map.values
res26: Iterable[Int] = MapLike(23, 23)

//打印所有的学生姓名和年龄
scala> for((x,y) <- a) println(s"$x $y")
lisi 40
zhangsan 30

3. iterator

• Same as in Java

scala> val a = List(18000,20000,22000)
a: List[Int] = List(18000, 20000, 22000)

scala> a.iterator
res29: Iterator[Int] = non-empty iterator

scala> for(i <- a)println(i)
18000
20000
22000