Scala 学习手册第三章

Scala 学习手册(Learning Scala)

表达式

定义：
在执行后会返回一个值的代码单元

scala> "hello world"
res53: String = hello world

scala> "hel" + "l" + "o"
res54: String = hello

定义值和变量

定义：

val <identifier>[:<type>] = <experssion>

val <identifier>[:<type>] = <experssion>

例子

scala> val value:Double = 3.43454
value: Double = 3.43454

scala> var data:String = "333Pi"
data: String = 333Pi

表达块

可以使用大括号结合多个表达式创建一个表达式块。

scala> val x = 5*20;val amount = x + 20
x: Int = 100
amount: Int = 120

scala> val amount = {val x = 5*20;x+10}
amount: Int = 110

scala> {val a = 1;{val b = a*2;{val c=b+4;c}}}
res55: Int = 6

注释：第二个以"x+10"作为返回值,第三个以"c"作为返回值

分行

scala> val amount = {
        val x = 5*20
        x + 10
     }
amount: Int = 110

if else表达块

Scala只支持一个"if"和可选的"else"块

if (<Boolean experssion>) <expression>

例子

scala> if(47 % 3 > 0) println("Not a multiple of 3")
Not a multiple of 3


scala> val x = 10;val y = 20
x: Int = 10
y: Int = 20

scala> val max = if(x>y) x else y
max: Int = 20

注意：如果if表达式没有相应的else表达式，则必须使用大括号。

匹配

匹配表达式

<expression> match{
    case <pattern> match => <expression>
    [case...]
}

例子

scala> val x = 10;val y = 20
x: Int = 10
y: Int = 20

scala> val max = x > y match {
        case true => x
        case false => y
        }
max: Int = 20


scala> val status = 500
status: Int = 500

scala> val message = status match {
        case 200 => "ok"
        case 400 => {
        println("ERROR - we called the service incorrectly")
        "error"
        }
        case 500 => {
        println("ERROR - the service encountered an error")
        "error"
        }
        }
ERROR - the service encountered an error
message: String = error

注意：=> 表示传递，后面跟多个表达式。如果一个case块中有多个表达式，可以通过大括号把它们转换为一个表达式块。

匹配替换式

case <pattern 1> | <pattern 2>.. => <one or more expression>

例子

scala> val day = "MON"
day: String = MON

scala> val kind = day match {
        case "MON" | "TUE" | "WED" | "THU" | "FRI" =>
        "weekday"
        case "SAT" | "SUN" => 
        "weekend"
        }
kind: String = weekday

注意：候选匹配必须能匹配上，不然就会报错误。

scala> "match me" match {case "nope" => "sorry"}
scala.MatchError: match me (of class java.lang.String)
  ... 28 elided

因为没有囊括所有的可能，导致匹配不上的错误。

用通配模式匹配

值绑定

分为值绑定、变量绑定：将把匹配表达式的输入绑定到一个局部值，然后可以在case块的体中使用。值绑定也是通配模式。

scala> val message = "OK"
message: String = OK


scala> val status = message match {
        case "OK" => 200
        case other => {
        println(s"Couldn't parse $other")
        }
        }
status: AnyVal = 200




scala> val message = "werwere"
message: String = werwere

scala> val status = message match {
              case "OK" => 200
              case other => {
              println(s"Couldn't parse $other")
              }
              }
Couldn't parse werwere
status: AnyVal = ()

可以看出，绑定和字符串内插相似。

通配符(_)

case _ => <one or more expressions>

例子

scala> val message = "Unauthorized"
message: String = Unauthorized

scala> val status = message match {
        case "OK" => 200
        case _ => {
        println(s"Couldn't parse $message")
        -1
        }
        }
Couldn't parse Unauthorized
status: Int = -1

不能像值匹配一样访问通配符

模式哨位匹配

值绑定模式增加一个if表达式，即case if 模式，从而可以为匹配表达式增加条件逻辑，只有当表达式返回true时模式才匹配。

case <pattern> if <Boolean expression> => <one or more expressions>

例子

scala> val response :String = null
response: String = null

scala> response match {
        case s if s != null => println(s"Received '$s'")
        case s => println("ERROR! Received a null response")
        }
ERROR! Received a null response

匹配变量类型

case <identifier>:<type> => <one or more expressions>

用于匹配类型

scala> val x:Int = 12180
x: Int = 12180

scala> val y:Any = x
y: Any = 12180

scala> 

scala> y match {
     case x:String => s"'x'"
     case x:Double => f"$x%.2f"
     case x:Float => f"$x%.2f"
     case x:Long => s"${x}l"
     case x:Int => s"${x}i"
     }
res60: String = 12180i

循环

主要是for循环

定义范围

scala> val x= 1 to 7
x: scala.collection.immutable.Range.Inclusive = Range 1 to 7

for循环

定义：

for (<idenifier> <- <iterator> [yield] [<expression>])

yield为可选，若是使用了yield,则将产生一个集合作为返回。若是没有，但是指定了表达式，调用时返回表达式，不能访问他的返回值。

例子：

scala> for (x <- 1 to 7) {println(s"Day: $x:")}
Day: 1:
Day: 2:
Day: 3:
Day: 4:
Day: 5:
Day: 6:
Day: 7:

带有yield关键词的

scala> for (x <- 1 to 7) yield {s"Day $x:"}
res62: scala.collection.immutable.IndexedSeq[String] = Vector(Day 1:, Day 2:, Day 3:, Day 4:, Day 5:, Day 6:, Day 7:)

迭代哨位

即for if模式

for (<identifier> <- <iterator> if <Boolean expression>)

例子

scala> val threes = for(i <- 1 to 20 if i % 3 == 0) yield i
threes: scala.collection.immutable.IndexedSeq[Int] = Vector(3, 6, 9, 12, 15, 18)



scala> val quote = "Faith,Hope,Charity"
quote: String = Faith,Hope,Charity

scala> for {
     t <- quote.split(",")
     if t != null
     if t.size > 0
     }
     {println(t)}
Faith
Hope
Charity

嵌套迭代

即多个迭代器

scala> for {x <- 1 to 2
     y <- 1 to 6}
     {println(s"($x,$y)")}
(1,1)
(1,2)
(1,3)
(1,4)
(1,5)
(1,6)
(2,1)
(2,2)
(2,3)
(2,4)
(2,5)
(2,6)

while 和 Do/while

while

scala> var x = 10;while(x > 0) x -= 1
x: Int = 0

Do/while

scala> val x = 0
x: Int = 0

scala> do println(s"Here I am,x = $x") while (x > 0)
Here I am,x = 0