ScalaTour 2.函数

/**
* 1. case class与模式匹配
*/
object TestFunction extends App{
def value(expr:Expr): Int = expr match {
case FibonaciExpr(0) => 0
case FibonaciExpr(1) => 1
case FibonaciExpr(n) => value(SumExpr(FibonaciExpr(n-1),FibonaciExpr(n-2)))
case SumExpr(a,b) => value(a) + value(b)
case _ => 0
}

println(value(FibonaciExpr(3)))
}

abstract class Expr
case class FibonaciExpr(n:Int) extends Expr{
require(n >= 0)   //PreDef中的方法，用于参数校验
}

case class SumExpr(a:Expr,b:Expr) extends Expr

/**
* 2. scala中函数对代码的简化
*/
object TestFun2 extends App{
val list = List(1,2,3,4,5,6)
println("list包含基数吗："+list.exists(_ %2 ==1 ))

val fileName = List("warn 2013 msg", "warn 2012 msg","error 2013 msg", "warn 2013 msgs")
println("cat file | grep 'warn' | grep '2013' | wc : "+fileName.filter(_.contains("warn")).filter(_.contains("2013")))
}

/**
* 3. scala的wordcount
*/
object TestWordCount extends App{
val file = List("warn 2013 msg", "warn 2012 msg","error 2013 msg", "warn 2013 msg")
def countInStr(str:String):Int = str.split(" ").count("msg"==_)
val num = file.map(countInStr).reduceLeft(_ + _)
println(num)
}

/**
* 4. 尾递归 ： 函数的最后一步返回函数本身，没有其他的操作
* foldLeft的尾递归实现
*/
object TestTailRec extends App{
val file = List("warn 2013 msg", "warn 2012 msg","msg 2013 msg", "warn 2013 msg")
def countInStr(str:String):Int = str.split(" ").count("msg"==_)
println(file.map(countInStr))  //List(1, 1, 2, 1)
def foldLeft(list:List[Int])(init:Int)(f:(Int,Int) =>Int) :Int = {
list match {
case List() => init
case head::tail => foldLeft(tail)(f(head,init))(f)
}
}
println(foldLeft(file.map(countInStr))(0)(_+_))  // 1+1+2+1 = 5 [科里化]
}

/**
* 5. 生成List的for循环
*/
object TestFor2List extends App{
val file = List("warn 2013 msg", "warn 2012 msg","msg 2013 msg", "warn 2013 msg")
def countInStr(str:String):Int = str.split(" ").count("msg"==_)
val counts = for(str <- file) yield countInStr(str)
val num = counts.reduceLeft(_+_)
println(num)
}

/**
* 6. Option类型
*/
object TestOption extends App{
def getProperty(param:String): Option[String] ={
val value = System.getProperty(param)
value match {
case null => None
case _ => Some(value)
}
}

val osName = getProperty("os.name")

println(osName.getOrElse("none"))      //1. Option类型的getOrElse方法
osName.foreach(println(_))             // 2. Some类型的值可以作为长度为1的List处理
osName match {                         //3. Option支持类型匹配
case Some(value) => println(value)
case _ => println("null")
}
}