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2.scala控制结构、函数、异常处理

scala，大数据

2.1条件表达式

（1）if表达式有返回值，如val result = if(x>0) 0 else -1 ,则result值可能为0或-1
（2）if表达式分支语句返回值的类型不一致时，则表达式返回值的类型是它们共同的父类型Any
val result = if(x>0) 0 else "error"
则result的类型为Any
（3）若if表达式没有分支满足条件，则返回Unit

2.2块表达式

（1）块表达式的值是最后一个语句的返回值，一个块中有多个语句时用逗号隔开。如：
val result = {val x=2;val y=3;pow(x,y)}

2.3循环

（1）while循环的实现方式和java中一样。
（2）scala中的for循环写法如下：
for(i <- 1 to 10){ println(i) }
（3）for推导式。如果for循环的循环体以yield开始，则该循环会构造出一个集合，每次迭代生成集合中的一个值，如：
for(i <- 1 to 10) yield i%3 //得到Vector(1,2,0,1,2,0,1,2,0,1)

2.4函数

一个函数包含了函数名称，参数和函数体等三个元素，如：
def abs(x:Double) = if(x>=0) x else -x
如果不是递归函数，不需要给定返回值类型

2.5参数

（1）默认参数。调用某些函数时可以不用给出参数，此时使用默认参数。如：
def check(path:String,name:String="home")
使用check("local")时传入的两个参数分别为local和默认的home
（2）带名参数。在提供参数值的时候指定参数名，带名参数可以打乱参数列表的顺序，但是带名参数和不带名参数共同使用时，需要将不带名参数放在带名参数之前。
（3）变长参数。参数列表可以为同类型的多个参数。如：
def sum(num:Int*)={ val count = 0 for(i <-num){ count +=i } }
函数得到的是一个序列，但是不代表可以传入一个序列。如sum(1 to 5)这种写法是错误的。

2.6懒值

当val被生命为lazy时，它的初始化会被推迟，直到其被第一次取值时。如：
val a = 0 lazy val b = a*10

2.7异常处理

scala中异常处理与java类似。一般写法如下：
try{ process }catch{ case e:Throwable => some process case _ => other process }finally{ }