|NO.Z.00014|——————————|BigDataEnd|——|Hadoop&Scala.V01|——|Scala.v01|Scala函数|if表达式|

一、if 表达式

### --- if表达式

~~~     Scala中 if 表达式有返回值。
~~~     如果if 和 else 的返回值类型不一样，那么就返回两个返回值类型公共的父类。

### --- 编程示例

~~~     # if 语句有返回值
val x = 10
val s = if (x > 0)
1
else
-1

~~~     # 多分支if 语句

val s = if (x==0)
0
else if (x > 1)
1
else
0

~~~     # 如果返回的类型不一致就返回公共的父类

val s = if (x > 0)
"positive"
else
-1

~~~     # 缺省 else 语句；s1/s2的返回类型不同

val s1 = if (x > 0) 1 等价 val s1 = if (x > 0) 1 else ()
val s2 = if (x > 0) "positive" 等价 val s2 = if (x > 0) "positive" else
()

### --- 编程代码

package yanqi.cn.part02

object IfDemo {
  def main(args: Array[String]): Unit = {
    //在Scala中不需要添加分号作为语句块的结束符
    val num = 20
    //在Scala中 if else语句是有返回值的，返回值就是最后一条语句的返回值
    if (num > 20) "zhangsan" else "jacky"
    //if语句可以嵌套
    if (num < 20)
      0
    else if (num == 20)
      1
    else
      -1
    //因为if else语句是有返回值的，所以可以直接将if else语句赋值给一个变量
    //注意：返回值不需加return关键字
    val result = if (num > 20) "zhangsan" else "jacky"
    //如果if else语句中返回值的类型不一样，那么Scala会自动推断出两者的公共父类型，作为表达式的返回值类开型
    val result2: Any = if (num == 20) "jacky" else 100
    println(result2)

    //如果if else语句中缺省了else语句块，那么默认else的值是Unit
    //Unit用“()”来表示，类似于Java中的void
    val result3 = if (num > 20) "jacky"
    val result4 = if (num > 20) "jacky" else ()
  }
}

### --- 编译打印

D:\JAVA\jdk1.8.0_231\bin\java.exe "-javaagent:D:\IntelliJIDEA\IntelliJ IDEA 2019.3.3\lib\idea_rt.jar=52540:D:\IntelliJIDEA\IntelliJ IDEA 2019.3.3\bin" -Dfile.encoding=UTF-8 -classpath D:\JAVA\jdk1.8.0_231\jre\lib\charsets.jar;D:\JAVA\jdk1.8.0_231\jre\lib\deploy.jar;D:\JAVA\jdk1.8.0_231\jre\lib\ext\access-bridge-64.jar;D:\JAVA\jdk1.8.0_231\jre\lib\ext\cldrdata.jar;D:\JAVA\jdk1.8.0_231\jre\lib\ext\dnsns.jar;D:\JAVA\jdk1.8.0_231\jre\lib\ext\jaccess.jar;D:\JAVA\jdk1.8.0_231\jre\lib\ext\jfxrt.jar;D:\JAVA\jdk1.8.0_231\jre\lib\ext\localedata.jar;D:\JAVA\jdk1.8.0_231\jre\lib\ext\nashorn.jar;D:\JAVA\jdk1.8.0_231\jre\lib\ext\sunec.jar;D:\JAVA\jdk1.8.0_231\jre\lib\ext\sunjce_provider.jar;D:\JAVA\jdk1.8.0_231\jre\lib\ext\sunmscapi.jar;D:\JAVA\jdk1.8.0_231\jre\lib\ext\sunpkcs11.jar;D:\JAVA\jdk1.8.0_231\jre\lib\ext\zipfs.jar;D:\JAVA\jdk1.8.0_231\jre\lib\javaws.jar;D:\JAVA\jdk1.8.0_231\jre\lib\jce.jar;D:\JAVA\jdk1.8.0_231\jre\lib\jfr.jar;D:\JAVA\jdk1.8.0_231\jre\lib\jfxswt.jar;D:\JAVA\jdk1.8.0_231\jre\lib\jsse.jar;D:\JAVA\jdk1.8.0_231\jre\lib\management-agent.jar;D:\JAVA\jdk1.8.0_231\jre\lib\plugin.jar;D:\JAVA\jdk1.8.0_231\jre\lib\resources.jar;D:\JAVA\jdk1.8.0_231\jre\lib\rt.jar;E:\NO.Z.10000——javaproject\NO.Z.00002.Hadoop\ScalaPro\out\production\ScalaPro;D:\JAVA\scala-2.12.2\lib\scala-library.jar;D:\JAVA\scala-2.12.2\lib\scala-reflect.jar yanqi.cn.part02.IfDemo
jacky

Process finished with exit code 0

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Walter Savage Landor:strove with none,for none was worth my strife.Nature I loved and, next to Nature, Art:I warm'd both hands before the fire of life.It sinks, and I am ready to depart

                                                                                                                                                   ——W.S.Landor

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