3、函数

这一期的视频主要内容是Kotlin中的函数
我们从这一期开始需要用到源文件，在IDEA中新建一个Kotlin文件的步骤如下：

1. 找到项目视图中的src文件夹
2. 在src文件夹上右键->New->Kotlin File/Class
3. 填写文件信息并确认

要运行Kotlin文件，文件中必须要有一个主函数：

fun main(args:Array<String>){
    
}

注意，Kotlin的所有函数都会返回一些东西（即使没有return语句），类似主函数这样的函数会返回一个type unit来代表没有值。type unit就是Kotlin认为的没有值，当一个函数返回type unit，那么就没必要写return

接下来是一个练习（必须要做，以后还会用到）：

1. Create a new Kotlin file.
2. Copy and paste the main() function from Hello World into the file.
3. Create a new function, dayOfWeek().
4. In the body of the function, print "What day is it today?"
5. Call dayOfWeek() from main().
6. Run your program.

官方答案是：

fun main(args: Array<String>) {
   dayOfWeek()
}

fun dayOfWeek() {
   println("What day is it today?")
}

下面继续练习：
In the body of the dayOfWeek() function, answer the question by printing the current day of the week.

这里有一系列提示：

1. You can use Java libraries (java.util) from Kotlin. For example, to get the day of the week:
2. Calendar.getInstance().get(Calendar.DAY_OF_WEEK)
3. Type in the code, then press Option + Enter in Mac, or Alt + Enter in Windows, over the red Calendar class to import the library.
4. Use a when statement to print a string depending on the day. Sunday is the first day of the week.

官方答案是：

fun dayOfWeek() {
   println("What day is it today?")
   val day = Calendar.getInstance().get(Calendar.DAY_OF_WEEK)
   println( when (day) {
      1 -> "Sunday"
      2 -> "Monday"
      3 -> "Tuesday"
      4 -> "Wednesday"
      5 -> "Thursday"
      6 -> "Friday"
      7 -> "Saturday"
      else -> "Time has stopped"
   })
}

接下来我们要学习的是命令行参数。
首先在IDEA中打开run->edit configuration，在program arguments中填入kotlin，之后确认
然后我们需要在程序中使用这个参数，将main函数修改如下：

fun main(args:Array<String>){
    println("我爱你~${args[0]}")

    val isUnit = println("This is an expression")
    println(isUnit)

    val temperature = 10
    val isHot = if(temperature>50) true else false
    println(isHot)

    val message = "You are ${if(temperature>50) "fried" else "safe"} fish"
    println(message)
}

上面的例子还说明了，在${}中可以使用表达式
那么再做一些练习题~
Create a main() function that takes an argument representing the time in 24-hour format (values between and including 0 -> 23).

In the main() function, check if the time is before midday (<12), then print "Good morning, Kotlin"; otherwise, print "Good night, Kotlin".
注意：
Remember that all main() function arguments are Strings, so you will have to convert this argument to an Int before you can apply the check.
高级一点：
Try to use Kotlin's string templates to do this in 1 line.

正确答案是（包含两种写法）：

fun main(args:Array<String>){
    if (args[0].toInt()<12) println("Good morning Kotlin")
    else println("Good night Kotlin")

    // 一行表示
    println("Good ${if (args[0].toInt()<12) "morning" else "night"},Kotlin")
}