這一期的視頻主要內(nèi)容是Kotlin中的函數(shù)
我們從這一期開始需要用到源文件，在IDEA中新建一個Kotlin文件的步驟如下：

1. 找到項目視圖中的src文件夾
2. 在src文件夾上右鍵->New->Kotlin File/Class
3. 填寫文件信息并確認

要運行Kotlin文件，文件中必須要有一個主函數(shù)：

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

注意，Kotlin的所有函數(shù)都會返回一些東西（即使沒有return語句），類似主函數(shù)這樣的函數(shù)會返回一個type unit來代表沒有值。type unit就是Kotlin認為的沒有值，當一個函數(shù)返回type unit，那么就沒必要寫return

接下來是一個練習(xí)（必須要做，以后還會用到）：

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?")
}

下面繼續(xù)練習(xí)：
In the body of the dayOfWeek() function, answer the question by printing the current day of the week.

這里有一系列提示：

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"
   })
}

接下來我們要學(xué)習(xí)的是命令行參數(shù)。
首先在IDEA中打開run->edit configuration，在program arguments中填入kotlin，之后確認
然后我們需要在程序中使用這個參數(shù)，將main函數(shù)修改如下：

fun main(args:Array<String>){
    println("我愛你~${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)
}

上面的例子還說明了，在${}中可以使用表達式
那么再做一些練習(xí)題~
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.
高級一點：
Try to use Kotlin's string templates to do this in 1 line.

正確答案是（包含兩種寫法）：

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")
}