3、随机数

今天的课程主要将怎样在Kotlin中使用随机数：
首先复习一下上一期的内容，写一个函数：

fun feedTheFish(){
    val day = "Tuesday"
    val food = "pellets"
    println("Today is $day and the fish eat $food")
}

之后再写一个：

fun randomDay():String{
    val week = listOf("Monday","Tuesday","Wednesday","Thursday","Friday","Saturday","Sunday")
    return week[Random().nextInt(7)]
}

其中，Random().nextInt(7)生成一个从0到6（不包含7）的随机数字
接下来又是练习题：

1. Create a main() function.
2. From the main() function, call a function, getFortuneCookie(), that returns a String.
3. Create a getFortuneCookie() function that takes no arguments and returns a String.
4. In the body of getFortuneCookie(), create a list of fortunes. Here are some ideas:
   "You will have a great day!"
   "Things will go well for you today."
   "Enjoy a wonderful day of success."
   "Be humble and all will turn out well."
   "Today is a good day for exercising restraint."
   "Take it easy and enjoy life!"
   "Treasure your friends because they are your greatest fortune."
5. Below the list, print: "Enter your birthday: "
   Hint: Use print(), not println()
6. Create a variable, birthday.
7. Read the user's input form the standard input and assign it to birthday. If there is no valid input, set birthday to 1.
   Hint: Use readLine() to read a line of input (completed with Enter) as a String.
   Hint: In Kotlin, you can use toIntOrNull() to convert a number as a String to an Integer numeric. If the user enters "", toIntOrNull returns null.
   Hint: Check for null using the ? operator and use the ?: operator to handle the null case.
8. Divide the birthday by the number of fortunes, and use the remainder as the index for the fortune to return.
9. Return the fortune.
10. In main(), print: "Your fortune is: ", followed by the fortune string.

还有额外练习：
Use a for loop to run the program 10 times, or until the "Take it easy" fortune has been selected.

标准答案是：

fun main(args: Array<String>) {
   println("\nYour fortune is: ${getFortune()}")
}

fun getFortune() : String {
   val fortunes = listOf( "You will have a great day!",
      "Things will go well for you today.",
      "Enjoy a wonderful day of success.",
      "Be humble and all will turn out well.",
      "Today is a good day for exercising restraint.",
      "Take it easy and enjoy life!",
      "Treasure your friends, because they are your greatest fortune.")
   print("\nEnter your birthday: ")
   val birthday = readLine()?.toIntOrNull() ?: 1
   return fortunes[birthday.rem(fortunes.size)]
}

扩展练习的答案是：

fun main(args: Array<String>) {
   var fortune: String
   for (i in 1..10) {
      fortune = getFortune()
      println("\nYour fortune is: $fortune")
      if (fortune.contains("Take it easy")) break
   }
}