Scala中 zip或者zipWithIndex的用法
问题：
你要遍历一个有序集合，同时你又想访问一个循环计数器，但最重要的是你真的不需要手动创建这个计数器。
解决方案：
使用zipWithIndex或者zip方法来自动地创建一个计数器，假设你有一个有序集合days，那么你可以使用zipWithIndex和counter来打印带有计数器的集合元素：
scala> val days = Array("Sunday", "Monday", "Tuesday", "Wednesday","Thursday", "Friday", "Saturday")
days: Array[String] = Array(Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, Saturday)
scala> days.zipWithIndex.foreach{case(day,count) => println(s"$count is $day")}
0 is Sunday
1 is Monday
2 is Tuesday
3 is Wednesday
4 is Thursday
5 is Friday
6 is Saturday
同样，你可以使用for循环来打印计数器和集合元素
scala> for((day,count) <- days.zipWithIndex) {
| println(s"$count is $day")
| }
0 is Sunday
1 is Monday
2 is Tuesday
3 is Wednesday
4 is Thursday
5 is Friday
6 is Saturday
zipWithIndex的计数器都是从0开始，如果你想指定开始的值，那么你可以使用zip Stream：
scala> for((day,count) <- days.zip(Stream from 1)) {
| println(s"$count is $day")
| }
1 is Sunday
2 is Monday
3 is Tuesday
4 is Wednesday
5 is Thursday
6 is Friday
7 is Saturday
Discussion
当有序集合调用zipWithIndex的时候，它会返回一个有序的二元组集合：
scala> val list = List("a", "b", "c")
list: List[String] = List(a, b, c)
scala> list.zipWithIndex
res3: List[(String, Int)] = List((a,0), (b,1), (c,2))
因为zipWithIndex是在一个已经存在的有序集合的基础上建立一个新的有序集合，你可以在调用zipWithIndex之前调用view：
-
scala> val zwv = list.view.zipWithIndex
-
zwv: scala.collection.SeqView[(String, Int),Seq[_]] = SeqViewZ(...)
就像上面这个例子里面看到的，它在原有的List基础上创建了一个lazy view，所以这个元组集合并不被会被创建，直到它被调用的那一刻。正因有这种特性，我们推荐在调用zipWithIndex之前先调用view方法。
zip和zipWithIndex方法都返回一个有序二元组集合。因此，你的foreach方法也可以写成下面这样，虽然这比起解决方案中的方法，可读性略差。
scala> days.zipWithIndex.foreach(d => println(s"${d._2} is ${d._1}"))
0 is Sunday
1 is Monday
2 is Tuesday
3 is Wednesday
4 is Thursday
5 is Friday
6 is Saturday
在之前的例子中我们曾经见过，可以通过一个for循环加range来创建这个计数器：
scala> val fruits = Array("apple", "banana", "orange")
fruits: Array[String] = Array(apple, banana, orange)
scala> for (i <- 0 until fruits.size) println(s"element $i is ${fruits(i)}")
element 0 is apple
element 1 is banana
element 2 is orange
