tuple

tuple 是可存取不同型態的一種物件，它是 immutable 的．格式像這樣 (element1 , element1 , ...)

scala> val nums = (1,"a",2D,3L)
nums: (Int, String, Double, Long) = (1,a,2.0,3)

使用 ._ 取得 tuples 的 element 並從 index 1 開始．

scala> nums._1
res34: Int = 1

其實 tuple 是根據 element 的數量，scala 幫你 new Tuple 的物件，例 : new Tuple2 , new Tuple3 , ... , new Tuple22．tuple 的 elements 目前最多只能存到 22 個，如果超過可能也要思考一下設計面是否恰當．

scala> val tupleObj = (1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23)
<console>:1: error: too many elements for tuple: 23, allowed: 22
       val tupleObj = (1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23)
                      ^

scala> val t22 = new Tuple22(1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22)
t22: (Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int) = (1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22)

tuple 如果只有兩個 element 的話，可以用 (element1) -> (element2) 產生．

scala> 1 -> "Daniel"
res1: (Int, String) = (1,Daniel)

tuple 的值也可以用變數存取．

scala> val (id,name) = 1 -> "Daniel"
id: Int = 1
name: String = Daniel

scala> id
res2: Int = 1

使用 filter find tuple :

filter 會根據寫的條件，找到需要的元素．

scala> val empList = List("Sam"->10,"Daniel"->20,"Jack"->30,"Ray"->40)
empList: List[(String, Int)] = List((Sam,10), (Daniel,20), (Jack,30), (Ray,40))

scala> def findName(name:String,empList:List[(String,Int)]) = empList.filter(emp => emp._1 == name)
findName: (name: String, empList: List[(String, Int)])List[(String, Int)]

scala> findName("Daniel",empList)(0)._2
res7: Int = 20

使用 map sum tuple :

map 會走訪 collection 的每個元素然後做處理．下面例子是取出每個 tuple 的第二個元素，然後是一個新的 List 最後再使用 sum 方法將 List 所有元素加起來．

scala> val wordsCount = List('A'->10,'C'->20,'E'->30,'Y'->40)
wordsCount: List[(Char, Int)] = List((A,10), (C,20), (E,30), (Y,40))

scala> def sum(wordsData:List[(Char,Int)]) = wordsData.map(_._2).sum
sum: (wordsData: List[(Char, Int)])Int

scala> sum(wordsCount)
res9: Int = 100

foldLeft 版本 (另外還有有fold、foldRight)

foldLeft 定義 : def foldLeft[B](z: B)(f: (B, A) => B): B
foldLeft 需要兩個參數，第一個是初始值(z: B)、第二個是一個函數(f: (B, A) => B)，然後回傳初始值的型態(B)

scala> val wordsCount = List('A'->10,'C'->20,'E'->30,'Y'->40)
wordsCount: List[(Char, Int)] = List((A,10), (C,20), (E,30), (Y,40))

scala> def sum(wordsData:List[(Char,Int)]) = wordsData.foldLeft(0){(sum,tup) => sum + tup._2}
sum: (wordsData: List[(Char, Int)])Int

scala> sum(wordsCount)
res11: Int = 100

總結

• scala 的 tuple 由於可以存放不同型態的物件，所以彈性很大，可以省略定義一些單純存放資料的物件，直接用 tuple 傳遞即可．