Scala中fold()操作和reduce()操作的區別

reduce()——規約操作，包含reduceLeft()和reduceRight()兩種操作。

fold()——摺疊操作，包含foldLeft()和foldRight()兩種操作。

兩者功能相似，不同之處在於：

fold()操作需要從一個初始值開始，並以該值作爲上下文，處理集合中的每個元素。

reduce()操作舉例：

scala> val list = List(1,2,3,4,5)
list: List[Int] = List(1, 2, 3, 4, 5)

scala> list.reduceLeft((x:Int,y:Int)=>{println(x,y);x+y})
(1,2)
(3,3)
(6,4)
(10,5)
res0: Int = 15

scala> list.reduceRight((x:Int,y:Int)=>{println(x,y);x+y})
(4,5)
(3,9)
(2,12)
(1,14)
res1: Int = 15

scala> val list = List(1,2,3,4,5)
list: List[Int] = List(1, 2, 3, 4, 5)

scala> list.reduceLeft((x:Int,y:Int)=>{println(x,y);x-y})
(1,2)
(-1,3)
(-4,4)
(-8,5)
res2: Int = -13

scala> list.reduceRight((x:Int,y:Int)=>{println(x,y);x-y})
(4,5)
(3,-1)
(2,4)
(1,-2)
res3: Int = 3

fold()操作舉例：

scala> val list = List(1,2,4,3,5)
list: List[Int] = List(1, 2, 4, 3, 5)

scala> list.fold(10)(_*_)
res0: Int = 1200

scala> list.foldLeft(0)((x:Int,y:Int)=>{println(x,y);x+y})
(0,1)
(1,2)
(3,4)
(7,3)
(10,5)
res1: Int = 15

scala> list.foldRight(0)((x:Int,y:Int)=>{println(x,y);x+y})
(5,0)
(3,5)
(4,8)
(2,12)
(1,14)
res2: Int = 3

上面fold(10)(_*_)，fold(0)中，10和0就是進行fold()操作所附的初值。